Finally figured it out...I think...lol

[markse68]

I've been fighting non roundness of stars with my setup for ages and I think I may have finally figured it out. I was really frustrated as I wasted 2 nights last week in really good clear conditions but my stars were horribly elongated particularly in B and G which looked like they were double stars so I've spent a bit of time experimenting tonight.

I think it's a focus issue! I have no idea why the focus should shift the stars in a linear fashion and right across the frame width, but it seems on one side of focus they go into weird shield shapes and finally horseshoe shapes, but on the other they elongate in a straight line eventually separating into 2!

I've been pretty thorough with my collimation so I don't think it can be that and I've checked my focuser is square to the ota so I don't think it is that. I had been focussing in the R channel as in R the stars are more bloated so I figured get them as small as possible in R. But very slight defocus in G and B caused this elongation. I have now focussed in G and the star shapes are much better.

Maybe it's some weirdness with my coma corrector- a Baader RCC1 but I've not read of such an issue (though tbh there's very little written about this corrector at all!)

I was hoping I'd get away without a robotic focuser but it looks like I really do need one even though the shift is tiny. But I will focus in G from now on anyway.

Any idea why my scope should do this though? It sounds like a misalignment but I'm pretty sure everything is aligned pretty well. Scope is an 8" f4.5 newt.

Mark

[skybadger]

That needs some photos please. 

[Stu]

@markse68 that sounds like it could be some form of astigmatism which could be caused by a number of things; primary mirror figure or how it is being held, or similarly with the secondary.

I’m sure others will comment as to whether I’m on the right track with that and if so, how to diagnose and remedy it.

This may help check it out…

http://www.loptics.com/articles/starshape/starshape.html

[markse68]

Yes I think you're right Stu and i think i’ve known that for a while and gone into denial not wanting to admit it- all i found was a work around that minimises the issue

Mark

[Stu]

3 minutes ago, markse68 said:

Yes I think you're right Stu and i think i’ve known that for a while and gone into denial not wanting to admit it- all i found was a work around that minimises the issue

Mark

😞😞

Well, I guess the scope isn’t going to make you happy unless sorted so hopefully you can find a solution.

Which make/model of scope is it? There are various tests you can do to check out where it is coming from, so hopefully it is something like how the primary is supported or how the secondary is held, rather than the actual figure/flatness of either.

Just trying to think, perhaps @Piero @Peter Drewor @Astrobits can be of assistance?

[markse68]

It’s a frankenscope Stu- it has what i believe to be a Vixen secondary and an Orion primary in Orion cell. 

for ages I’ve been blaming the mount but it’s the same with 10sec and 300sec exposures so i don’t think it can be a tracking issue.

Mark

Edited September 27, 2021 by markse68

[markse68]

Next clear night i’ll rotate the primary 90 deg and recollimate to rule it out. It’s probably the secondary like it was with my fullerscope.

Mark

[Piero]

Hi @markse68 , 

To me what you described seems like astigmatism mainly (elongated stars), but also some spherical aberration (bloated stars).

Astigmatism does not come from miscollimation of the primary mirror unless this is severely miscollimated, which is not the case here. A focuser axial misalignment causes the stars not to be on focus at the same time and / or unbalanced field illumination (for instance if the secondary tilt is very off).

 

The photo of your secondary mirror does not look good to me. It seems like it has been cemented to its support. That alone can cause astigmatism and this astigmatism can become even more severe as the temperature drops. Being the surface not flat, the rays can reach focus at different distances and for a star, at different positions, likely causing some spherical aberration.

At this point, I would focus on the astigmatism, discarding the chromatic aberrations that you saw.

