f ratio?

[algol]

ok guys really confused now, so a higher F ratio is better for planetary viewing? or so i've read on web

so a f8 120mm refrac is better for planets than my f5 130pm newt? do i have that right? more of a DSO nut at min though.

[blinky]

As far as I am aware it is Focal Length that is important. The longer the length the more magnification you get, since Mag = Focal Length of Scope / Focal Lenght of Eyepiece.

So your refractor is better as it is longer therefor giving a greater magnification.

[algol]

ah but the limiting mag is smaller than the newt.

240 in the refrac max and 260 max in newt.

[lunator]

Algol

I wouldn't worry about this to much. The longer focal length scope will allow you to acheive higher mags with longer focal length eyepieces or without the use of barlows.

The 130 will be fine on planets. My F6 Newt is a great balance between a widefield and planetary scope.

Cheers

Ian

[algol]

thanks ian, kinda whati imagined anyway.

i find all this stuff confusing. as im sure everyone does at first.

[Macavity]

Heheh. If you look at a good MANY scopes though, you seem to find they have a focal length of about 4 feet (sic), 48", 1250mm (OK approx!). I reckon this is so we (standard-ish human-sized?) folk can "reach" the bloomin' thing!

Not that there isn't something in all the F-number stuff - Especially for the extremes. But many scopes are a lot more "general purpose" than the "ancient sages" (sometimes) say? Someone remarked too, with a few notable exceptions, most DSOs are less than a degree in extent, and within the field of many a "Planetary scopes" too.

[Ajohn]

F8 has long been regarded as a good general purpose focal ratio suitable for all observing. One of the big effects is eyepieces once you get down below that life starts getting tougher for them. eg On an F9 refractor I can compare eyepieces costing £50 with another costing over 5 times as much and can't really see any difference. At F6 things are entirely different. That can't be a hard and fast rule though as eyepieces vary by manufacturer. eg I've noticed that skywatcher super plossls don't get the best out of my sons 120mm f8.3 refractor. Vixens products are noticeably better. All of this applies most at high magnifications indicating that it pays to spend a bit more on eyepieces for that sort of use.

The same sort of argument also applies to the other end of the scale where as big a view as possible is wanted. That will be got from a wide field eyepiece giving an exit pupil of maybe as much as 5mms. (diameter of scope / magnification) There are one or two cheaper eyepieces in this area but the more extreme ones are going to be more expensive so it comes down to how many £ one wants to spend to get a good wide field view. On a budget it's worth remembering that erfles and other eyepieces were designed for use at F5 to F6 in the first place, modified ones even faster than that. Clave plossl eyepieces will also work very well at F6 but oh the price.

Some one mentioned the angular size of nebulae ect. That's another factor. The off axis resolving power of telescopes goes down with faster focal ratios it also goes down for the same focal ratio as the diameter goes up. Sounds daft but an F8 8ins newt gives a wider field of good definition than an F5. The downside is that the 2ndry mirror needs to be much bigger in order to use it and that introduces other problems.

The design of the scope also makes a difference. Cassegrains are the same as newts, schmitt cas can be a bit better the ritchey variant even better still. All of these need larger 2ndry mirrors though unless they are specifically designed for narrow field viewing. The mac can also be better but on the inventors original designs that eliminated coma the glass shell on the end was very very thick. Refractors are some what different. There are designs that give lower aberrations but cost goes up dramatically with size and there is 2ndry colour unless it's an apo.

Some might like a nice clear cut answer but there isn't one. Telescopes are a case of trade offs, improving one thing often spoils something else. Over all though refractors do have the edge but get too expensive in larger sizes.

John

[Kaptain Klevtsov]

Primers here. http://stargazerslounge.com/index.php/topic,8470.0.html

Basically the f/ ratio means nothing as its just a ratio. You need a long focal length for planets as that gets you higher magnification with each eyepiece, therefore you can use a more comfortable eyepiece for the same magnification if you start off with a long focal length.

A 200mm Newt. at f/5 for example gives the same image scale as a 120mm f/8.3 refractor as they both have the same focal length.

Kaptain Klevtsov

[Ajohn]

Not wanting to argue Kaptain but it does mean something. 2 things. How long an exposure will take and how obtuse and angle the light rays going into the eyepiece are. (Hence the use of the word stess.) The exposure aspect is simple. F2 with half the exposure in comparison with F4 - using camera type numbers. It also sets the minimum size of spot a lens or telescope can give.

John

[Kaptain Klevtsov]

I totally agree Ajohn, but I'd missed the camera bit out as he'd mentioned planetary viewing. I was trying to say that something like this

http://www.firstlightoptics.com/proddetail.php?prod=ev90az3

which is f/10, will not be as good as this

http://www.firstlightoptics.com/proddetail.php?prod=sw300pota

which is f/5.

Without the aperture there really is very little data given by the f/ ratio (except the theoretical spot size ) and so its meaningless to say that a high f/ number 'scope is good at or bad at anything at all.

