WARNING: scope assembly practices may be vexing many SCTs & Maks

[mental4astro]

Hi all,

The following is my experience with a brand new SCT OTA. If you have at any time removed the corrector plate retaining ring from your SCT or Mak, the following is also very important to read as it may give valuable info on looking after your scope.


Some of you may have been aware of my recent quest to find a high quality Cassegrain. What triggered this was the brand new Meade SCT that I had bought that just refused to give a good image at anything over 200X.

For two years I struggled to get this scope to work. At first I thought it was just a bloody bad run of two years of poor seeing as the image not only boiled, but there was also a curious scintillation that I had never seen before in any other scope that was visible only when the magnification was ramped up.

What proved to me that it was not poor seeing, but a flawed scope was when Wavytone brought by his gorgeous 9" Santel Mak and we compared both scopes side-by-side. No issue now with any seeing as both scopes were under the exact same sky. This was a pure and simple drag race.

Seeing though not perfect was good enough to show a stable image in Wavy's scope at 300X with only a mild wave caused by seeing, yet my Meade was plagued with the same soft seeing wave and also this curious and maddening scintillation!

We carefully examined the optics and sure enough, the Meade was astigmatic! REALLY astigmatic!!! :mad2::mad2::mad2: Flaming bloody great as the scope was now no longer under warranty.

Last week I had some time to look over this Meade by removing the corrector plate and seeing if there was anything I may be able to do with the corrector or secondary.

When I attempted to remove the screws that secure the corrector plate retaining ring, I found ALL the screws were all WAY over tightened! How the heck the corrector plate did not crack is a mystery. The corrector plate however did not come away unscathed as there were two small scallop-shaped chips that were hidden underneath the retaining ring immediately where one of these screws pushes down on the retaining ring. Thankfully it is not in the optical path, but it should not have happened. The secondary mirror showed nothing obvious that could cause any strain. I replaced the corrector plate, this time setting the screw just firm, not tight, to secure the retaining ring.

First light with the adjusted retaining ring, and guess what? NO MORE ASTIGMATISM, and no more scintillating image!!! :D I was finally able to push this scope to 400X, and the only issue was the prevailing seeing conditions. The image is now back to that of a standard SCT. But this is something that neither I nor anyone else should EVER have to be concerned about or have to adjust on our scopes.

I had emailed Meade about this crappy assembly job of theirs, and oddly enough I got no reply from them! Makes me wonder at the poor training the assembly workers get, the time pressures they are put under, and the number of corrector plates that get smashed because of this in the factory.

So, if you are struggling to get a sharp focus from your SCT at high magnification, are struggling to collimate it and collimation changes when you swap between inside and outside focus positions, and you see that the optics are astigmatic*, you may be well served checking that the screws holding the corrector plate retaining ring are not over tight. These screws need to be JUST FIRM. Over tighten them and the pressure that this puts on the corrector plate will be enough to distort it and hence screw up the image the scope produces.

* How to check for astigmatic optics.

If your optics, SCT, Mak or Newt, are not astigmatic, the image that you should see of a star both inside and outside of focus should be of a round doughnut with a dot in the middle:

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If your optics are astigmatic, the symptom is that the out of focus star doughnut is ovoid in shape, and the long axis of this oval shifts by 90° when you go from inside focus to outside focus:

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Collimation may also not hold when you shift between inside and outside of focus, though this may be also due to mirror shift in an SCT or Mak.

If you do think about adjusting the screws that hold the retaining ring, you ONLY NEED TO LOOSEN THESE SCREWS. You just want to back them off until loose and once all loosened you just re-set them with gentle fingers to just firm to hold. Tighten them too much and the optics will distort. You do not need to remove the screws or the retaining ring or the corrector plate. If these screws were too tight, it will relieve any strain that the corrector plate was subjected to, and in turn relieve any resulting distortion.

Note that if your SCT is still under warranty, and you see the tell tale signs of astigmatism, do not undertake this adjustment yourself. Return the scope from where you bought it, and have them do this adjustment. The scope had been given to you not fit for purpose and needs to be fixed.

Alex.

