REFLECTORS FOR PLANETARY WORK

[Kaptain Klevtsov]

Phil, the optical axis of the secondary needs to be central in the tube, but that makes the mirror appear to be offset away from the focuser. This is because the light being reflected from the primary mirror is cone shaped, so bigger closer to the mirror. The edge of the secondary closest to the primary therefore has to be further from the 'scope's axis than the edge further away from the primary mirror. This makes the secondary appear to be off centre, but its correct.

Kaptain Klevtsov

[Vega]

Remember however that no reflecting telescope will yield as good an image for lunar and planetary work as a refracting telescope does because of the optical design

I have beg to differ. I find Chromatic Aberration in refractors give an inferior view to a well collimated reflector both optically and photographically on the likes of the moon/saturn. Of course if he is referring to APO refractors then its a different kettle of fish.

Anyone else find this ?

Matt

[themos]

This makes the secondary appear to be off centre, but its correct.

"Off centre" in the primary's axis or in the focuser tube axis? or both?

[Ajohn]

Can't agree with Kaptain unless the 2ndry mirror isn't at 45 degrees. That sounds like "sky and telescope" set up I described. The person who came up with it should have been strangled at birth. The gains just aren't worth the trouble it causes.

I centralize the 2ndry with a tape measure. That can get it to better than 1/5mm. You could use callipers. On the view down the focuser you should see the 2ndry circular and the whole of the mirror plus some of the tube also central and circular. All round and central. It's best viewed with something in the focuser with a small hole central in it. Moving the eye nearer or away and altering the focuser will alter the field of view so it can be arranged to just show the 2ndry. That will centralise the 2ndry to the focuser. You can then use the 2ndry tilt adjustment to centralise the mirror and tube in the 2ndry through the same hole. You may find as you do that you will have to go back and re centralise the holder and the 2ndry mirror. You can also get some idea that the focuser is aligned by racking it in and out. If you can't see all of the mirror it means that the 2ndry isn't catching all of the beam from the main mirror - you haven't got an 8ins scope only the size that you can see. I drop a circular stiff paper mask over the mirror with circles on it when I find that or take the mirror cell off and tape one in place - easier to get it central. If you can't see the tube at all then the 2ndry is going to miss some of the off axis light gathered by the main mirror. Like most things about telescopes that is a compromise. A good setting for general observing would catch all of the light from the moon which is 1/2 a degree across from memory. You won't see much tube. If you wanted the scope for star fields then a slightly larger flat might be a reasonable idea but the limit is just how much coma you will tolerate. An 8ins F5 scope will give a fan shaped spot size of 0.1mms with a field of view of 1 1/3rd degrees which measures 0.9ins at the focus. The flat size has to be worked back from that depending on how far out of the tube it needs to be.

The next thing to do is to tweak the main mirror as per normal. You can also check the squareness of the focuser and maybe pack it to improve things without using a star. Simply get a short focus eyepiece that give an insane level of magnification and look at a terrestrial scene with it. Defect or perfection should be even across the field. 50 to 100x per inch of aperture is in the right ball park. 100 is probably to much. To give you an idea my son's 120 skywatcher refractor will give a 330x+ slightly fuzzy view. My own 5ins apo will give a much less fuzzy view but it's noticeable that additional detail isn't been shown as a result of increased magnification. I can't comment on reflectors as this only became apparent about 5 or more years ago following a problematic startravel pass on why I bought it I should have known better. It's a good way of checking a scope out the only problem being that there is a very high chance that reflectors wont be set up properly even straight out of the shop. Having said that though I did sort out one of those strange newton cats that Dixons etc used to sell a long time ago. I had to manually bend the focuser into place. Having set it up it as outlined above it gave very good views. All of the final little tweaks were done against terestrial object.