Kaptain Klevtsov

[Ajohn]

Agree entirely Kaptain. Fact is that an f5 newt would give as good an on axis image as an f10 providing that they are both made to the same effective accuracy. The image on the f5 would be 1/2 the size of the image on the f10. It isn't just the size that make life hard for an eyepiece on the F5.

John

[rusirius]

ok guys really confused now, so a higher F ratio is better for planetary viewing? or so i've read on web

so a f8 120mm refrac is better for planets than my f5 130pm newt? do i have that right? more of a DSO nut at min though.

But I've also heard that refractors make better planetary scopes because unlike SCT's they do not have a central obstruction. Apparently this causes the loss of some detail? The central obstruction on a newt or mak is a lot smaller so maybe it doesn't affect the view so much? Has anyone else heard this?

[Whippy]

Rus, the size of the central obstruction (CO) depends on the particular design and focal ratio of the scope. The smaller the obstruction, the less the effect that it has on contrast. Much debate surrounds this but it's generally accepted from what I've read that any CO under 25-30% doesn't cause any loss in contrast. Ie: a longer focal ratio reflector say, f8 has smaller CO than one at f5 and has much better contrast. That's why you see Mak-Newts like the Intes Micro M715 that are termed 'planetary' scopes that not only have a high f ratio (f15) even comparative to other Maks but also have a smaller CO for better contrast.

Tony..

[Ajohn]

Think you have that the wrong way round whippy. For the same field angle the f5 needs the smaller obstruction. The catch is that the off axis aberrations of the f5 will be worse. As an idea the 1 degree field size of an 8ins F5 is about 0.68ins and an F8 1.1 ins.

20% is regarded as the ideal obstruction size. From memory that diverts 20% of the light out of the aery disc into the rings which ins some ways is worse than mirror surface errors. 30% is definitely a bit of a disaster and will have a very noticeable effect. Few compound telescopes achieve 20% - I don't know of any commercial ones 25 to 30% is the norm. A newt stands the best chance. Sometimes manufacturers quote figures like 12 1/2% but that is by area and is entirely miss leading.

One of the major understanding it all problems is that manufacturers tend to try and justify and make what they have look good. Others seem to aid them in that too. Message is that refractors and newts have a lot going for them.

John

Think I should have called myself "ThereAintNoSuchThingAsaTelescopeWithoutProblems"

Or "ItsAllaCompromise"

[lunator]

ok guys really confused now, so a higher F ratio is better for planetary viewing? or so i've read on web

so a f8 120mm refrac is better for planets than my f5 130pm newt? do i have that right? more of a DSO nut at min though.

But I've also heard that refractors make better planetary scopes because unlike SCT's they do not have a central obstruction. Apparently this causes the loss of some detail? The central obstruction on a newt or mak is a lot smaller so maybe it doesn't affect the view so much? Has anyone else heard this?

Rus

This is a fallacy. The central obstruction may have an impact on the contrast but a larger aperture scope will show smaller details.

My scope has a Daws limit of 0.57" a 4in scope will have one of 1.1".

Cheers

Ian

[Ajohn]

I saw some hard data on daws limit some time ago. Actual photo's and (yuck) maths. More recently found a comment that it was actually down to Rayleigh too - not so sure about that.

It applies to splitting stars and in real terms shows a double humped light curve. One of the points that was made was that spider defraction was a better indicator of a double on photo's. If correct this indicates that it can't apply to low contrast fine planet detail.

Be interested to know if anyone has visually split a star at the limit or better still imaged it.

John

[FLO]

ok guys really confused now, so a higher F ratio is better for planetary viewing? or so i've read on web

so a f8 120mm refrac is better for planets than my f5 130pm newt? do i have that right? more of a DSO nut at min though.

But I've also heard that refractors make better planetary scopes because unlike SCT's they do not have a central obstruction. Apparently this causes the loss of some detail? The central obstruction on a newt or mak is a lot smaller so maybe it doesn't affect the view so much? Has anyone else heard this?

Rus

This is a fallacy. The central obstruction may have an impact on the contrast but a larger aperture scope will show smaller details.

My scope has a Daws limit of 0.57" a 4in scope will have one of 1.1".

Cheers

Ian

Contrast, particularly acutance (edge contrast) can give the 'impression' of greater resolution/detail, which is where the myth comes from.

[GazOC]

When you look through a scope with a central obstruction you don't see a black hole in the middle of the image, but its effects are spread out across the entire field. This dims the image slightly and can cause a lost of contrast, a lot of planetary detail is low constrast so the smaller refractor can give more constrasty and pleasing planetary vews then a larger Newt/ Cat but it cannot resolve more detail.

For instance, if you look at Saturn though a small apo you will often see the different coloured banding on the planet look more impressive and vivid than in a larger Newt/ CAT, this is the low constrast detail. If you then try looking at Cassini (and Enkle if you have a large scope)division then you will see that the larger scopes are more suited to resolving these smaller, high constrast details.

[lunator]

John

I have split doubles down to the Dawes limit of my scope and I have also a paper written by Chris Lord about how high contrast objects can allow you to see them below the Dawes limit.

This does not change the fact that an 8 inch wil have better resolving power than a 4" scope.

