REFLECTORS FOR PLANETARY WORK

[philsail1]

To answer Phil's questions:-

The f/10 'scope would be easier to collimate accurately and not as sensitive to collimation so the view would be slightly better. The f/10 would be easier on the eyepiece design so cheaper eyepieces could be used. The f/10 would have double the magnification for a given eyepiece so you could use longer focal length eyepieces for the same size image, giving more comfortable eye relief.

The curved vanes would get rid of the diffraction spikes and make the effects of the diffraction less noticeable.

For planets you don't need expensive eyepieces as you don't need a wide view.

Flocking and curved vanes are about all you can do to the f/5 for improving the views as far as I know.

Kaptain Klevtsov

Thanks Captain K. That clarifies things a lot for me. I'm glad that I took all your, and other members' advice last year and bought the Skywatcher 200mm f5 scope.

regards,

philsail1

[philsail1]

Sorry - I had to leave SGL there - my brother called round to see me!

Thanks to "Gaz O'c" for explaining the advantages of "curved" spider vanes. Wouldn't mind exploring this further with trying to obtain a set for my Skywatcher 200mm Newt. I wonder whether any improvement in views would be enough to justify the expense of buying a new set of curved vanes (assuming I can obtain a set for the Skywatcher).

Kaptain K. Thanks again for your answer on the "f" numbers. Oh! and I've already flocked the inside of the Newt, but for the life of me I cannot recall whether it has made that much difference! At least it was a cheap improvement. (The most noticeable improvement to the scope, (but not directly optical) was to fit the single speed "Crayford" focuser I bought from "FLO" last year. This has made a really marked improvement in making focusing dead easy - and getting it spot on!

Regards,

philsail1

P.S. Lovely thin crescent Moon tonight. The "Earthshine" was beautiful.

[KaStern_Former_Member]

Good evening Stargazers,

I would like to write some words concerning the first post of this thread.

Refractors and reflectors are different telescopes because they are designed to do different tasks ...

I do not agree to this. Reflector and refractor are mainly differing because the use completely different light gathering techniques:

- a refractor refracts light

- a reflector reflects light

In a refractor the lenses refract the light, but unfortunately light of different wavelength are not refracted in exactly the same way.

In a single bi-convex lens the deep blue (short wavelength, say about 435nm wavelength) light is focussed nearer to the lens than blue

(480nm) wich is focussed nearer to the lens than green, than yellow-green (546nm) and that farest away would be deep red (about 680nm).

The stronger the refractive pover of the lens, the worse the longitudinal colour aberration. Therefore single lens refractor was extraordinary long.

The englishman Chester Moor Hall was the first who constructed an achromatic refractor, but Dollond got the patent for it, because he published

the new type of refractor, whereas Hall did not do that.

In this 2-lens Refractor wich was further improoved by the german Fraunhofer the first lens converges light (positive lens) the second lens

diverts light (negative lens).

The lenses are made from different glas types, one crown glas (mostly BK7) and one flint glass (SF2, FS). So how do they manage to be achromatic?

Harrie Rutten / Marten vanVenrooij give a short answer in their book "telescope optics":

Suppose we make the positive lens half the focal length of the negative lens --- but pick the glass so the dispersion of the negative lens is twice that of the positive lens. The combination will be achromatic. Why? he negative lens (because of the higher dispersion) compensates for the short focus of the and long focus of the red rays with errors of the opposite sign, bringing the rays to focus together.

Rutten/van Venrooij further point out that

in the real world this does not works so well, because the lens has to be corrected for spherical aberration at the main wavelength (normally 546nm)

and bent so that the off axis coma will be low or better no coma present. So there remains a longitudinal colour aberration.

In modern times glass makers invented modern glass types (ED Glass, SD Glass, Fluorite Chrystal lenses) so combined with the matching glass there

will be very very little longitudinal colour aberration left. And in addition manufacturers take more than 2 lenses to achieve maximal aberration control.

As a trade-off these apochromats are very expensive, especially with "big" apertures, say 5" or more.

A reflector is made with 2 or more Mirrors. A Newtonian Reflector uses 1 image-forming mirror, but it needs the secondary mirror, a small flat

mirror, to direct the rays out of the tube where the eyepiece is situated.

The primary mirror is a paraboloidal mirror. A spherical mirror will only work in Newts with small apterture and small f/ratios (f/15 is smaller than f/5).

