Help with identifying optical aberration of my system - Forums

Topic starter

Mar 7, 2021 · Edited: Mar 8, 2021

Hi All,

I bought a used quadruplet from Teleskop Services last July (with a discount of course) and the stars have never been sharp pinpoints, but I feel like the problem is increasing more and more. It was my first refractor and my starts in the astrophotography field, so I never thought of sending it back. And again, the problem was lass pronounced.

This is a 71/347 Imaging Star quadruplet from TS optics, and I feel like it is a collimation issue, but I am really not sure if that's the problem with my stars getting a coma shape (same direction on all the image). And even if that's the problem, I don't know if these small refractors have collimation screws ? (can't see any of them on the front lens… maybe the built-in flattener can be tweaked ? how ?)

I post a sub of my session last night : 1 minute on the Owl nebula.

I don't think it is a polar alignement issue because all the shapes of my stars is quite constant over sessions : and I am polar aligning every night, so if there was a random error, I would expect elongation in various directions.

You can check my posts on Astrobin, almost everything was made with this scope.

Any feedback on what can cause this stars shapes and any tips to collimate my refractor would be welcome.

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Jérémie

Topic starter

Mar 7, 2021 · Edited: Mar 7, 2021

Well, I couldn't post my pic, I try again here with a smaller resolution.

Focus was made just before this first picture of my serie of subs

Jérémie

Topic starter

Mar 7, 2021

Jérémie

Topic starter

Mar 7, 2021

Same problem tonight…

Jérémie

Topic starter

Mar 7, 2021

Jérémie

Topic starter

Mar 7, 2021

Jérémie

Topic starter

Mar 7, 2021 · Edited: Mar 7, 2021

The Airy disk seems non symmetrical… (out of focus)

John Hayes

Frequent contributor

Mar 7, 2021

That looks like coma, but it's hard to tell what else might be there from that defocus image without some more sophisticated analysis. The first question: Is that a star image from the precise center of the field?

John

Jérémie

Topic starter

Mar 7, 2021

Hi @John Hayes !
This star showed out of focus is not dead centered, this is the brightest star you can see on the picture two messages above, a bit on the right of the center of the picture (should be Polaris).
They all have that shape, without any central symmetry in the image.
What would you suggest as tools to make better analysis of my images ? Something like a Ronchi test ?

David Redwine

Mar 7, 2021

It is very hard to say based on the images you have posted, but if all of your stars have the same distortion it might be a (large) tilt problem. The plane of the sensor is not perpendicular to the light path. Can you feel any vertical motion in the focus mechanism when you try to wiggle it up and down?

CS

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John Hayes

Frequent contributor

Mar 7, 2021 · Edited: Mar 7, 2021

The problem with refractors is that they generally don't have anything to adjust to fix problems like this. One guess might be that you have air spaced components in the primary that are not well centered. With a well designed cell, that shouldn't be possible but maybe it's possible in this design. Since you've mentioned a field flattener, that's probably the best place to start. If that component isn't well centered or spaced, you'll get problems. What have you done to check the spacing and centration of the flattener? Is it possible to rotate it to see if the direction of coma rotates with the flattener? The last alternative is that there might be a figure problem with one of the surfaces–and you'll never fix that.

My advice in fixing problems like this is to first try to reduce the number of variables. If possible, it would be helpful to know how the objective images without the field flattener. It will have a curved field but you could see if the stars are unaberrated on-axis. If so, you know that the problem isn't in the objective; otherwise, you'll need to get the objective fixed. I'd put my money on some problem with the field flattener but you need to rule out the objective first. Of course if the field flattener is built-in, there isn't much you can do and in that case, I think that you are going to have to return it to the manufacturer to get it fixed. In my view, that level of aberration is unacceptable.
John

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andrea tasselli

Frequent contributor

Mar 7, 2021

Most likely it's a tilted element within the lens system. The first port of call is to remove the flattener, if possible, and test the scope again and see if the problem persists. Normally, and I speak from experience, the effect of lens tilting can be minimised but never removed. This if the lens cell has adjustment screws, which yours nearly certainly doesn't have. The adjustment procedure is easy enough with a Cheshire EP. If you are still within the warranty grace period then return the thing and do it presto. These scopes are not easy to fix for inexperienced users. And nearly impossible to without a proper collimatable cell.