The only thing that can go wrong with all of this is that the 2ndry holder isn't actually a 45 degree type and will not adjust to 45 degrees. On the one I had that was like that I had no other alternative other than to try and set it up knowing that the field wouldn't be square to the focuser or remachine the 2ndry holder. I chose the latter. The trouble with not at 45 degree idea is that such things become folk law - so and so says blah blah and wow IT APPEARED IN SKY AND TELESCOPE it must be right and people do it and never ever fully check out the basic concept. Now the internet is about there is an awful lot of duff info about. This isn't by the way. Maybe I should start a website or write a book. The problem with that is to be successful one needs to offer something new - that's the basic problem and the source of much guff. There are also a lot of commercial influences about too.

John

PS Having spent years writing software I make some really strange typo's sometimes. Fingers' brain and words sometimes get out of sync. If it makes what I've typed up incomprehensible let me know and I will correct it.

[beamish]

Just read this and now feel utterly knackered !!

[Viking]

If you read all that, then could you provide a brief summary for the inflicted!!! :insects1: :insects1:

John

[Ajohn]

I take part in a couple of forums on different things. Following a thread can be a right pain and often offer conflicting views. Some people brows through them and pick a view and search that persons posts and read em. Strange thing is few people ask for clarification if they don't understand or want to query something.

Problem with answering questions is that it's easy to spend hours on a forum and after a while the same old questions keep cropping up. Nope I can't summarise mine. If you are collimating a scope for instance you do need to understand what you are doing. There is lots of info available via google some even with video's and diagrams. I've found them somewhat lacking in respect to the points I've made but browsing through a few of them should enable anyone to understand what they are trying to do.

By the way I should have added that there shouldn't be any problem tweaking collimation with a laser thingy once things are more or less right. The more right the better. Not so if things are way out. That statement might start a heated debate but I think that they are largely a bit of a waste of money and am glad I got mine for free. An artificial star would be a better investment. Those aren't too difficult to make either.

John

[Astroman]

Remember however that no reflecting telescope will yield as good an image for lunar and planetary work as a refracting telescope does because of the optical design

I have beg to differ. I find Chromatic Aberration in refractors give an inferior view to a well collimated reflector both optically and photographically on the likes of the moon/saturn. Of course if he is referring to APO refractors then its a different kettle of fish.

Anyone else find this ?

Matt

Some of the best planetary views I've had have been through long focal length reflectors. I'll add that I've observed Jupiter and Mars through the Clarke 24" Reflector at Lowell, and a 10" F/6 from the desert floor was much superior. I've also observed Jupiter through a 6" Takahashi F/9.3 and it was superb, of course, but no better than the 10". My own choice for planetary observing is my 6" F/9.3 newtonian. Absolutely amazing details on Jupiter and Mars, (yes, even now that it's tiny) and completely color free.

I do not agree that it is just the design of the telescope that makes the difference, but the quality of the optics. Full stop.

P.S. The views I had of Saturn last weekend through the 20" F/4.3at 500+x were stunning.

[KaStern_Former_Member]

Hello John,

I also under stand that Pentax for instance quote strehl ratios over the field of the scope.

can you please provide a link to that Pentax page ?

Thanks, Karsten

[Kaptain Klevtsov]

Can't agree with Kaptain unless the 2ndry mirror isn't at 45 degrees. That sounds like "sky and telescope" set up I described. The person who came up with it should have been strangled at birth. The gains just aren't worth the trouble it causes.

John

Must be the way I explained it (or didn't ) then.

Grab a traffic cone and a hand saw and cut the top off at 45 degrees. You'll notice that the cut starts at one side of the cone high up, and lower down on the opposite side.

Get the saw again and cut the cone flat, taking nothing more off from the point where the diagonal cut was lowest i.e. just straighten it so that its cut horizontally. The cut surface should, when seen from the top, be circular. Next, do the same thing with the bit that you cut off when you first cut the cone diagonally. Straighten that up too and you'll have a circle also. But a smaller one.

The edge of the secondary closest to the primary mirror is intersecting the light cone at a bigger diameter than the edge furthest away from the primary mirror. That's because the cone is tapered, they often are.

The same thing goes for the focuser, if you can look through a sight tube with a small hole in it and see the whole secondary centred in the tube, its an optical illusion as the secondary is angled so the bit furthest from your eye necessarily has to be further off axis to appear the same as the bit thats closer to you.