I have some other questions: 

1. Could you confirm that stars elongate when defocused on one side, and broadly elongate at 90 degrees when defocused on the other side? (basically they have a kind of "X" pattern)
2. Does a bright star show astigmatism when placed on-axis, using an eyepiece? 
3. Same question, without Baader RCC1
4. Could you take some photos of the primary mirror cell with the mirror please? In particular, I would like to know 1) if you can say whether the mirror can rotate on it or this rotation is somehow constrained due to the 3 mirror edge supports, 2) if the clips above the 3 mirror edge supports (the ones preventing the mirror from flipping forward) somehow touch the primary mirror
5. do you think the secondary mirror can be detached from the holder? A dental floss can be used for detaching it if it is glued using silicon, but not if it is glued with contact cement (I think). 

Edited September 27, 2021 by Piero

[Piero]

In any case, it is good to be critical, Mark. Denying a problem certainly does not help and only adds frustrations. Any mounting issue can be fixed with not too much effort / cost.  

[markse68]

Hi Pierro, thanks for your input and yes you’re right- I won’t be happy till i get to the bottom of this- it’s very very frustrating!

To answer your questions in order-

1: no it’s not classical astigmatism- the stars elongate until they split into a double image on one side of focus and on the other they irregularly bloat and turn into comatic horseshoe shapes. I wonder if this is coma corrector weirdness though?

regarding the bloat- i only noticed that the red/L channel has larger stars than the other colours- the other colours just distort weirdly.

2: I’ve never looked through this scope 😳 I haven’t got adapters to get an ep in focus. But this “astigmatism” seems to be even across the whole frame

3: I was trying to test without rcc1 last night but the clouds rolled in. I think the same thing happens but i can’t use oag live view to rack focus in and out because of severe coma and using main camera is painfully slow and tedious (30secs per frame!)

4: I have adjusted the cell very carefully so that it doesn’t clamp the mirror anywhere and allows very slight movement but that is minimised to prevent collimation shift when moving the scope. I’ll see if i can find more pics. It’s quite a nice adjustable cell but relies on supporting mirror in several m6 nylon screw tips but it’s only 8” mirror so should be ok?

5: I did this already  It was glued hard with silicone and no way to get in there with floss. I ended up soaking it in acetone overnight which weakened the silicon and was eventually able to pry the secondary out with minimal damage. it’s now supported on 3 small pads of 3m uhb tape.  You can see it here: 

Thanks for your help Pierro,

Mark

edit- found more pics of primary cell:

Edited September 27, 2021 by markse68

[Peter Drew]

I'm happy to agree with everything that @Piero says as it saves me a lot of typing.  Production methods these days ensure that native astigmatism in optics is rare, most observed cases are mechanically induced.  An easy tests is to rotate the primary, which should be free to do so, this would show if the astigmatism is in either component,  I also think the secondary attachment is suspect.    🙂  

[Peter Drew]

Your last post arrived just as I was sending mine which mainly negates it.  Still worth rotating the primary to eliminate the culprit before looking further along the system for a solution.   🙂

 

 

[markse68]

2 minutes ago, Peter Drew said:

Your last post arrived just as I was sending mine which mainly negates it.  Still worth rotating the primary to eliminate the culprit before looking further along the system for a solution.   🙂

 

 

yes i should have done that ages ago 🤦‍♂️

Next clear night is the plan 👍

Mark

[markse68]

3 hours ago, skybadger said:

That needs some photos please. 

Sorry missed your post Skybadger- I’ll try to find some representative subs up later 👍

Mark

[Stu]

54 minutes ago, markse68 said:

yes i should have done that ages ago 🤦‍♂️

Next clear night is the plan 👍

Mark

Checking your thread on changing the way the secondary was mounted Mark, did that make any difference at all? I assume not given this thread…..?

[markse68]

At the time i thought it had Stu but now i think that was just wishful thinking.

The trouble is this seems to be very focus sensitive and if i luck out and get the focus just right then it can appear minimal but a few degrees of the fine focus knob the wrong way and it becomes very evident. At least that’s what i think is happening. And the filters although described as parfocal, aren’t perfectly (and i think they can’t be for all combinations of optics, ccs etc)

Mark

 

[Piero]

2 hours ago, markse68 said:

Hi Pierro, thanks for your input and yes you’re right- I won’t be happy till i get to the bottom of this- it’s very very frustrating!