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Jérémie

Topic starter

Mar 8, 2021

@John Hayes @David Redwine @andrea tasselli Thanks guys

Unfortunately, the flattener is built-in, and to be honest I am not experienced enough trying to access it : they don’t provide blueprints of the stuff, so I don’t know the purpose of the few screws I see on top of the tube, more or less where the flattener is, near the slots to put your guidescope. I am not sure how to access the flattener correctly, even by removing the focuser (and there are baffles in the tube, I don’t want to mess with them if I am not sure on how to put it back).

The focuser is quite rigid, not sure there is tilt, but I can try with a tilt adjustment ring (will receive one for my RC). But I would expect to get uneven stars with a tilt problem, because there would always be at least a point where things would focus correctly with small stars on a corner and bigger ones opposite of them ? But I am not expert in optics...

I think I am going to contact TS Optics and see how much they take to adjust that, but I am afraid there won’t be guaranty as it was a used refractor they were selling. I should have carefully checked that at reception :-)

andrea tasselli

Frequent contributor

Mar 8, 2021

If you are going to receive a tilting camera gizmo you might as well attempt to improve upon you current situation. Just to spell it out you have a tilted optical system and significant chromatic aberration (as expected from a fast ED doublet). You need a Cheshire EP, such as this:

https://www.harrisontelescopes.co.uk/acatalog/skywatcher-cheshire-collimating-eyepiece.html?gclid=Cj0KCQiAs5eCBhCBARIsAEhk4r5y4OxT4GK4avJ7nSRFia0qrLRH5q25z54fHFE3TPFuhNaJJT8HmKsaAjiREALw_wcB

(Note that I am not endorsing the seller, just took one out at random)

The testing should be done in daylight and with a source of strong sunlight at hand, say a window facing the sun. Insert the Cheshire into the adapter attached to the tilting unit you received (they are 1.25" and the adapter should have at least a T2 thread). Put the scope cap on. Turn the Cheshire EP angled slot toward the source of light and look into the EP end. You should see a number of reflections from each of the elements in the optical train. They should appear not overlapping if there is a tilted element. Adjust the tilting unit screws so that you bring the non-overlapping reflections to overlap. Small movement should do, hopefully. If, however, the issue is with a wedged lens then the adjustments (which can only minimise the issue) must be done at night and in focus and they are not trivial. Software such as CCD Inspector will make it a much easier task (warmly recommended if your are going that route).

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Jesus Magdalena

Mar 8, 2021

There is no doubt that the telescope is not well collimated, it is best to send the technical service to have it adjusted.

Greetings.

Jérémie

Topic starter

Mar 8, 2021

I sent an email to Teleskop Services with the pictures sooner today. I’ll see what they suggest :-)

John Hayes

Frequent contributor

Mar 8, 2021

Jeremie,
From what I can see, adjusting the sensor tilt isn’t going to solve the problem. One of the big problems with buying optical equipment is that without good test data, you never know what you are getting and that’s true whether it’s new or used equipment. The problem with used equipment is that when you run into problems like this, you really don’t have much recourse to get it fixed. The good news is that even though you may have to pay to get it repaired, your final price should be a bit less than the cost of a new scope and (if they do a good job with the repair) you’ll end up with “like-new” performance. Hopefully, you can get the company to stand behind their product to get it working properly. Good luck and let us know how it goes.

John

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Jérémie

Topic starter

Mar 8, 2021

Yes, fingers crossed @John Hayes

I totally subscribe to what you are saying : the right way to buy optical goods would be through companies that provide test data and quality insurance (and you have to be able to perform your own tests at reception), but with the small price tag of these probably rebranded refractors I am afraid you can’t expect the same level of support and quality than when buying from Planewave :-)

But we will see, I have always had good customer experiences with German companies (in my professional life) as well as with some orders of astronomy goods (Berlebach to mention them). Let’s see with Teleskop Services.

I’ll tell you the outcome of that story.

Douwe79

Mar 10, 2021

Interesting thread. I have the same issue with a second hand refractor.

You should indeed thoroughly test the back focus distance.

CCD inspector is a useful program to check both your curvature and tilt in X and Y axis.