Not hard to set up, as you do it by eye and that makes it work, for the focuser tube, but more awkward to offset the secondary relative to the primary mirror axis. The easy way is to put your eyeball so that the secondary covers the primary perfectly. Your eye should be more or less at the focal point of the mirror and it should be on axis. The secondary shouldn't though.

Kaptain Klevtsov

[beamish]

By the way I should have added that there shouldn't be any problem tweaking collimation with a laser thingy once things are more or less right. The more right the better. Not so if things are way out. That statement might start a heated debate but I think that they are largely a bit of a waste of money and am glad I got mine for free. An artificial star would be a better investment. Those aren't too difficult to make either.

John

I'll certainly agree with that, can't figure out why I don't sell my laser "un-collimator" ? Perhaps it's the Eastern European heritage !

[Kaptain Klevtsov]

Found a picture.

http://www.bbastrodesigns.com/OFFSET.JPG

Kaptain Klevtsov

[Sam]

Thanks for posting that image KK - a picture is worth 1000 words, and that certainly explains things quite well - just looked into my tube and noticed the secondary mirror is positioned slightly away from the focuser - now I know why! excellent!

Sam

[Ajohn]

I would look very carefully at that picture and really think about what it means. Optically it doesn't make sense. As I pointed out duff ideas become folklaw and spread. Any one who has a scope like that is likely to run into serious difficulties if they attempt a major recollimation.

But they do it so it must be right mustn't it

John

[Ajohn]

Thanks Kaptain - I did understand. That sky and telescope article from the 70s (roughly might be 80s) went through it in detail. The reason that it cropped up was a move from the normal F7 or 8 newts to much shorter f ratios. It was argued that because of the more obtuse angle of the cone of light of the main mirror light was being lost - important light because it came from the edges of the mirror (area goes up R^2) all down to the standard collimation set up that I've described. It's also described on many web pages so watch out.

One of the problems with an offset is that the standard set up is the only one that will give an all square result. That's focal plain, tube, focuser, mirror and tube. That can be fixed to a certain extent by not using a 45 degree tilt and from memory also moving the mirror slightly of the axis of the focuser and or tilting the focuser to compensate. There is still a problem though as the mirror is circular, we hope for a circular final field of view that uses all of the main mirror and the usual ellipse used to form the 2ndry will only do that if it's centred on the beam of the main mirror and at 45 degrees. Maybe they use another shape on some scopes now but I've never heard of it. If they haven't done that some of the light is going to be missed in any case - also from the edge of the mirror. The other factor is that most scopes do have a field of view (unlike the picture) but fully illuminating the field of view can mean a very large 2ndry so like most things about telescopes a compromise is used. That might be a 30% drop in illumination at the edge of a field that in a newt's case will be limited by coma. More limited in respect to angle the lower (faster) the F ratio and the bigger the scope. I mentioned one field of view which is suitable for film imaging and should give good results on ccd too. The illumination drops off because as the beam from the object make more of an angle with the axis of the scope and the image moves towards the edge of the field of view eventually the 2ndry wont catch all of the light. My argument is that if you take all of this into account it just isn't worth offsetting the secondary. It's a drop in the ocean on most scopes and isn't worth worrying about especially as it make collimation more difficult. I'm more inclined to say that it's like doing something into the wind.

The net effect following the usual collimation procedure on that newt I mentioned was to only use 2/3rds of the mirror.

Finally if you think about the "view" Kaptain has described at some angle of light entering the telescope the 2ndry is going to miss light of the main mirror. More so in one direction than another. But Mel has put something on his web page so it must be the right thing to do mustn't it?

In some ways I think it's best to know absolutely nothing about telescopes and just use them. Me I got interested a long time ago trying to find that perfect telescope. It's a hobby of mine. The lack of explanation about the good points and bad points of the various scope that can be bought and the various compromises that are used really does get up my nose as a result of that hobby.

John

[Sam]

I would look very carefully at that picture and really think about what it means. Optically it doesn't make sense. As I pointed out duff ideas become folklaw and spread. Any one who has a scope like that is likely to run into serious difficulties if they attempt a major recollimation.