To answer your questions in order-

1: no it’s not classical astigmatism- the stars elongate until they split into a double image on one side of focus and on the other they irregularly bloat and turn into comatic horseshoe shapes. I wonder if this is coma corrector weirdness though?

regarding the bloat- i only noticed that the red/L channel has larger stars than the other colours- the other colours just distort weirdly.

2: I’ve never looked through this scope 😳 I haven’t got adapters to get an ep in focus. But this “astigmatism” seems to be even across the whole frame

3: I was trying to test without rcc1 last night but the clouds rolled in. I think the same thing happens but i can’t use oag live view to rack focus in and out because of severe coma and using main camera is painfully slow and tedious (30secs per frame!)

4: I have adjusted the cell very carefully so that it doesn’t clamp the mirror anywhere and allows very slight movement but that is minimised to prevent collimation shift when moving the scope. I’ll see if i can find more pics. It’s quite a nice adjustable cell but relies on supporting mirror in several m6 nylon screw tips but it’s only 8” mirror so should be ok?

5: I did this already  It was glued hard with silicone and no way to get in there with floss. I ended up soaking it in acetone overnight which weakened the silicon and was eventually able to pry the secondary out with minimal damage. it’s now supported on 3 small pads of 3m uhb tape.  You can see it here: 

Thanks for your help Pierro,

Mark

edit- found more pics of primary cell:

 

 

Thanks for your reply, Mark. I have some other critical questions / checks. These look like small details but they are not at all and can have a lot of impact.

Primary mirror

Your mirror is supported by 9 points. Could you check that all points are supporting the mirror? I have never played with an Orion Optics mirror cell, but I would believe that the 3 isosceles triangles (1 triangle => 3 points) can float (not rotate!) a bit. Could you confirm this, please? You can run these checks by moving the mirror on its cell and check that no triangle is stuck for some reason. 

Secondly, from your photos, the nylon pads on the edge supports seem to touch the mirror tightly. They should not press onto the mirror, only prevent a lateral shift. Also, could you make sure that the mirror can somehow "slide" vertically and that those nylon pads do not block this movement? It is important that the mirror (when the cell is inclined) does not sag on its lateral supports and it can still move vertically along the mirror cell axis. If you feel that this movement is hard to achieve, then very slightly loosen those nylon screws mounted on the edge supports, so that the mirror still does not shift laterally, but it can have some movement. Those pads should not cause any friction. 

Secondary mirror

I saw your other post. To be honest, I don't know whether that secondary mirror is sound. It also seems cracked at the back, as if it fell at some point. There is a chance that it is not flat.

Does the holder touch the secondary laterally? Usually, when gluing the secondary to its support, three blobs of silicon are applied at about half way from the centre and the edge (in your case you glued it at the very edge because your holder has a hole at the centre. Also the blobs are not really thin. This is important because with the decrease in temperature, the glass contracts and the silicon must not warp the secondary figure.  If too thin, there is also a risk that this warp occurs as the silicon dries.

 

 

 

>>> 1. "no it’s not classical astigmatism- the stars elongate until they split into a double image on one side of focus and on the other they irregularly bloat and turn into comatic horseshoe shapes."

That's not a surprise, you have a combination of aberrations, not just astigmatism. The problem is to break them down. You certainly have some astigmatism in there though, and as it has major effects, it is better to fix this one first.

 

>>> 2: I’ve never looked through this scope 😳 I haven’t got adapters to get an ep in focus. But this “astigmatism” seems to be even across the whole frame

>>> 3: I was trying to test without rcc1 last night but the clouds rolled in. I think the same thing happens but i can’t use oag live view to rack focus in and out because of severe coma and using main camera is painfully slow and tedious (30secs per frame!)