[Saturnine]

Keep producing your insightful, informative articles Alex. I'm sure many of 'us" out there can and will benefit from the knowledge and wisdom that you are imparting to the IIS community.

Jeff

[beren]

:thumbsup: Nice work on the fix. A while back I pulled apart a used SCT tube that I brought used here ( previous owner had flocked the secondary housing and outgassing had dulled the secondary mirror) I was nervous about taking the corrector plate off etc but actually it wasn’t to hard with patience and due care. I contacted Meade a while back and they where pretty fast to get back, I had a Meade rep by the name of John Piper help me out, he is also a cloudy nights forum member if you want to PM him there.

[Merlin66]

Over the years of using various SCT’s I’ve come across this a few times...
A quick Ronchi test usually shows the issue...

[mental4astro]

Thanks lads :)

I don't expect anything from Meade - they supplied a scope with a damaged corrector plate. To me it speaks volumes their silence and the damaged corrector plate. If I was in the USA, then something may have come from them. But here out in the Sticks, not a chance. Poor assembly practices from the factory, this should not happen, but fixable. A damaged corrector plate that was stuffed in-factory and hidden (the two little flakes of chips had been removed so they knew about the cock-up), not acceptable.

Ken, yes, a Ronchi eyepiece is a good tool too!

[bojan]

Yes, this is a common problem, and not only with SCT's.

I had similar exrecise couple of years ago witth my old russian MTO 1100A lens.. and Rubinar 10_1000 as well.
After "relaxing" the primary mirrors, they are now behaving like totally another, much better lenses.

Right now I am into similar "repair" of Samyang 500mm f/6.3 catadioptric.. it is very cheap lens and almost not worth the trouble, but I regard this as the challenge - from discussions with other owners, I know my specimen is almost unique (coma), others are not showing coma at all. or are much worse.

The challenge now is how to build the reference light source (artificial star). I am thinking about using that MTO and laser (without colimation lens) as light source.

This setup should behave like a star placed at infinity.

[mental4astro]

Good luck with your project, Bojan!

By the way, the scintillation effect that I mentioned is due to the corrector plate being distorted in many different directions, so the photons were being scattered in all sorts of directions. Two things were the consequence:

1, if seeing was not perfect, then this was exacerbated with the photons being thrown around in every direction, so the image appeared to vibrate and shimmer beyond what seeing does;

2, the image will not focus sharp at high magnification - it can't, the figure of the optics is stuffed so the photons are not all going where they are supposed to.

Alex.

[Wavytone]

+1 Alex . I’ve seen similar before on two SCT’s - both displayed hideous astigmatism which had their owners perplexed... in both the secondary was badly crimped in its cell - relax that and all good.

Ditto the corrector on your M715 being strained by over-tight retaining screws.

As a rule all optical elements should be just sitting in place without force applied.

[bojan]

BTW...

Cheap Ronchi gratings are available from here:
http://www.willbell.com/atmsupplies/ATM_Supplies.htm
And even cheaper here:
https://stellafane.org/tm/atm/test/tester-2.html#RonchiScreen

[multiweb]

Yeah the corrector ring has to touch only the glass as a retainer, no pressure. You can apply a lot of pressure across 10 screws like a flange. Easy to overdo. Another thing to check is the shimming of the secondary cell in the corrector glass. You need to let it a little loose. As the corrector gets cold the ID shrinks and the mirror cell pushes back again the glass so you get trefoil if you pack the gap too tightly or with something other than cork that doesn't give.

[silv]

oh, 2 lost years - plus, this past Mars season counts for 5 lost years. :'(

Saw an extreme example yesterday of how in lens scopes tight collimation screws lead to weird features. Screenshot attached. Article here http://interferometrie.blogspot.com/2014/08/esprit-tuning-how-we-finetune-esprit80.html?view=sidebar
But I guess if one has a scope like that there's no doubt about a hardware issue and one won't lose precious time.

[mental4astro]

Oh my golly gosh!

Talk about an improvement!!!