But they do it so it must be right mustn't it

John

You clearly have strong feelings about this issue which is understandable after the time and effort it sounds like you've put into the area over the years. I certainly respect the depth of knowledge you've clearly attained in telescope design and have found this topic quite interesting, now that the originator isn't posting anything random anymore. I can assure you I have carefully looked at the picture KK posted and fully understand what it means. Obviously since my telescope is "a scope like that", like all the other reflectors made in that same factory, then there is a few of us who are going to have major problems when we do a major collimation. I would, therefore, invite you to offer an explanation on how we would rectify this seemingly major flaw in our telescopes or is it something that is relatively minor and not worth worrying about?

Sam

[Ajohn]

Hi Kastern. I came across the pentax spec when I though I might buy a 3ins pentax refractor some time ago. Information was hard to find - for instance the english pentax site I found has no reference to the telescopes at all. I have just had another search but can only find reference to on axis strehl. This link does show a plot though on another scope http://www.trifid-optics.com/telescopes-a-m.htm. ( and dome interesting scopes)

I have a feeling that it was a reference to the sudf but all I can find now is an off axis max spot size. That too would be a very useful addition to all telescope specs.

Note one of the mods comments about quality and the telescope type doesn't matter. Bit saddened by that. For planets maybe. Extended objects no way.

Lastly the nearest I have found to short and universal and good in all respects even on 2ndry size is cape newise. See this link

http://www.newise.co.uk/

Well thought out fast scopes, uk made and look what's happened!

John

[beamish]

I know, I own one !!

Karlo

[Ajohn]

Sorry I can't help because I can't get my hands on one to see exactly what they have done. My answer on the meade was to remove the 2ndry and remachine the holder to 45 degrees. I would have sold it by now but I missed by about 2 degrees as the equipment I had wasn't adequate. I should be getting round to correcting that eventually. (not a sales pitch, I just have too many and must get rid of some.) On this scope the 2ndry couldn't be tilted to 45 degrees it was meant to be used at some other angle. The previous owner had clearly messed things up by way of a laser collimator.

My advice would be to leave well alone. Just tweak the main mirror against a star under high magnification. Hopefully the 2ndry offset and maybe even it's angle will have been set correctly in the factory. If you do look down the focuser through something with a small hole in it and can only see part of the mirror - one side missing - try adjusting the tilt of the 2ndry. Only adjust the offset as a last resort. If the 2ndry isn't central in the focuser that's difficult because I can't remember all of the detail of the article. My recollection is that is was down and back away from the focuser which does make a bit of sense given the aim. In that case the 2ndry will not be at 45 degrees when it's set correctly. If a scope has been messed up it's a case of playing around till it's right. It may well be possible to set the scope up as I've outlined but going back may turn out to be difficult.

As to if that sort of set up is worth worrying about well I don't think so other than situations where the settings are overdone and cause the focal plain to be very tilted. It's just that it makes life a pain and in real terms doesn't achieve that much if anything and probably makes things worse elsewhere. You will also probably find that all newts are now like that.

Also on tweaking on a star. I'm fed up with waiting for a clear night to see what I have bought. There is a simple way of making an artificial star. The bottom of hp inkject cartridges has a small metal plate with some really small holes in it. It can be removed, with care, all but one hole blocked off with paint and lit from behind. One way of doing that would be to mask of a torch with something that blocks all light leaving a small hole and sticking the hp inkject plate to it. The holes in the plate are less that 0.030 mms dia and should be viewed from a distance that makes them less that the resolving power of the telescope if possible. It should be easy to get diffraction rings under high magnification and is a much more convenient way of collimating or evaluating a scope. I would use a torch bulb not a led torch. I knocked up a fibre optic artificial star and there are some strange optical effects that seem to be down to the led. Turns out that the hp inkjet way is a better way of doing it. Another way with more info in general is on

http://observatory.mvastro.org/library/Star_Test/ArtStar.html

Interestingly since starting a discussion on these with other people on an atm forum there are now lots of links on these on the web. I used an 0.008mms fibre optic cable to hopefully get a smaller star but some of the light leakes into the 0.125mms cladding. Sad because that would have been wonderful.