^^^ testing on axis is critical. You can discard the rest of the field as this can contain additional aberration (e.g. quality of the CC, coma, eyepiece edge correction, and so on). Testing on axis is where you need to focus. If you have collimated your telescope properly, you should not see coma on axis (although the coma-free region is quite small given the f/ratio of your telescope). 

 

 

Other test to run when you take the telescope out: 

a) all my posts here assume that the optics have cooled down properly. If not, you are going to have an additional level of astigmatism due to the mirrors cooling down.

b) Record the orientation of the on-axis star elongation upon changes in telescope altitude ONLY. In particular, you need to record whether these elongations remain somehow consistent with the OTA axis or they change orientation depending on the altitude. See sketch below. This test can disentangle whether the astigmatism comes from the secondary only or the primary only. Forget the horseshoe, just test with stars defocused on the side where they become elongated. That's sufficient.

c) you would really need an eyepiece for these tests.. Is there a chance you can make a temporary extension tube to reach focus?

 

Test checking astigmatism axial orientation: 

Edited September 27, 2021 by Piero

[markse68]

Hi Pierro, thanks for all the suggestions- we are forecast another unforecast clear night so I think I'm going to be quite busy!

To answer your first questions-

Primary Mirror- the side screws aren't tight against the mirror, it just looks that way as the mirror had slid up against the one in the picture. I have them adjusted to just clear the sides so indeed the mirror can be lifted till it hits the top clips and falls back under its own weight. I'd guess about a thickness of paper clearance. Maybe I should back them out a little more though as you can't rotate the mirror without it binding as it's not precisely circular. The triangles are free I think but I will double check when I remove the cell later to rotate the mirror for further tests.

Secondary Mirror- I haven't glued it with silicone- that was how it was originally fixed. I stuck it with small pieces of foam double sided tape that should have enough give in them to prevent stressing the mirror. The silicone that was holding it was applied large and thin so it wasn't ideal for stress free support! The damage you see was caused by me extracting it from the holder but I did this trying to solve the issue which existed before I did this so I don't think I have caused it. It's not cracked but the thin rear edge chipped off. The mirror might touch the holder but only glancingly- it's not a tight fit. I do suspect the secondary though- testing with the primary rotated 90 should give some insight on that

I remembered that I do have a 2" barlow that I could unscrew the element from and use as an extension to use an eyepiece so I'll try that too- would be much more convenient than waiting for the camera. I'll do as you suggest with the altitude testing, and the primary rotation of course

Thanks Pierro,

Mark

 

 

[markse68]

So I’ve spent a few hours investigating this and as suspected I think, i need a new secondary. I wasted too much time using the camera to diagnose it but as you rightly pushed me towards Pierro, an eyepiece instantly resolved the issue. The only reason i had never looked through this scope was i tried initially and the focal plane was so far out that i had no way of securing an eyepiece. I should have bought something way back then and i’d have seen the problem straight away. 🤦‍♂️

I used my Big Barlow having taken the lens out- It doesn’t half look funny though and very uncomfortable viewing 😂

I confirmed it by rotating the cell 120 deg and back again, collimating each time and the astigmatism always aligns with the ota- ie the long axis of the secondary. So it can’t be the primary- phew! And it does look like classic astigmatism with the defocussing either side forming a 90 deg cross as it breaks from the airy disk. And scope’s been out plenty long enough for it not to be thermal issues i think.

Thanks for all the input folks and particularly Pierro for pushing me in the right direction

Disappointed but relieved to finally know what’s wrong. I feel a new project coming on- I don’t like that secondary holder 😉

Mark

 

[Piero]

9 hours ago, markse68 said:

So I’ve spent a few hours investigating this and as suspected I think, i need a new secondary. I wasted too much time using the camera to diagnose it but as you rightly pushed me towards Pierro, an eyepiece instantly resolved the issue. The only reason i had never looked through this scope was i tried initially and the focal plane was so far out that i had no way of securing an eyepiece. I should have bought something way back then and i’d have seen the problem straight away. 🤦‍♂️

I used my Big Barlow having taken the lens out- It doesn’t half look funny though and very uncomfortable viewing 😂

I confirmed it by rotating the cell 120 deg and back again, collimating each time and the astigmatism always aligns with the ota- ie the long axis of the secondary. So it can’t be the primary- phew! And it does look like classic astigmatism with the defocussing either side forming a 90 deg cross as it breaks from the airy disk. And scope’s been out plenty long enough for it not to be thermal issues i think.

