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Help with tilt or whatever this is

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#1 klaussius

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Posted 15 July 2020 - 03:12 PM

Hi.

 

For a while now I've been dealing with a confusing issue, and I'm a bit at a loss here.

 

My rig is what's in my sig. In short, an F5 150/750 newt with a CC that normally works very well all the way to the edge, a Canon EOS 650D, a 70/400 guide scope and a NEQ5 mount with dual axis drives.

 

A few times already I've had imaging sessions come out like this:

 

tilt_analysis.jpg

 

I normally don't notice that in the tiny screen I use to check subs while capturing. I should get me a bigger screen or some more extreme zooming capabilities. In any case, I'm lost as to what causes it.

 

Upon closer inspection, the issue is there in individual subs, so it's not a registration issue.

 

CCD inspector does report tilt, but I have not been able to correct it. I'm suspecting it might be something else that is being detected as tilt, or just tilt that I don't know how to correct. I need some help here.

 

ccd_inspector_sml.jpg

 

A few of my suspicions in case this is something else, and why I've "discarded" them:

 

* Collimation: A star test looks fine, and I've checked collimation frequently and found no major deviations. What really makes me think it's not collimation, is that some days it's fine, and some days it's not, without me ever changing the collimation. I can imagine it getting worse without intervention, but fixing itself on its own? Unlikely. One point about collimation I'll explore after exploring the other points though.

* Field rotation: This time I had PA down to 1-2 arcmin with PHD's drift align. I intentionally leave about of 1' of error because I have a lot of DEC backlash. Hard to be precise though because PHD's measurements were all over the place, there was some major shifting going on. Also unlikely because stars are not pointing to the guide star. My guide scope was pretty centered on the frame, it's fov encompases about 1/3rd of the imaging FOV.

* Differential flexure: I do have some. But, then again, this is visible in individual subs. Flexure shouldn't be substantial in a single sub right?

* Mount issues: The mount has been quite unstable lately, hence the above point about PHD having a hard time measuring drift. Guiding was at 1.8" total, really high, but the stars are round in center-right, and it's always like that. There are round stars in some corner, usually around center-right or bottom-right. If guiding was the issue it would be trails all around. Guiding may be to blame for the high HFD though.

* Tilt tilt tilt: My main suspect. The fact that it's not reproducible is actually common with fast systems. I don't consider my system fast at F5, but maybe it's fast enough to suffer this? I've been very careful lately to make sure everything is snug and flush. If this is tilt, I have no idea how to avoid it. Or how to verify that it actually is.

* Focus: Could focus drift manifest in this way? I think not because I do focus to my best ability with a bhatinov mask and it's present from the start (ie: just after focusing), but the stars in the corner do look out of focus rather than trailing, so perhaps?

 

The last thing re collimation, is that my focuser does cast a shadow on the defocused star test. Turns the donut into a kind of D shape. AFAIK, that would cause some excentricity, but not this elongation. Right?

 

I'm thinking this may be something that was always there, but I've just started noticing it because I'm getting better guiding than before.

 

If this is actual tilt, how does one turn what CCD inspector shows into corrective action? All my tests lately have turned random results, I get no correlation between what I do and what I get.

 



#2 GregsCNAccount

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Posted 15 July 2020 - 04:15 PM

You said CCD Inspector reports tilt.  Did it also indicate any collimation error?  I have been playing with doing my collimation using CCD Inspector's real-time multi-star collimation viewer lately with my 8" f/4 Newtonian and Explore Scientifics High Resolution Coma Corrector.  Prior to this I did the best collimation I could using a barlowed laser.  With the laser, CCD Inspector says it's usually close, 10-20 arcseconds.  CCD Inspector also reports tilt after the laser.  Using the real time multi-star collimation I have been able to get it to under 2 arcseconds.  Then CCD Inspector shows a nearly flat field with almost zero tilt.



#3 klaussius

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Posted 15 July 2020 - 04:25 PM

Actually I had no idea it had that capability. And I cannot find anything like that.

 

Maybe we're talking about different tools. When I say CCD inspector, I mean the tool ASTAP calls CCD inspector within ASTAP.

 

But if I read you right... could this be minute collimation errors? Minute enough to vary randomly session to session?



