What's wrong with my telescope?

[StuartT]

This is a Meade LX-90 (20cm SCT) and I recently collimated it. But it's still not right. Here is a defocused image to show what I mean.

The centre of the field looks ok, but at the edges I am getting 'D' shapes.

What could be the explanation?

Edited March 18, 2022 by StuartT

[michael8554]

I'd say that's due to coma, which is not unusual for an uncorrected SCT.

Michael

[900SL]

It's either that, or somebody has tipped a box of cheerio's in the tube

[RT65CB-SWL]

I think it still needs a little tweak looking at the centre ones. Though the bright one at eleven o'clock looks OK.

[StuartT]

55 minutes ago, michael8554 said:

I'd say that's due to coma, which is not unusual for an uncorrected SCT.

Michael

should it be that severe though?

It is an old scope, I guess. About 2004

[ollypenrice]

11 hours ago, StuartT said:

should it be that severe though?

It is an old scope, I guess. About 2004

This won't make any difference. What you're seeing looks normal to me, too, and that's why the manual places so much importance on collimating with the star in the centre. If you have an EP with crosshair I'd use that for centering and re-centre between each adjustment. You're a bit off near the middle as well, as others have said. (The secondary shadow is slightly displaced to the upper left to my eye, and the best star shapes are slightly left of the image centre.)

Olly

[michael8554]

Putting collimation to one side for the moment, look at the star shapes of an in-focus image.

Progressively more teardrop-shaped stars toward the edges of the frame are a sign of coma, inherent in the basic SCT design.

Michael

Edited March 19, 2022 by michael8554

[StuartT]

Thanks for this. 

So is there any way to get rid of the coma?

[Stuart1971]

5 minutes ago, StuartT said:

Thanks for this. 

So is there any way to get rid of the coma?

Don’t worry about it, it’s perfectly normal, just get collimation bang on in the middle and you are good to go, this is inherent of uncorrected SCT’s, especially the older ones, I have an 8” 1997 model, and it’s the same…it’s due to the design of these scopes…👍🏼

[vlaiv]

19 minutes ago, StuartT said:

Thanks for this. 

So is there any way to get rid of the coma?

https://www.highpointscientific.com/starizona-sct-corrector-4-0-63x-reducer-and-coma-corrector-sctcorr-4-1

[Stuart1971]

2 minutes ago, vlaiv said:

https://www.highpointscientific.com/starizona-sct-corrector-4-0-63x-reducer-and-coma-corrector-sctcorr-4-1

And there are much cheaper versions, but they all reduce the imaging circle, and with todays large sensor cameras, it will not work very well, as the area that is currently suffering from coma, will just be vignetted out anyway….if that a word….😂😂

[Sunshine]

Am I understanding this "correctly" that this "corrector" doesn't actually "correct" anything, it masks out the more severe coma affected outer edge of the FOV correct?

correct me if I'm wrong, whoa! there are many "corrects" here. If i am correct, please refrain from answering with "correct" 🤣

Edited March 19, 2022 by Sunshine

[vlaiv]

21 minutes ago, Stuart1971 said:

And there are much cheaper versions, but they all reduce the imaging circle, and with todays large sensor cameras, it will not work very well, as the area that is currently suffering from coma, will just be vignetted out anyway….if that a word….😂😂

I don't have any experience with SCTs and their reducers / correctors.

Above one seems to correct up to APS-C size - not sure if claims are correct and how good it is.

Then there is this (I just ran into it due to bit of researching on what can be used for SCTs to correct the field):

https://www.teleskop-express.de/shop/product_info.php/info/p732_Baader-Alan-Gee-f-5-9-Telecompressor-Mark-II-for-Schmidt-Cassegrains.html

Quote

The Alan Gee has been designed upon recommendations by Roland Christen (Astro Physics) especially for Schmidt-Cassegrain telescopes. It increases the aperture ratio from f/10 to f/5.9 and reduces the necessary exposure times significantly. It also corrects the field curvature inherent to all SC telscopes and makes pinpoint stars up to the edge of the field.

However, here is interesting answer from Baader team:

Quote

And we must admit - this product was designed 30 years ago - when people used emulsion film and wondered how to evacuate film layers prior to every exposure...
With modern CMOS-cameras we see the end of this product coming - since the original SC-optical design does not support airy disc sizes smaller than ~ 8 micron. So no SC-compressoer has a chance to live up to the tiny pixel sizes.
For this reason Celestron had redesigned the SC to incorporate a matched corrector, situated within the optical train at a position we cannot attain with the Alan Gee. So whatever distance you choose - modern imaging devices will mercilessly exhibit the limitations of the system as a whole.

It seems that no SCT corrector will do the right job (with exception of that Starizona one that is expensive) and that EdgeHD is the way to go for good field definition if one wants to use SCT.

[vlaiv]

5 minutes ago, Sunshine said:

Am I understanding this "correctly" that this "corrector" doesn't actually "correct" anything, it masks out the more severe coma affected outer edge of the FOV correct?

correct me if I'm wrong, whoa! there are many "corrects" here. If i am correct, please refrain from answering with "correct" 🤣

They actually correct things. They correct for coma and field curvature.

Here are couple of spot diagrams of Starizona corrector vs no corrector and Starizona vs regular one (Celestron F/6.3 reducer):

It might not give pin point stars but at least they are round up to say 27mm diameter (which is APS-C size +/- a mm).