Thanks for all the input folks and particularly Pierro for pushing me in the right direction

Disappointed but relieved to finally know what’s wrong. I feel a new project coming on- I don’t like that secondary holder 😉

Mark

 

 

That's very good, Mark. You tested this very well   It's kind of shocking that, if you do know what / how to test, a half decent night is all what it takes. 

 

What secondary do you need? I do have a 3.1" Antares secondary + Astrosystems holder on sell in the classified section and abs, but that is for a 12" f4 - 15" f5 for visual. I have no idea what it is recommended for imaging with your telescope, but my suspect is that you need something smaller, maybe 2.4" at most. 

[markse68]

Morning Pierro

I have awoken with a new resolve and real feeling of relief! And i’m really kicking myself for not looking through my scope when i first got it- would have saved so much time and frustration!

Your secondary would have been perfect thank you but alas the one that’s in there is 60mm. But i will treat it to a finest quality secondary- either Antares or Protostar. I put a Protostar into my Fullerscope and it totally transformed that scope into a planet killing double star dissecting precision instrument  I think that the secondary is just as important as the primary- maybe more so! The whole image area is crammed into a much smaller space and any defect will destroy it!

Thanks Pierro,

Mark

Edited September 28, 2021 by markse68

[Stu]

17 minutes ago, markse68 said:

Morning Pierro

I have awoken with a new resolve and real feeling of relief! And i’m really kicking myself for not looking through my scope when i first got it- would have saved so much time and frustration!

Your secondary would have been perfect thank you but alas the one that’s in there is 60mm. But i will treat it to a finest quality secondary- either Antares or Protostar. I put a Protostar into my Fullerscope and it totally transformed that scope into a planet killing double star dissecting precision instrument  I think that the secondary is just as important as the primary- maybe more so! The whole image area is crammed into a much smaller space and any defect will destroy it!

Thanks Pierro,

Mark

Excellent stuff Mark, I look forward to hearing about the scope with the new secondary 👍👍

[Piero]

26 minutes ago, markse68 said:

Morning Pierro

I have awoken with a new resolve and real feeling of relief! And i’m really kicking myself for not looking through my scope when i first got it- would have saved so much time and frustration!

Your secondary would have been perfect thank you but alas the one that’s in there is 60mm. But i will treat it to a finest quality secondary- either Antares or Protostar. I put a Protostar into my Fullerscope and it totally transformed that scope into a planet killing double star dissecting precision instrument  I think that the secondary is just as important as the primary- maybe more so! The whole image area is crammed into a much smaller space and any defect will destroy it!

Thanks Pierro,

Mark

It happens to spend some time when things do not work. Yeah, it is frustrating, but it's part of the learning curve. 

I spent about 6 months to nail down the issues of the mirror cell of my 12" f6 - which was bought new by me 😕 . It gave me a very hard time and frustrations, particularly considering that all those issues were caused by design errors of the telescope maker. Said this, at least they are fixed now and the telescope is functional.

Then, when you make your own telescope (for me, it's the one in my profile photo), you design it so that all these issues cannot be present..  

 

60mm or ~2.4" - I suspected correctly then! Lol! No problem, Mark.  

Edited September 28, 2021 by Piero

[Astrobits]

Glad you're more or less sorted. I've been too busy the last few days to pick up on this thread but Piero has helped you.

As soon as I saw the secondary mounted that way I knew it would be a ( the) problem.

Nigel

 

[CraigT82]

Good to see the detective work is paying off!