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.
You do not have to remove anything (except eyepiece).
Secondary imperfections will not be dominating factors.
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?
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?
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?
If it really is that simple, you could pretty much make one with gaffer tape!
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.
Would this be enough to identify over-tightened corrector screws? And collimation issues?
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.
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
Have a look at this webpage, 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..
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?
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.
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.
Aah, thank you!
[Edit]...and also to Bojan - just saw your new link. Cheers!
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!
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.
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.
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).