 

[Adam J]

20 hours ago, StuartT said:

This is a Meade LX-90 (20cm SCT) and I recently collimated it. But it's still not right. Here is a defocused image to show what I mean.

The centre of the field looks ok, but at the edges I am getting 'D' shapes.

What could be the explanation?

Its over corrected coma, but just because you can see it out of focus doesnt mean its a problem in focus. But am also seeing that you have a slight collimation error. 

Adam

Edited March 19, 2022 by Adam J

[ollypenrice]

1 hour ago, Sunshine said:

Am I understanding this "correctly" that this "corrector" doesn't actually "correct" anything, it masks out the more severe coma affected outer edge of the FOV correct?

correct me if I'm wrong, whoa! there are many "corrects" here. If i am correct, please refrain from answering with "correct" 🤣

As Vlaiv says, it does correct and really quite well, though not perfectly. It's worth reading about its highly eccentric inventor.  https://en.wikipedia.org/wiki/Bernhard_Schmidt

Olly

[Zermelo]

On 19/03/2022 at 16:45, ollypenrice said:

its highly eccentric inventor.  https://en.wikipedia.org/wiki/Bernhard_Schmidt

I never knew he was Estonian. Though Wikipedia somehow omits him from its long list of notable Estonians.

Hey, the Struves were also Estonian!

[Peter Drew]

I don't think it is anything to do with coma, I believe it is due to vignetting of the outer field by the internal shade tube, normal for this design.  The main purpose of the front corrector element is to introduce spherical aberration of equal power but opposite sign of that of the spherical primary thereby cancelling it out.  Although it is essential it is of relatively low optical leverage and has no practical focal plane so introduces negligible chromatic aberration.    🙂

[vlaiv]

44 minutes ago, Peter Drew said:

I don't think it is anything to do with coma, I believe it is due to vignetting of the outer field by the internal shade tube, normal for this design.  The main purpose of the front corrector element is to introduce spherical aberration of equal power but opposite sign of that of the spherical primary thereby cancelling it out.  Although it is essential it is of relatively low optical leverage and has no practical focal plane so introduces negligible chromatic aberration.    🙂

Why do you think that?

Here is simulation of defocus + coma versus actual image:

Actual image shows a bit astigmatism as well which I did not simulate (that is why outer stars have "almond" shape - because combination of coma, astigmatism and defocus).

 

[Peter Drew]

My belief is based on all baffle tubed Schmidt-Cassegrain and Maksutov-Cassegrain telescopes that I have used display this effect visually with defocused stars that are off- axis.  Supporting the advice to make sure that a star is centered during collimation procedures.    🙂

[vlaiv]

17 minutes ago, Peter Drew said:

My belief is based on all baffle tubed Schmidt-Cassegrain and Maksutov-Cassegrain telescopes that I have used display this effect visually with defocused stars that are off- axis.  Supporting the advice to make sure that a star is centered during collimation procedures.    🙂

I see why you might think that, but consider this - which one is original aperture then:

this:

or this one:

In either case - central dot is strangely displaced from original aperture - and that happens with coma, so actual effect is combination of coma and astigmatism in SCT rather than shadow of baffle tube (which would show as decrease of brightness rather than blockage as defocused pattern is image of entrance aperture - everything away from entrance pupil will be "out of focus").

[paulastro]

It's the reason I use refractors. 

[Peter Drew]

Interesting.  To be honest, looking back at the OP image and subsequent ones none of them does exactly show the effect I was describing.  It would be interesting to know the field of view of the OP image as it appears to be larger than expected for visual.  I have several catadioptric telescopes and due to their long focal lengths and small visual fields I've never noticed objectionable coma.  Next opportunity I get I'll try an 8" version for a recap.    🙂

[vlaiv]

19 minutes ago, Peter Drew said:

Interesting.  To be honest, looking back at the OP image and subsequent ones none of them does exactly show the effect I was describing.  It would be interesting to know the field of view of the OP image as it appears to be larger than expected for visual.  I have several catadioptric telescopes and due to their long focal lengths and small visual fields I've never noticed objectionable coma.  Next opportunity I get I'll try an 8" version for a recap.    🙂

From the list of equipment in signature - image is most likely from APS-C sized sensor - that is ~28mm diagonal, so any coma on the edges would be the same as at the edge of 32mm eyepiece (which has field stop of 27mm).

Here is level of coma in 8" SCT:

0.25 degrees at 2000mm of focal length equals to 8.72mm radius (while above would be at ~14mm radius - almost as twice far away).

Problem is that 32mm plossl won't magnify image sufficiently to properly resolve coma at that level. If we look at spot diagram - coma is 4-5 times larger than airy disk size and airy disk size for 8" scope is 1.28".

32mm eyepiece will give x62.5 magnification and 1.28" magnified x62.5 is 80 arc seconds or 1.33 arc minutes. Limit of resolution of human eye is about 1 arc minute - so coma size is going to be barely resolved at 6-7 arc minutes - and certainly not very apparent.

In images on the other hand it will be visible as long focal length often means high sampling rate - like 1"/px or less with SCTs - and coma will here be at least 8-10px in size.

 

[StuartT]

On 29/03/2022 at 18:11, vlaiv said:

From the list of equipment in signature - image is most likely from APS-C sized sensor

 

Yes, you are correct @vlaiv
My Starizona SCT corrector has now arrived and I located a 90mm extension tube, so I hope to try this out soon. I'll let you know how I get on!