I would recommend a chapter written by Christopher Taylor who runs the Hanwell observatory in Oxfordshire in Bob Argyle's book. He shows that a reflecting telescopes can match refractors in terms of resolution performance and the main reason refractors are preferred is due to their robustness.

Cheers

Ian

[Ajohn]

The point I was making on dawes was that say we have 2 scopes of the same F ratio - so the minimum size of the spot of light (airy disc) that they can show is the same. Then say one is twice the size of the other so it's field size will be twice as big too. It can there for get more spots of light in the same angular view=better angular resolution.

What I should do is work back from the angular airy disc size to see if this is the same as the dawes limit or limit that is quoted for the scope.

On diffraction patterns Texereau suggests a way anyone can see perfect examples.

Just need a clear bulb. In front of that place a card with a hole in it made with a pin pushing the point right through. Then take another piece of card and make another hole in it but this time don't push the point right front. View the other card through that and see a diffraction pattern. The set up can be made faster (lower F ratio) by pressing the pin further through the 2nd card. This will make the central spot smaller. It works but things need to be fairly lined up and the holes need a little bit of care to ensure they are round. Hold the 1st card up to the light at arms length. Works for me but I'm 6ft2. Should also work for others too. If not try more distance.

The 2nd card is reducing the diameter and hence resolving power of the eye. I believe that there are eye defects which can show an irregular pattern but turn each of the cards before panicking.

John

[lunator]

Ajohn

Thanks for the info. It's an interesting idea to try out.

Algol

This thread has got a bit theoretical but I just wanted to re-interate that your scope is a perfectly capable planetary scope.

Cheers

Ian

[algol]

thanks again ian, how did you know i had loundered out of my depth?

[Ajohn]

Have agree with that Ian.

Trying to help prevent thoughts of a scope change yet.

Don't expect huge lick observatory type views out of any scope that you are likely to buy. Many people do when they start.

You will get the best planetary images with decent eyepieces that give a magnification of between 130 and 160 with a 130mms scope. You will get that with 4 and 5 mm eyepieces. Having an F5 scope you will need good quality eyepieces especially for high magnifications. I'm assuming that you have skywatcher super plossls and their barlow. I would be inclined to forget the barlow and buy a decent eyepiece instead. The ideal would be one of the nagler zooms - bit expensive but they do come up on the 2nd hand market. (Search http://www.astrobuysell.com/uk/) Alternatively go for the best Vixen eyepiece you can afford. There are a number of planetary eyepieces about but I have no experience of them. Vixen is an extremely good bet on a budget. If you have concerns about the quality of your scope err on the longer focal length eyepieces. If it turns out cracking then get something more powerful. A very keen planetary observer would have a range of eyepieces covering 180 to 120X the idea being to use the highest power that conditions allow. Actually that's much easier on smaller scopes.

Thing to remember about eyepieces is that scopes often go eventually but all of the good eyepieces you buy tend to stay.

John

[lunator]

Algol

I have spent several years getting to understand resolution limits etc as part of my double star research and it does get complicated. I just wanted to make sure you were comfortable with the idea that your scope is a good one.

Cheers

Ian

[philsail1]

Dear All,

Wow! these threads start off with a simple question (sometimes from a beginner), then quickly "soar" off into the realms of mathematics, physics and optical design and engineering!

Nothing inherrantly wrong in that of course, but for simple folk like myself, who just look through their scopes and hope to see a nice view.

Of course, once "the bug" gets you, you do start wondering if that image could be made a "little" bit better, a bit clearer, sharper, bigger, more contrasty, etc, etc.

With regard to making "improvements", that is the beauty of this forum - as one can virtually guarantee he/she will obtain an answer to any question they may pose.

But can we amateurs really optically improve our scopes that much more than the manufacturers - without spending ££££££?

Optical manufacturers must be well aware of the "potential" of telescopes they "Could" produce, but because of commercial viability, they are limted by what they are "obliged" to produce.

Does it boil down to simply this then:-

If you want the best image money can buy, you must be prepared to spend lots of it!

Or is it:-

We mere mortals must be satisfied with the equipement available for a modest budget, and can only make, what simple modifications we can, to improve "slightly" on the manufaturers efforts.

Or! Can we improve a modest scope by buying the best quality eyepieces we can afford for that particular scope?

Or, would we be wasting our money buying very expensive eyepieces for a "particular" scope which (because of its optical design (ie. an 8" reflector or a 4" refractor) which will not improve it one iota?

This leads me onto "The eyepiece Survey" I'm compliling (see Lounge & Review sections for details!)

If we knew what eyepieces would give the best results for a particular scope, we could be happy in the knowledge that spending a large amount of money on a Hyperion, Nagler or Pentax, would indeed give us the highest quality image we are seeking with our particular scope?

So, could you all take a few minutes to "pm" me with what eyepiece(s) you find gives the best images with the scope(s) you have.

When finished (when I've gathered a good cross section of as many different scope designs as possible) I will put the list on the tutorial section of SGL for all to study or download.

Many thanks,

philsail1