I've just been looking at the Moon (clouds noe rolling n otherwise I would not be here right now...), and I just caught a glimpse of the tiny rile that runs down the centre of Vallis Alpes (Alpine Valley)!!! :eyepop: :eyepop: :eyepop:

Just a fleeting glimpse due to seeing not being too good, but it was repeated in those breath taking moments of clarity!

WOW, WOW, WOW!

I have my 7" Intes Mak set up too, and it shows it too! Both scopes pushed to 400X! 400 with a 7" scope! :D

God damn! What a rush!

Alex.

[astroturf]

Wow
Interesting reading
I remember a few years ago at waterfall,I had my pride and joy Meade 10" sct gps all set up and viewing

this guy rocks up with a much cheaper chinese 8 " sct,

we tried each others ep's and his made for better viewing all round at about 1/3rd of the cost

He bagged out Meade and their quality etc, while I was quietly pissed off and not showing it

I've always thought it should be a bit sharper on planets etc and it seems hard to get pinpoint stars (thought it might have been the red wine on some occasions)

when the Sydney cloud clears I'll take a good look at all of this

do you have to remove the corrector plate altogether or is it enough to reset the screws to a reasonable tension?

cheers
Bryan

[mental4astro]

Bryan, like I said in the second last paragraph of my first post, you don't need to remove the screws, ring or corrector. As long as the screws are loosened this will relieve whatever strain is on the corrector. You can fiddle a little tbe ring to make sure it is loose too, but no need to remove any item. You then go about re-sitting ghe screws a little at a time.

Do this before you set up. Better still during the day. This way there is no pressure to get observing. No need to rush this.

Alex.

[mental4astro]

One VERY EASY "taste test" of optical quality that I came to discover in my Cassegrain quest actually makes use of less than good seeing conditions. It is the wave like movement of an image at high magnification that shows up any distortion within the optics.

If the optics of your scope are good, and with SCT's & Maks this means the corrector plate is strain free, under less than perfect seeing and with magnification being maxed out, the image will flutter like a flag in the breeze, and the overall image will resist blurring out.

With my final cat shoot out, this was the final test that differentiated three Cats that excelled under very good seeing conditions. Under poor seeing conditions, the scopes with better optics* better resisted the image blurring out. One C8 just couldn't hack the poor seeing and was blurry continuously. The other C8 the image stability came and went. The Intes Mak, the image of course showed the flag-like flutter, but the image did not blur. This simple test was again proven by the new condition of my Meade.

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*Knowing now what I've found out about strained corrector plates, I'd love to be able to revisit those two C8s and examine the state of the retaining ring and re-set the screws that hold this ring in place.

Keep one thing in mind - today's mass production SCT's and Maks are fabricated in China. Their manufacturing techniques have improved out of sight. The quality of their optical elements are routinely outstanding today. One thing that has not changed is that humans assemble these instruments. And with the production pressures that the assembly workers are put under, and that these people may not understand optics, no matter what their training may tell them about careful torquing of the retaining ring screws, these people WILL OVER TORQUE THESE SCREWS ALL THE TIME. Remember, next to none of these people use telescopes in the same way that we do, so they will not understand just how delicate glass really is, and that when dealing with wavelengths of light, a small amount of undue pressure will have grave consequences on image quality. SCT corrector plates in being so thin are especially at risk to distortion.

If you think that your SCT is not affected by a distorted corrector plate, I would suggest that you rethink this. If you struggle to get the scope to focus, if the image blurs significantly under poor seeing, if the image displays scintillation at high magnification, if the defocused star image is ovoid in shape and not perfectly round both in inside and outside of focus, if your planetary photos are soft and lack crispness & experience a lot of flaring, it will be an idea to examine the condition of the retaining ring screws.

Alex.

[mental4astro]

To gauge if your 7" and larger Mak or SCT is up to scratch, here's a few target objects that you can try. Best thing about these items is they are either planetary or lunar as it is all about extended object resolution which is much finer than the Dawe's Limit ( see this link for more info on the actual resolution of telescopes (http://www.iceinspace.com.au/forum/showpost.php?p=1398775&postcount=10) and associated thread).