The reason is that the sperical mirror suffers from spherical aberration. The incoming rays of light parallel to the optical axis of the telescope

wich hit the mirror in the center will focus farer away from the mirror than the rays that hit the mirror. So the mirror surface has to be "bend

to the back" (or better: it has to be polished that way) that the rays focus together. All different wavelength are reflected the same way,

that is one big advantage of the reflector. The disadvantage is the strong off-axis coma of a Newt. The diffraction limited field is narrow.

this impies that the Newt hs to be collimated very well. Decollimation means that the optical axis of the telescope does not go thru the center

of the eyepiece. In the center is an off-axis part of the image plane, and therefore the contrast transfer of the scope may be significantly lower!

Unfortuately the small secondary mirror is situated in the incoming wavefront of light. This causes aditional diffration of light (light is too diffracted

at the outer border of the optics, refractor and reflector). The additional diffraction causes a slightly lower MtF curve at the middle spatial frequencies.

To understand this please have a look at the Website if the frenchman Thierry Legault:

http://www.astrosurf.com/legault/obstruction.html

Have a look at the MfT-cuves. One can conclude that a telescope for planetay studies (remember that many planetary surface details are low contrasty)

demands for a telescope with small secondary mirror obstruction. Or, for bigger aperture than a "perfect unobstructed telescope" (wich is not existent).

Remember that the unobstructed achromat suffers from longitudinal colour aberration and the spherical aberration is only corrected at the main wavelength,

so contrast transfer is lowered significantly. The quality apochromat does it better, ca is only marginal and the spherochromatism is not bad as

long as the focal ratio is not too big.

In addition the human eye is not equally sensitive at all wavelength. In bright light (Moon, Venus, Jupiter Mars and Saturn) it is very sensitive

around 550 nm and towards gren light and yellow and bright orange, but sensitivity drops fast towards blue and red. So the wavelength have

to be weighted.

William P. Zmek did that some month ago in a americam forum. He came to the conclusion that a 6" reflector with 30% linear central obstruction

and a 6" f/15 achromatic Refractor will have about the same modulation transfer, their MtF-curves are comparabel.

A good 6" apochromat will be much better when planetary viewing is the game. A long focal length Newt (long is better because of the possible

smaller secondary, the bigger diffration limited field of view, and collimaition tolerances) with a small secondary comes close to an apochromat,

but to equal the mft of the apochromat the Newt needs 1" more aperture.

Diffraction:

Even when a scope is made so wonderfully good that theoretically all rays focus in one point, there is diffration unsharpness.

A star at very high magnification will mot be seen as a mathematically small point, it will be seens as a tiny dot (the airy disc),

surrounded by one, two or three rings of light (depending on the brightnes of the star and the aberrations of the telescope).

This is mainly caused by diffraction.

For example most observers will see the airy disk of a 4" teleskope when magnification is 150x. The bigger the aperture the smaller the airy disc.

With a 8" Telescope it will need 300x to see the airy disc with the same angular resolution.

So the aperture is as important as high optical quality and as perfect collimation.

In addition the thermal situation of the scope is important too. When you bring the scope from the warm house into the cold night

it will need to cool down a long time. During this period the mirror form slightly changes, and so do the lenses. In a reflector tube

where the mirror is at the downside and heats up the air above the warm air causes tube currents. This is one of the main disadvantages

of the reflector compared to the refractor(!) but it is seldom referred to.

There are many different types of reflectors. Some use lenses too, like Maksutov-, Klevtsov-, Schmidt- Types of reflector telescopes.

Every type of teleskope has its own typical advantages and disadvantages. The more the User knows about them the better he can choose

the type of scope that will give him the best views (or imaging abilities) for the kind of observing (or imaging) he wants to do.

Unfortunately this sort of discussions often cause "heated discussions" among the amateurs

You can rad a lot of books or you can do lots of side-by-side-comparisons. In the end it is up to you to choose the right scope... for YOU.

Regards, Karsten

[FLO]

... The most noticeable improvement to the scope, (but not directly optical) was to fit the single speed "Crayford" focuser I bought from "FLO" last year. This has made a really marked improvement in making focusing dead easy - and getting it spot on!

Quality focusers are also better at holding the eyepiece on the optical axis so collimation is more accurate and you are making better use of the telescopes 'sweet spot'

[GazOC]

Thats a great post. Thanks Karsten.

[Ajohn]

The problem you have Phil is that most of the scopes on offer are designed to be short and convenient. Cynically speaking more can be packed on a container ship in china and the smaller mounts that are needed too. There is also the fact that very few manufacturers actually specify just how accurate their mirrors are.