Unfortunately, with a built-in flattened there is not of adjusting you can do. But at least with CCD inspector you can determine if you have a flat (enough) field and/or tilt. They provide a 30 day free test version. It’s extremely easy to use. Take a 30 second guided shot, import it in CCD inspector and analyze.

Good luck!

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Jérémie

Topic starter

Mar 10, 2021

Thanks @Douwe79 . Will have a look at CCD inspector, though I am not sure I will be able to interpret the results (it just gives you an idea of the curvature of your field, correct ?)

For the time being, I still didn’t receive any response from Teleskop Service.

I had to « Photoshop » each stars of my image to remove the coma from my picture : that’s painful ! (The technique is to make a custom brush with a doughnut shape, with a blured external side, and use it with the Clone tool... and then click click click everywhere you have a star, adjusting the size...).
https://astrob.in/uob07t/0/

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Douwe79

Mar 10, 2021

Jérémie:
Thanks @Douwe79 . Will have a look at CCD inspector, though I am not sure I will be able to interpret the results (it just gives you an idea of the curvature of your field, correct ?)

For the time being, I still didn’t receive any response from Teleskop Service.

I had to « Photoshop » each stars of my image to remove the coma from my picture : that’s painful ! (The technique is to make a custom brush with a doughnut shape, with a blured external side, and use it with the Clone tool... and then click click click everywhere you have a star, adjusting the size...).
https://astrob.in/uob07t/0/

Correct! And CCD inspector is easy to interpret

Jérémie

Topic starter

Mar 14, 2021 · Edited: Mar 14, 2021

No news yet from Teleskop Services, but in the mean time, I have taken some pictures of my little refractor and I am wondering what is the purpose of some of the screws we see on it ?

On the picture I am talking about the 2 made of brass and the 2 smaller black ones, all about where the flattener sits within the tube.

I don’t find any manual about this refractor, so I really don’t know what they are made for... maybe centering the flattener ? This would be good news because it means the refractor can be collimated / tweaked...

Any idea from people who would happen to have the same one (TS Optics Imaging Star 71/347) ?

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Jérémie

Topic starter

Mar 22, 2021 · Edited: Mar 22, 2021

Hi !

As a follow up on my adventures in collimating my little refractor : I am now in contact with Teleskop Services and they have looked at my pictures, suggesting there may be tilt or "pinching". They propose to rule out the tilt hypothesis by trying a tilt ring (which I haven't yet tested, because tonight was time to prepare the diner for the kids, so I just had time to test my new artificial star on my "classical" optical train).

If that's not tilt issue, they will either explain me how to try to fix the « pinching » myself, or suggest I send them back the refractor (but they are a small company and it may takes some weeks before it comes back from Germany).

Nevertheless, I would really appreciate if some experts (@John Hayes if you happen to see my post :-)) can give me some thoughts on these new tests with the artifical star, below (I tried to present the tests nicely).

I think on some of the inside focus Airy disk - very close to focus - to see what is called "pinched" optic - read something about that on Cloudy Nights (https://www.cloudynights.com/topic/393450-pinched-optics/).

andrea tasselli

Frequent contributor

Mar 22, 2021

I don't know what they say on CloudyNights but the classical definition of pinched optics is Fresnel pattern with a triangular-ish shape, i.e. a circle squashed on 3 sides (see H R Suiter's Star Testing Astronomical Telescopes, page 27, or get Aberrator and simulated the pattern for yourself). To me it looks like a classical tilted lens, same as mine in fact.

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Jérémie

Topic starter

Mar 22, 2021 · Edited: Mar 22, 2021

andrea tasselli:
I don't know what they say on CloudyNights but the classical definition of pinched optics is Fresnel pattern with a triangular-ish shape, i.e. a circle squashed on 3 sides (see H R Suiter's Star Testing Astronomical Telescopes, page 27, or get Aberrator and simulated the pattern for yourself). To me it looks like a classical tilted lens, same as mine in fact.

I agree on the definition, and when you look closely on the airy disks very close to focus (inside focus), they really seem squashed in a triangle : we can clearly see three points where the curvature is higher (radius of curvature lower), though it is not completely obvious. I attach the subset below, feel free to zoom. But indeed, farther away from focus, everything looks equally roundish.

Nevertheless, I am happy to hear it could just be tilt : how would you use the tilt ring in that case ? The brighter part is top right, so I should unscrew the pull screw and tighten the push one on this side, correct ?