One can buy artificial stars too - a bit overpriced for what's in them. These usually have adjustable brightness but needs must. Some tissue over the bulb would do at a pinch if the star is too bright.

John

[philsail1]

Kaptain K. Thanks for the explanation as to why the secondary mirror looks "off set." I can understand how and why "B" should be greater than "A," but I also take on board (after some re-reading) what "Ajohn" says.

I have just applied a new "spot" to the centre of the 8" mirror (used one of those circular punch hole reinforcers). I measured the centre spot with a ruler marked in millimetres, and checked it with a paper circle with a small hols in the centre. Found the original paper circle was off centre by about 1.5mm. Perhaps this is why my "Cheshire" collimator kept drifting off centre as I slowly turned it through 360 degrees. I re-collimated my scope using the Cheshire, and then just doing a double check with my home made film canister collimator. Both seemed to show the scope to be well collimated - but I could be interpreting what I see completely wrong! (after you see the photo below which was taken through the central hole of the "film canister" collimator).

(click to enlarge)

You can see the new circle I stuck on the centre of the main mirror as the bright circle. You can also see the outer edge of the secondary mirror, and the three clips of the main mirror. I don't know what the shadowy second circle is though - unless its the reflection of the end of the eyepiece?

The secondary mirror looks a bit off centre to me - Is this what you mean when you say the mirror looks "off set?"

Echoing Sam's comments, I can appreciate "Ajohn's " frustrations over the technicalities of secondary mirrors, but would ask "what is the best way we non technical people can colllimate our Newtonian scopes, taking into account what you have said about the positioning of the secondary mirror?"

Regards,

philsail1

[Ajohn]

There isn't any way of avoiding the technicalities. I notice form your photo that the main mirror doesn't look round but that might be down to photo upload distortion or the the way the camera was held. I thought about suggesting that as a check but it's not a very sensitive one and I don't want to mess anybodies scope up. I will try and find some links when I have time that explain the technicalities. Piccies help a lot. Just words is a bit difficult to say the least.

Don't forget that final collimation should be done against a star for round diffraction rings by just tweaking the main mirror. That can be difficult which is why I outlined how to make an artificial one. A star is the definitive test. Unfortunately that can show other problems too. Distortion can even be down to the flat itself.

John

[Macavity]

[part serious] I suppose the ultimate in "off-axis secondaries" is THIS (rather nifty) little scope?

http://telescope-service.de/OrionUSA/reflectors/reflectors.html#Clant

But (obviously) here they have gone out of their way to make the Primary mirror assymmetric too!

I guess, for me, this merely typifies the sometimes difficult, but GENERAL problem of deciding WHICH errors are significant - And worth working on, and which are not. Why a mirror is figured to a fraction of a wave, but we think we can "collimate things" with plastic film containers. Also too reminding me that, when I first saw the above "off centre" diagram, in a telescope manual, the sometimes (more often than not?) complete lack of EXPLANATION to accompany such diagrams. I.E. was the illustration Genereric? Quantitative? But actually one reason (among the usual others) why I went for a "nice symmetrical, solid" MAK. That said, I was a bit "miffed" my short tube refractor no longer had an adjustable lens cell, so I "shimmed it", with the aid of a Cheshire eyepiece anyway. But I later sensed, any "success" I may have had, was probably negated by the general (fair enough on that budget!) quality of other components and their assembly. There is the splendid notion of "suffering from delusions of accuracy" - Common to most "scientists" - amateur or professional, beginner or expert...

[philsail1]

Hello "Ajohn," and thank you for your reply. I think the main mirror on my 8" Newt is round, as I tested it using an 8" circular piece of paper (Made with a simple pencil and school compass).

I will do a star test (as I always do) when the weather clears, and I will try to get some photos to post on here for possible interpretation. When I've conducted star tests in the past, the circles (in and out of focus) seem OK, but I am very inexperienced in this field. After testing, all I tend to look for is that my scope gives me (what I consider!) a good clear and sharp view of whatever I'm looking at. So far, the 8" Newtonian seems to be well on par (for clarity and sharpness) with my TAL100RS refractor. I have done a star test on the TAL, and this scope too, seems to give "even" diffraction rings when both "in" and "out" of focus - using (as in the Newtonian) a 6.3mm orthoscopic eyepiece.