You could rate your scope by choosing to view a planet at certain exit pupils. That's the diameter of the scope divided by the magnification the eyepiece gives. It may seem an odd way to do it but it takes care of resolution too. Basically a more or less perfect scope will give a good image of a planet with an exit pupil in the range of 0.7 to 0.8 mms or so providing the seeing conditions are reasonable eg cold clear winter night. If you find that the image is fuzzy and much clearer at 1mms or above the scope aint so good for one or more of the reasons mentioned. Also with an exit pupil in the range of 0.4 to 0.6 you should see good defraction rings around stars. There's plenty of info on the web about how those should look. Oddly these figures are stiff but ok on an 8ins or smaller scope but may be too much for a 10ins. The magnification needed on a 10ins scope is starting to be a problem due to atmospheric effects. These may even mean that there are only brief instance where the image is clear on smaller scopes too.

On eyepieces I can use cheap ed's at high magnifications on the f9 apo. Naglers don't make any difference. On another f6 scope I have a nagler is clearly better. Much better. Cheapest solution here is the nagler zoom. Still expensive though. Vixen do a longer focus zoom that works well on short focus scopes and the moonfish group do a cheap 2ins wide field eyepiece that also works well on short focus scopes. There is also a modified erfle about that may even be better. The moonfish was difficult to use on the f9 scope as my eye needed to be dead central not so on the f6.

Don't bother with the bent etc spiders - they will just remove defraction spikes from around stars. They do that by smearing them all over the image - not good for planets or anything else really. You can get a the same effect by cutting oval holes between the vanes in a cardboard mask and placing it on the end of your telescope. You could also check the size of your 2ndry mirror but fixing that problem usually means moving the focuser too.

John

[KaStern_Former_Member]

I missed to write to some things:

The f/10 'scope would be easier to collimate accurately and not as sensitive to collimation so the view would be slightly better. The f/10 would be easier on the eyepiece design so cheaper eyepieces could be used.

Kaptain Klevtsov said it very good !

Curved spiders:

Curved vanes don't put the diffraction into the sides of the OTA, they merely spread them out more evenly across the image. Curved spiders actually cause more overall degrading to an image than straight vanes as they are have to be thicker.

And this too is very well said! Curved spiders really give no advantage for planetary viewing. But when you try to split doublestars with very

different magnitudes it can happen that the faint star sits on a spike of the bright star caused by the straight spider vanes...

For this type of viewing I recommend the 180° arch.

I think I'm going to get some flocking for my 8" - but I'm not to worried about the spider vanes - I kind of like the odd diffraction spiked star.

Flocking is a good thing. Reflected light coming from the tube walls lower image contrast. This really is a point when you want to view our moon

or when Streetlights are near to your observing site.

Quality focusers are also better at holding the eyepiece on the optical axis so collimation is more accurate and you are making better use of the telescopes 'sweet spot'

I second this.

Getting hungry now. I think I should walk into the kitchen and surch for a tin-can with ...............spam. Oink

Good night to all, Karsten

[saturn5]

Good evening Stargazers,

I would like to write some words concerning the first post of this thread.

Refractors and reflectors are different telescopes because they are designed to do different tasks ...

I do not agree to this. Reflector and refractor are mainly differing because the use completely different light gathering techniques:

- a refractor refracts light

- a reflector reflects light

Regards, Karsten

Are you totally sure of this Karsten? and is this loron post another cut and paste from http://loron91423.tripod.com/id7.html

[FLO]

That's my understanding too. It is worth noting that because a Newtonian's mirror reflects light it does not behave like a prism so is free of the false colour fringing seen through refractors. Essentially, all Newtonians are APOs 8) There are of course other things to consider, such as central obstruction and light scatter...

Now that this thread has become a 'discussion' it has become useful and worth bookmarking

[KaStern_Former_Member]

philsail1

you seem to have issues with your f/5 Newt. May I ask wich aperture and manufcturer it is ?

Could an f5 reflector's view be significantly (and noticeably) improved by changing the four vane "spider" for a curved three vane assembly?

Are the spider vanes thin? If so: No, there will be no significant improvement.

Also could an f5 reflector's views be significantly (and noticeably) improved by the use of very high quality eyepieces?

I think yes, it would improve the views. First: The normal Plössl, Ortho, and Kellner eyepiece suffer from the big f/5 f/ratio.

On-axis they do add a little spherical aberration when used in a f/5 Newt, and off-axis the suffer from eyepiece astigmatism

so the off-axis stars are not really looking good.

You can do two things:

- add a good barlow. I recommend a 2.2x University Optics Klee Barlow wich is designed for Newts, or a 2x or even a 3x Televue Barlow.

- or take modern eyepieces of the negative-positive sort, most of them will show no or only little off-axis astigmatism and most of them will

work fine on axis. But they can be really expensive, if you want them have a wide apparent field of view and be of really good quality :shock:

If your plössl are o.k. you might try a quality barlow first.