Also, perfect seeing conditions is not mandatory for these. On the contrary, some thermal distortion is GREAT as it will really test the optical quality of the scope and an indicator that some tweaking of the retaining ring may be an easy fix, or even collimation. You will be maxing out the scope's magnification with these too.

Planetary
* Encke Division in Saturn's rings.
* Fine festoons and smaller cyclonic swirls on Jupiter. Colour contrast an additional trick.

Lunar
* Craters Armstrong, Aldrin and Collins - if you cannot see all three, especially Collins the smallest at 2.5km diameter, you have a very sick scope! You should be able to resolve knife-edge details as thin as 150m, and finer with scopes 10" and larger.
* The thin rille that cuts down the middle inside Vallis Alpes (Alpine Valley)
* Fine rille inside crater Campanus

Struggle to see any of the above, even with seeing conditions say at 7/10, then the opitcs need some attention, including collimation.

And of course the above test targets also apply to all 7" and larger instruments, not just SCT's and Maks.

Alex.

[Wavytone]

BTw in case anyone is worried about loosened screws falling out under vibration (eg while being transported) there’s an old solution - use a cotton bud to apply a dab of nail polish around the head of the screw. This provides a visible reminder that it’s there, and it breaks easily if you need to adjust the screw again later.

Just don’t use glue or worse, Loktite... I have seen scopes with that.

[AstralTraveller]

I wonder how many scopes have been checked in the last 4 days?? Probably hundreds. Pleased to report that my screws were all appropriately just touching the retaining ring. Since it is second (actually third) hand I don't whether to thank Celestron or the care of the previous owners.

[morls]

Great thread, thanks Alex and all other contributors :thumbsup: :cheers:. I'm off to check my mak...

[mental4astro]

Only stating the obvious, but if you do adjust the screws, double check the collimation as it may need re-tweaking.

Alex.

[Stonius]

Good idea. Thanks for the links - now you got me thinking I should get some Ronchi gratings to test my scopes with, but I don't really understand the experimental setup.

I had a look at this video which is short and gets the general idea across, especially when it pulls back at the end https://youtu.be/FUBTlR82U68

My question is, do you *have to remove the mirror, or can it be used on an assembled OTA (given that it will display an aggregate of the primary and secondary imperfections)?
If so, I'm suprised they don't just make a ronchi 'eyepiece' with integrated LED for testing in the field, or at least package them as 2" mounted filters.

Thanks again for the links

Markus

[bojan]

You do not have to remove anything (except eyepiece).
Secondary imperfections will not be dominating factors.

[Stonius]

Having done some more reading, my understanding is that the grating is usually placed at the centre of curvature (which would be twice whatever the focal length of the scope is).

I take it that's not an issue having it at focus instead?

Cheers

M

[morls]

I've attached a grab from an article liked to in another thread here on IIS (can't remember which just now, will track it down).
I'm thinking of making a simple Y-shaped aperture mask and testing on a star (if and when the skies clear here...)
Would this be enough to identify over-tightened corrector screws? And collimation issues?

[Stonius]

If it really is that simple, you could pretty much make one with gaffer tape!

Markus

[Merlin66]

A ronchi grating used to observe an artificial star (or real star) will quickly show any issues.
Very simple, very effective.
Markus, the Centre of curvature methods is used during fabrication of mirrors. Testing with a star “at infinity” give the same results.

[morls]

A peace sign inside a wire coathanger opened out!

[Wavytone]

Not IMHO. Those masks are for accurately finding the exact focus particularly for AP.

The test for optics being crimped and collimation is a star test - a very bright star mag 1 NEAR THE ZENITH - and using an eyepiece that gives 80X per inch or 3X per mm of aperture, look at the star very slightly defocussed. If its a scope with a secondary obstruction (newtonian, SCT, Maks) you should be able to see the Poisson dot in the centre too.

Do not try this on stars below 60 degrees elevation, as atmospheric effects will be evident. You also need to do this with a mount that tracks well, and to have the star centred in the field of view.

Circular rings but non-concentric = needs collimation (and I'd fix that on the spot).