I've never tried to "teak" my Newtonian's mirrors in the dark - it would be just too much for me in the dark. (But I may try this on a summer's evening, when it's a bit warmer!). I may have a go in trying to make an "artificial star" for indoor use.

"Macavity," I like the look of the unusual "off axis" Orion Newtonian in your reply. I wonder how popular this kind of scope is in the USA. They don't seem to be very prolific in this country - I don't recall seeing any advertised in any of the British companies I've looked at over the past four years or so. Has anyone on SGL got one?

I noticed when I was viewing Mars last Thursday night (when we had a brief period of clear weather between 6.30pm and 11.00pm - after which it clouded over and rained heavily!!) that there were the usual four "spikes" emanating from the view of the planet when using a high magnification. When I looked though my TAL100RS the view was "clean," though the view was smaller on account of the magnification limitations of the 4" scope. Perhaps the way to go is with the "off set" Newtonian?

To return to the technicalities of collimation, and mechanical accuracy of mirror manufacture, configuration and assembly, all we "amateurs" can do is "hope" (and trust) that the scopes we buy are indeed manufactured to the extremely fine tolerances required for the scopes to perform 100% to their designed limits. But even we amateurs are constantly seeking to improve the performance of our scopes (perhaps under some delusion of our ability to get everything 100% accurate!). It's fun trying though!!

regards,

philsail1

[Macavity]

"Macavity," I like the look of the unusual "off axis" Orion Newtonian in your reply. I wonder how popular this kind of scope is in the USA. They don't seem to be very prolific in this country - I don't recall seeing any advertised in any of the British companies I've looked at over the past four years or so. Has anyone on SGL got one?

I think, given sufficient budget, decent observing site, weight lifting capabilities, I might've seriously looked at some of these "other" designs. I was particularly impressed by Jan-March SPA Mag's article on Schmidt-Newtonian and Maksutov-Newtonian scopes. The former being available down to F4/5 (for wide field DSOs) and the latter, F6/7 for general purpose, and upto F10 (with a 13% central obstruction completing [a basic illusion of?] a "disapperaring secondary") ergo excellent contrast (on lunar/planetary). Always nice to spend others' money though...

[Ajohn]

On the round mirror what I meant was that the main mirror will only look round if the camera is square to the beam coming out of the 2ndry. Also it can be used to roughly set the 2ndry at 45 degrees PROVIDING it's on the axis of the focuser. Could be used to set other angles too.

I note the small % secondary quoted bet it's % area. That's a common dodge many manufacturers use. It should be by diameter. Even the mystic 20% optimum spreads light though. An optician called Dall did achieve 10% but I'm still trying to figure out how he did it.

I doubt that the off axis newt uses a compensated primary but it may as the easiest way to do it is to make a large mirror and cut segments out of it. The catch with these scope is the F ratio. That ones ok as it will work just like an f13.6 /2 = F6.8 scope as far as the eyepiece is concerned. Some off axis scopes are fascinating. Try searching tct telescopes (that's tilted component telescopes) and schiefspiegler. One of the designs is very diffraction limited over a wide field. (The stevick paul) Problem though is that the design causes the exit beam to hit one of the mirrors. There is also a Brunne (may be spelled incorrectly) that has fairly good performance of a decent field but isn't really diffraction limited. They can be bought commercially too. Some of these scope are probably the ultimate weapon for planets etc. As to distorted mirrors for off axis scopes search yolo. I say no more on that one. A web place called weird telescope (should be a brook high site if it's the right one) has loads of info in these scopes. Also free software to evaluate them - winspot. The major manufactured source is in Germany but there are other.

The film canister with a hole in it by the way will set things up to a fraction of a degree. It amplifies errors - best have that hole in the middle though. It's a bit like a faucoult test that can be used to test mirrors. On that one you could knock something up at home that can measure errors on the surface of a mirror to better than 1/20th of the wavelength of light and even see smaller ones than that.

John