Is it a case of "if you want the best views, you have to buy the best (optically constructed) telescope money can buy, or can standard "affordable" telescopes be improved (optically) by spending money on them!

It depends on the quality of your primary mirror. If it is good enough (say: diffration limited) then it is worth spending money for that scope.

The secondary has to be good too. Often it is not good, and a chage to a quality secondary mirror can imporve the views significantly!

I propose that you should meet an experenced observer. He can check the collimation of your scope and try to startest it when seeing conditions permit.

If he comes to the conclusion that the mirror(s) is/are o.k. you can try to get the best out of your telescope.

If not you can save your money and buy a better telescope.

Regards, Karsten

[KaStern_Former_Member]

Saturn 5,

Are you totally sure of this Karsten?

sorry, my english is not that good. Maybe I better should have writen:

Reflector and refractor are mainly differing because the use completely different image forming techniques?!

Regards, Karsten

[Ajohn]

Not wishing to start a "heated argument" as Karsten mentioned but: The mtf argument in relation to reflectors and refractors is clearly miss leading and not born out in practice for VISUAL observing. This can only be down to one of 2 things. MTF is usually a purely software analysis unless it's backed up with actual observations of a test card with the correct range of spatial frequencies on it. The other is contrast. Anyone who is interested in photography and buys say a nikon lens capable of resolving 200 lines per mm on the film and then actually tests it will know just what I mean. For astro scopes strehl ratios over the field are far more important. (bet I've miss spelled that.) It's a sad fact that all conventional reflecting telescopes aren't too good off axis in that respect. More so with lower F numbers. Poorly made mirrors will not be much good in that respect on axis either.

Me I use both but have to conclude that the refractor even a straight achromat has much to offer over typical reflectors especially at the cheaper end of the market. Few people emphasise the problems associated with making accurate parabolic mirrors. The Fraunhofer refractor as already mentioned has all spherical surfaces and an air space. Easy to manufacture reliably and well corrected across a reasonably sized field providing it's f8 or more. Even skywatcher use them. Ok bright objects have a bluish purple haze but that isn't really a problem for visual observing.

John

[philsail1]

Many thanks for your "in depth" replies "Ajohn" and "KaStern."

My Newtonian scope is a standard "Skywatcher" 8" with a Parabolic mirror. I have a single speed Crayford focusser fitted which (as Steve from FLO says) does keep any eyepiece used, locked in the centre of the reflected light path.

I'm using a mixture of "cheapish" eyepieces:- 10mm and 25mm Kelners. 6mm & 18mm Orthoscopics (these do give good images of the planets and stars). Also use, several Antares Plossls, and have a Celestron Ultima 2x Barlow (but this does not seem to work so well with some of the eyepieces). Have a very good TAL 25mm Plossl (which gives very good images), and a 6mm TAL plossl, which is not so good.

I have flocked the entire inside of the scopes' tube, and matt blacked the inside of the focuser tube (as carefully as I can without compromising its smooth action).

The scope has ultra thin 0.5mm four vane spider. (I've matt blacked the vanes).

I have a "Cheshire" collimation tool, which works fine (the scopes main mirror has a small central paper disc on it).

The only thing I've noticed after collimating, is that when I slowly turn the Cheshire through 360 degrees whilst in the focuser, the collimation appears to drift out of alignment. I re-collimate at another point, turn the collimation tool, and the scope again goes slightly out of collimation. Could this be, my focuser being slightly off angle to the scopes tube. Could it be an anomaly in the scopes tube construction, or even the central spot on the main mirror being "out?" It is only a fraction of a millimetre, so does it matter.

I am happy with the crispness of the views I'm getting with the scope. (The 8" seems to be about equal in sharpness as my 4" TAL100RS - though the TAL does show a darker background). Also of course, the 8" gives much brighter views and will take higher magnifications.

I simply asked the earlier questions in response the "Lorons" initial post.

I take on board "KarSterns" recommendations of buying a better quality Barlow (more suited to an f5 Reflector) and perhaps some better quality eyepieces such as "Hyperions" or "Pentax" (I would need to take further advice on their suitability before spending what would be a greater sum of money than I initially paid for the scope!

I'm making notes of all your comments - to be acted upon at when "funds" become available!

Regards,

philsail1

[KaStern_Former_Member]

Hello John,

The mtf argument in relation to reflectors and refractors is clearly miss leading ....

For astro scopes strehl ratios over the field are far more important. ...

Poorly made mirrors will not be much good in that respect on axis either ...

you mix up things that should be strictly divided. First of all:

The Term Strehl was invented to discribe how well an Optic was manufactured compared to the theoretically perfectly made sample of that Optic!