If you have everything concentric but aren't able to see the diffraction pattern note that some scopes simply aren't particularly good and will not produce a decent diffraction pattern, no matter how much you tinker.

if you are seeing a clear diffraction pattern and the rings are not circular or astigmatism = uneven supports or perimeter stress (dobsonian primary) or crimping (SCT corrector or secondary) assuming the optics are otherwise reasonably good.

Examples on this page (scroll down)
https://www.telescope-optics.net/diffraction_pattern_and_aberrations .htm

[bojan]

Have a look at this webpage (https://www.telescope-optics.net/ronchi_test.htm), I think the setup is well explained.
Basically, you are looking at the primary mirror through Ronchi grating, when scope is pointed at bright star..

[morls]

Apologies if this is taking the thread too far from the original intention, but I'm wondering about this, from the webpage Bojan linked to:


"A very worth mentioning is the possibility of using Ronchi grating in front of the telescope focus (Ronchi "eyepiece"), using a bright star as the light source. With two intercepted lines (L=2), it will detect down to about 1/7 wave P-V of spherical aberration in the telescope objective at f/10. For faster objectives, the sensitivity can be easily increased with a good Barlow lens.
In conclusion, while the geometric Ronchi test certainly has its limitations, they do not prevent its use as a quick, simple and reliable - within given limitations - test for surface quality of both, single optical elements and telescopes (with the latter, Ronchi grating is placed in the infinity focus zone)"

Are both of these instances referring to a test with the grating over the corrector, i.e. between scope and star, or is the Ronchi "eyepiece" a grating between (primary) mirror and eyepiece?
Does the f/15 of my scope make this a less suitable test for spherical aberration?

[Wavytone]

Steven a Ronchi grating work fine with your scope. The grating has to be close (slightly inside or outside) of the focus.

Lastly relax and DO NOT futz with your mak. All of the recent SW maks I’ve seen have been quite well assembled and there is no scope for you to improve it - and more likely you will make it worse if you try.

SCTs are whole different kettle of fish and many have either been assembled poorly at the factory, or have been fiddled with by owners who meant well but in their enthusiasm have over-tightened things that should never have been so tight. Not only can this degrade the image quality, in some cases scratching, chipping or fracturing the glass.

It is these SCTs that Alex is alluding to. Many SCTs do not perform optically as well as the probably should and there may be a simple reason - as Alex found with both of his SCTs.

But he simply didn’t know they weren’t working as well as they could until they were critically star tested at 400X. This only arose because I was pushing my MK91 to 500-600X one night. But the MK91 is no average scope.

[Stonius]

Aah, thank you!:thanks::thanx:

[Edit]...and also to Bojan - just saw your new link. Cheers!

[mental4astro]

Hey, folks, don't go about modifying on your scopes without first verifying that there is something that needs modifying!

Unless you first identify the tell-tale symptoms, leave your gear alone!!! You need to be certain that you have reason to consider doing changes.

It was only because I noticed problems that motivated me into considering checking things out.

You should not undertake any changes without first identifying problems. How are you then going to then verify that you have improved things afterwards, by how much, or if nothing has changed indicating other issues???

This verification of symptoms includes identifying astigmatism and doing the high magnification testing with the Moon. The conditions that the Moon offers for verifying optical quality are totally unique as a diagnosis tool! Don't dismiss the Moon as being a blight - IT is a severe task master when you max out the magnification of your scope!

Alex.

[The_bluester]

I am going to have to go out and check mine now.



I have to remove the corrector anyway. We had a seriously dew infested night after going to bed a while ago and when I moved the scope in the morning to position it to dry out without the sun falling on the optics a trickle of water ran down the inside of the tube and splashed the corrector! That and the secondary holder is slightly too loose, you can easily rotate the secondary in the corrector plate.

[morls]

Thanks for the advice Wavy. So far the only mechanical adjustment I've done is collimation, and I think it's performing really well. I definitely haven't seen any signs of crimped optics while star testing. I like 'passive' tests like star test and Ronchi because they help me learn about how the scope behaves - and what I'm actually looking at in terms of mirror and corrector - without making any changes mechanically (aside from focusing of course).