Strehl does not say anything about something else!

In addition, Strehl will be measured for the axial point.

Imagine a Newtonian with nearly perfect primary and nearly perfect secondary Mirror. The measured Strehl may be 0.99 wich is perfect.

So how would be the modulation transfer of this scope for the axial point?

The trueth is, you cannot say. You do not know how big the secondary mirror is. Ths scope may be a long focal length Newt for visual use

with a tiny secondary mirror, lets say a 10" f/7 Newt with superlow focusser and a 39mm small secondary mirror.

The Modulation transfer for the axial point is very good. Compared the a "perfect unobstructed telescope" (wich is not existent)

the MtF curve of this telescope is only slightly lowered.

Or is the above measured Strehl 0.99 Newton a 6"f/4 Newt with a 70mm secondary for photographical use?

The modulation transfer of this Newt for the axial point is significantly lower due to much more diffraction.

The MTF curve is strongly lowered. In addition, beacause of the smaller 152mm aperture the MTF is dramatically

lower than with the 254mm Newt! Please see the following link:

http://www.astrotreff.de/topic.asp?TOPIC_ID=69031&whichpage=1

Please scroll down until you see the MTF diagram. It is in german, but more or less self-explaining.

User Kurt is a very experienced amateur telescope maker, retired ingenieur an he knows a lot about optics.

I give a translation:

Vergleich = comparison

Spiegel = Mirror

Kontrastübertragung = modulation transfer

normierte Ortsfrequenz = normed spatial frequency

Spiegel mit Fehlermix = mirror with mixed surface errors/defects

The big 20" mirror is a bad one, some spherical aberration, some astigmatism, some roughness, Strehl 0.5

As you can see the MTF curve (red line) is significantly lower compared to a perfect 20" Mirror (blue line with red dots).

But if you compare to a perfect 10" Mirror the 20" "Lemon" Mirror can give the same modulation transfer at 0.05

(wich translates to 5 arcseconds spacing of structures, for example planetary surface structures).

But at 0,5 wich translates to 0,5 arcseconds spacing of structures where the modulation transfer of the perfect 10" mirror drops to zero

the poorly made 20" mirror is able to transfer 20" modulation (given that the atmoshere is perfectly stable).

So waht I wanted to explain is:

- Strehl is a very good Term to describe how well an Optic was manufactured, the is what Strehl was invented for!

- Strehl alone does not say anything about the resulting MTF transfer

- aperture has to be known as well as how big is the central obstruction

If you have all this you cal calculate an MTF curve wich is best suited for displaying how the optic can transfer contrast

on the optical axis (wich is in the center of the eyepiece when collimation is perfect).

For astro scopes strehl ratios over the field are far more important.

Once again: Please do not mis-use the term "Strehl"! This comes from american boards, but it is worth nothing.

What you want is to know how much contrast the opics can transfer in the field, off-axis.

Of course, you are right:

MTF on-axis does not tell about how this optic can transfer contrast for imagepoints far away from the optical axis.

A Newt wich can transfer image contrast nearly perfect on-axis suffers from from coma the more you get away from the axis

and the "faster" the Mirror is. For example:

A 8" f/8 Newt gives a much bigger diffraction limited field than a 8"f/4 Newt in wich the diffraction limited field is extremely small.

An achromatic refractor gives better off axis images. But:

Aperture and f/ratio differ. There is no 8"f/4 Achromat. If there was such a scope it would on axis suffer badly from

longitudinal colour and sperical aberration even at the main wavelength. It would unsuitable, like a 8"f/4 Newt with spherical Mirror would be.

So one should allways compare scopes with the same aperture and f/ratio.

Please see the following link: http://www.astrotreff.de/topic.asp?TOPIC_ID=68699&whichpage=2

Please scroll down until you see the MTF Curves made bei the User "Birki" using the opticak design program "Oslo".

The question in that thread was how good a 8"f/6 Refractor (with special glass N-FK51 und N-KZFS4) would be

compared to an 8"f/6 Newt with 25% Obstruction.

With the refractor the MTF on-axis was lower than with the Newt, even with one surface of the refractor beeing an ashere.

This is mainly caused by logutudinal colour aberration.

Due to the asperisation the refractor showed off-axis coma, see the spot diagrams when you scroll down further.

There you see too the off axis MTF Diagrams for Newt and big-short-refractor.

Birki tried another Refractor, now with the typical achromat BK7 and F2 glass. This was even much worse.

In that thread of the german board www.astrotreff.de the conclusion was that a 2-lens refractor that big and that short

is far beyond the boarder limits of the optical system "refractor".

It would be necessary to employ 2 more lenses to get a good result.

When you look at the Apochromats of well-known manufacturers xou will see that they take 3 lenses and f/9 for a 8" Apochromat.

To coma back to what you wrote:

It's a sad fact that all conventional reflecting telescopes aren't too good off axis in that respect. More so with lower F numbers.

More so with lower F numbers is valid for both refractors and reflectors.

Even if you take a 4"f10 achromat one thing remais to be limiting off axis performance:

Field curvature. Visually this is no problem, the eye can accomodate most of the typical curvatures.

To be fair one should allways compare comparable scopes when one wants to gind out the differences inherent in the different telscope types.

When you take a 6"f/15 achromat and say "this is offf-axis fine" then take a 6"f/15 Newt to compare the off-axis views.

Then you deal with manufacturing precision:

Few people emphasise the problems associated with making accurate parabolic mirrors.

Nowadays it is no longer that difficult to make a parabolic mirror with a strehl better than 0.8

The manufacturing and testing techniques are known and even in far-east they manage to produce some fine mirrors.

I have an ATM 8"f/6 Newt with a taiwanese Mirror in it. That one was tested interferometrically

(9 measurements, mirror was rotated after 3 measurements to find out about astigmatism)

and the Strehl is 0.9 bue to a very slight astigmatism.

I agree that it might b one of the godd mirrors, but since I often met other stargazers I like

to perform a startest with their Newtonians/Dobsonians. And I seldom found a bad mirror, most of the time I find bad collimation.

After fixing that the views are significant better.

I admit that with fast Mirrors, say f/4 it gets much more difficult to make high quality paraboloids.

The Fraunhofer refractor as already mentioned has all spherical surfaces and an air space. Easy to manufacture reliably and well corrected across a reasonably sized field providing it's f8 or more. Even skywatcher use them. Ok bright objects have a bluish purple haze but that isn't really a problem for visual observing.

Easy to manufacture sphere, but the lens-cell can cause problems, the lens-spacing to, the lenses can hage a wedge,

the lenses can be decentered,...

I have seen many of those, but I cannot recommend them for planetrary viewing.

The contrast is very much lowered because of the plue-ish and purpl-ish haze.

I recommend a good 6" or 7" true apochromat. And I recommend a 8" or 10" Newt.

I could compare my 8"f/6 to a 7" TMB at the german telescope meeting ITV.

Both performed similar on Jupiter. Only few people will believe this, but they may ask

the owner of the TMB, Hannes Hasebergen, about this.

This was how a well-made ATM Newt compares to a well-made apochromat.

In real live a de-collimated low-cost Newt with a too narrow and too short tube, not cooled down, using bad eyepieces,

will be compared to smaller longer achromats or apochromats and the result od the comparison can be totally different.

Regards, Karsten

[KaStern_Former_Member]

Hello Phil,

I have flocked the entire inside of the scopes' tube, and matt blacked the inside of the focuser tube (as carefully as I can without compromising its smooth action).

The scope has ultra thin 0.5mm four vane spider. (I've matt blacked the vanes).

I have a "Cheshire" collimation tool, which works fine (the scopes main mirror has a small central paper disc on it).

all this is fine. But did you ever check that the center mark is really precisely centered?

A friend of mine had a center mark wich was 5mm out of the center(!)

The only thing I've noticed after collimating, is that when I slowly turn the Cheshire through 360 degrees whilst in the focuser, the collimation appears to drift out of alignment.

That is bad. I hope that the chesire is precise and straight. If not you will need another one.

It is only a fraction of a millimetre, so does it matter.

No, a fraction of a millimetre is no real issue. But if it is some millimetres that your scope would be decollimated.

Can you met with an experienced observer and check the collimation and the chesire with him?

This issue is really important. It should be solved before spending much money for new eyepieces and barlows!

The 8" seems to be about equal in sharpness as my 4" TAL100RS - though the TAL does show a darker background). Also of course, the 8" gives much brighter views and will take higher magnifications.

When seeing conditions are good (this is not often the case) the 8" should clearly outperform the 4" Tal, wich is a nice scope.

Regards, Karsten

[philsail1]

Thank you for your reply KaStern.

I'll check that the centre spot is exactly that - central! (If it isn't I'll simply peel it off and stick one of those white paper punch hole reinforcing circles on the mirror!).

I do know of a local more experienced astronomer than me, so when the weather gets a bit warmer, I'll take my Newtonian over and let him have a look.

Regards,

philsail1

[KaStern_Former_Member]

Good evening Stargazers,

I hope that I do not bore you, but I would like to add some thoughts.

First thought:

It is not as easy to get a reflector work as well as a good refractor.

A reflector need to be very well collimated.

The diffraction limited field of view is small in most reflectors. So collimation has to be dead-on to avoid loss of contrast.

A refractor has to be collimated too. But once collimated a quality refractor hold the collimation well.

A quality Newt hold collimation well too. But from time to time it will need a twitch on the collimation screws.

Second thought:

Thermal situation is much more critical with a Newtonian Reflector vs an Achromat or Apochromat.

The mirror heats up the air surrounding him. The warm air is lighter than the colder air so it rises up,

through the tube and causes tube currents. Especially when the tube is narrow, like it is in a typical

8" low-cost Newt with only 230mm Tube diameter, this can be really annoying. During wintertime

cool down can take much time.

The achromat lenses heats up the surrrounding air too, but the warm up-rising air soon leaves the light path.

When the lens radii are long like they are in a long focal length achromat they do not cause any problems at all.

With big 3-lens short f/ratio Apochromats this can be a little more of a problem.

If you add an effective cooling fan to a Newt things get much better, but the best seems to be to take a bigger tube diameter.

For example 260mm for a 8" Newt.

Another thing is that the Primary mirror of a dobsonian Telescope is situated only few centimeters above of the ground.

During the cool down of the atmosphere this region of the Air is very unstable.

A long focal length achromat has the objective above of this trouble-zone. 1.5m or 2m abive the ground the air is much more stable.

Third thought:

The newt often is a f/6 or an f/5 Newt wheras the achromat is an f/10 or an f/15.

Most eyepieces like Plössl, Ortho, Kellner, König, Erfle work fine in f/10 or f/15 scopes.

In an f/5 all of them show severe off-axis astigmatism. f/6 is not much better.

In addition the eyeoieces on-axis begin to form a slightly bigger "blurr". They begin to add a little sperical aberration.

So with a fast Newt eyepieces get much more critical. You can add a quality barlow wich cures the off-axis astigmatism

and helps the eyepieces too on-axis. Both of the Klee barlow is a Newton-barlow, it was designed to corect for off-axis coma

when used in Newts down to f/4. This information is suppressed on the american board CN.

I met the Designer Mr. Klee some weeks ago and he explained the design of the Klee Barlow to me.

In addition Harrie Rutten (the co-Author of the Book telescope Optics") stated on the german board www.astronomie.de

that the Klee Barlow should not be used with Refractors, becuse it was designed to be used with Newts.

I could try to find the link to his post.

I have two 2.2x and two 2.8x Klee Barlows at home and my observations seem to confirm this.

But it is a little too early now, I want to test them with about 20 different eyepieces.

The Klee Barlow is NOT good for fast Apochromats, it would ADD coma there(!)

Fourth thought:

Some people like much more to observe with a binoviewer. This is easy with most refractors,

but it is more or less difficult with a Newt.

In addition some people like more to sit behind the telescope than besides of the telescope.

So in the end it is up to every stargazer to find the best telescope for HIMSELF !

The more he knowss, the more experienced he is, the easier will it be to find the right scope(s).

Many own more than 1 Scope

Thanks for reading, Karsten

[MartinB]

Hats off to you Karsten, top man! :hello1: :hello1:

[saturn5]

Geeez,after reading all that i need to adjust the polar alingment of my 12 y.o Highland Park. :downtown:

[Kaptain Klevtsov]

Bore us? This is good stuff Karsten, thanks for the information.

I might add something later, after I've thought more, but it seems to be good information from line one to the end.

(I'm biased as I like reflectors as they have no false colour and are cheap to buy in big sizes)

Kaptain Klevtsov

[FLO]

Excellent post indeed :salute:

[KaStern_Former_Member]

Hello Stargazers,

thank you very much for your friendly comments!

Kaptain Klevtsov it would be good when you add your thougths.

I have some more thoughts wich might be worth to add,

but it will take until next week or so.

Good night to all of you, Karsten

[Ajohn]

Thanks Kastern. I am aware of that but feel that the strehl ratio gives a much better feel for the performance of the scope than MTF. I also under stand that Pentax for instance quote strehl ratios over the field of the scope. It's a pity more manufacturers do not do the same. I spend some time playing with oslo looking for that perfect scope. The results are interesting.

On the collimation problem it sound like your focuser isn't square to the axis of the beam coming off the 2ndry. I advocate spending a lot of time making sure that the 2ndry is accurately aligned centrally in the focuser and the it's also in the centre of the tube. Lots of time before bringing the main mirror into play. I usually take that out or mask it. And ensure that I can see the entire bottom of the tube central in the 2ndry too. This is a very important step. There is a catch though. Many moons ago a man wrote to sky and telescope pointing out that on fast scopes and f5 was really fast then the 2ndry mirror should be below the axis of the focuser and tilted at slightly less than 45 degrees. The idea being to account for the fact that the beam is wider as it gets nearer to the mirror. ie The top of the 2ndry is further away from the mirror than the bottom. Some manufacturers seem to have taken this on board. Set such a scope up as is usually suggested and there is a fair chance that the 2ndry will not intercept all of the beam from the main mirror. I came across a case where it was missing 30% of it. I think the article also advocated positioning the 2ndry slightly away from the centre of the scope and away from the focuser. If you have such a scope and have collimated it in the normal way the 2ndry holder is likely to look rather tilted when it's done. I would suggest you go back to setting the 2ndry as outlined and then set up the main mirror. If you still have a problem the optical axis is not square to the tube of the scope and the focuser needs packing to make that square. If you have the sky and telescope type set up then concentrate on being able to see the whole of the bottom of the tube with the 2ndry will have to be below the axis of the focuser. Then the focuser needs packing out to suit and the 2ndry will be central in the focuser again. It shouldn't need much packing and is likely to take several attempt. The key is being able to see the bottom of the tube evenly central in the 2ndry just as before. I fail to under stand why certain people thought this was a wonderful NEW idea. Must think our ancestors were stupid. My answer was to remove the 2ndry holder and remachine it. It still slightly out other wise I would have sold the scope by now. I'm too honest to a fault.

I note kasterns comments on a better scope. May be able to put that into perspective but need to talk about the airy disc again. That's the little dot a star forms hopefully surrounded by nice round rings of light under high magnification. Lord Rayleigh came up with a limit for optical accuracy many many years ago but it's still valid today. He basically showed that to produce an acceptable image there must not be more tha 1/4 of the wave length of light error in the wavefront. What that means in respect to a mirror is that the maximum error mustn't exceed 1/8 wave. At this point 20% of the light that should be in the airy disc will mostly be in the dark area surrounding it. This is regarded as a virtually perfect star image. This assumes that the mirror is smooth to a much finer degree too. Moving on - an image of a planet can be considered as a lot of airy discs so even at this level there is a loss of contrast. Some say that 1/15 or 1/20 wave maximum error is needed for planetary observation. That's what might be called research grade. The only major manufacturer that I'm aware of who quotes figure is Orion UK. Oddly they use a so and so wave PV which stands for peak to valley. A more meaningful figure is maximum slope error and another for ripple. PV is much better than RMS that some used some time ago. That's 1/2 the square root of the true figure. These days hardly anybody quotes anything at all and I can't agree that 8ins F5 mirrors are easy to make that well in volume commercially. Tests of major manufactures products by private individuals crop up from time to time that show a whole wave error. There are a number of people about who will make 1/10 wave mirrors but search the web and check the cost. That's is a worth while limit. Then comes the 2ndry. The optimum on that score is 20% of the diameter of the main mirror. I have never ever come across a commercial compound telescope that meets this. Those Japanese 0.965 or whatever eyepieces make a lot of sense in this respect. It can and may be met on a newt. Again the central obstruction takes more light out of the airy disc what ever it's size but does get truly negligible at about 10%. Many people quote % area. It makes the telescope look a lot better on paper providing the reader is ill informed.

John

[Ajohn]

By the way some might think that I'm advocating refractors. I'm not because there are serious cost implications in getting one where a 5ins is likely to perform as well or better than a decent 8ins reflector. It has to be an apo. The old longer focus ones are often very cheap on the 2nd hand market though and well worth buying. A newt has a lot going for it that can be improved even further with a coma corrector. People should also realise that they can make the mirror themselves. Personally I would go for an 8ins F6 7 or 8 but F5 can be done. You can take a look at Oldham Optical if you want to buy a 1/10th wave mirror. He uses a thing called a Dall null tester on his mirrors and makes some comments about other methods of testing. Having made one I would be inclined to agree.

John

[philsail1]

Good morning "Ajohn!" (I'm up - bit of arthritis in my right shoulder).

Just read your post.

Re the secondary being central inside the telescope tube.

Am I correct in saying that if I can see the three clips of the main mirror when I look through a simple "film canister" collimating tool, the secondary will be in the centre of the tube.

Also, could one use a pair of internal calipers to check each edge (top, bottom, right and left side) of the secondary to ascertain if its central?

Could the secondary's mirror angle be checked by shining a torch down the tube and let the light be reflected up through the eyepiece?

Regards,

philsail1