#4 GregsCNAccount

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Posted 15 July 2020 - 05:25 PM

No, I am talking about the CCD Inspector program put out by CCD Ware.  I am afraid I am completely unfamiliar with ASTAP.

 

Collimation is very important and the faster the scope the better it has to be.  Since an out of focus star on your scope has an odd shape due to protrusion of the focuser into the light path, judging collimation from an out of focus star may be compromised.  That's the beauty of the CCD Inspector's ability to report collimation while in focus, obstructions don't come into play.  Plus I doubt that the eye observing a nicely symmetrical  out of focus star can judge collimation errors down in the 1-2 arcsecond range, especially if the seeing is not perfect.

 

Do you leave the imaging setup untouched between sessions?  Unless you have 100% threaded connections in the optical path, including connection to the focuser, and have a beefy focuser, things can shift very slightly.  Do you put a cover over the rig?  The cover could put slight pressure on the imaging train and cause it to shift slightly.  I didn't change anything from the night I got to less than 2 arcseconds, but the next night it showed 8-9 arcseconds and had to be tweaked.



#5 klaussius

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Posted 15 July 2020 - 06:20 PM

No, I am talking about the CCD Inspector program put out by CCD Ware.  I am afraid I am completely unfamiliar with ASTAP.

 

Collimation is very important and the faster the scope the better it has to be.  Since an out of focus star on your scope has an odd shape due to protrusion of the focuser into the light path, judging collimation from an out of focus star may be compromised.  That's the beauty of the CCD Inspector's ability to report collimation while in focus, obstructions don't come into play.  Plus I doubt that the eye observing a nicely symmetrical  out of focus star can judge collimation errors down in the 1-2 arcsecond range, especially if the seeing is not perfect.

 

Do you leave the imaging setup untouched between sessions?  Unless you have 100% threaded connections in the optical path, including connection to the focuser, and have a beefy focuser, things can shift very slightly.  Do you put a cover over the rig?  The cover could put slight pressure on the imaging train and cause it to shift slightly.  I didn't change anything from the night I got to less than 2 arcseconds, but the next night it showed 8-9 arcseconds and had to be tweaked.

I assemble and disassemble everything each session. Focuser to CC isn't threaded, and may be the source of tilt. I've been careful to make sure it's snug when I image, but maybe that's not enough.



#6 klaussius

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Posted 18 July 2020 - 10:03 PM

I re-checked collimation, made sure everything including the secondary was properly collimated (to my ability), and went out to test it.

 

I did make changes, because I rechecked everything, but I don't think it was far off initially. I think the secondary was a bit off though.

 

It's cloudy out so I had to shoot test subs whenever I could spot a hole in the clouds.

 

The defocused star looks fine enough:

 

defocused_star_test.jpg

 

The stars on those test subs look fine and round enough.

 

ASTAP's CCD Inspector tool reports very little tilt on some, but still a lot in others:

 

IMG_5565_inspector.jpg

 

IMG_5577_inspector.jpg

 

Maybe ASTAP's CCD inspector isn't such a precise tool?

 



#7 klaussius

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Posted 01 August 2020 - 10:57 AM

An update for those that may be suffering from similar issues.

 

I think collimation is back to the top of the list.

 

After collimating the best I could, I mounted the OTA and re-checked, and suddenly saw collimation was a way off immediately after mounting. It seems something shifted as I was mounting the OTA.

 

I think my primary moves in its cell when changing altitude (ie: when pointing the scope higher). It could be the secondary, I'm not sure. Or the focuser, though it seems perfectly stiff. With this knowledge I started moving the OTA around while watching through the cheshire, and I witnessed the center spot on the primary shift as I changed altitude. I suspect the primary since it's not gradual, but rather a sudden shift.

 

In any case, that shift is likely to affect collimation, and it makes sense then that I would collimate perfectly and still get "tilt", since I'm used to collimating the scope horizontally, but most capture is done more vertically than horizontally.

 

I tried collimating the scope at 45° inclination and I still got the same tilt, but it does look like collimation may be off even if everything looks alright, as the primary moves in its cell.

 

Since this started happening one day, and before that I had never had issues like these, maybe something got lose. It would make total sense. I'll post an update if I manage to solve it. I never pulled an OTA apart so I'm not sure I know what I'm doing. I'll have to do some experiments to properly diagnose where the shift is coming, but it is substantial so I'm highly suspicious of it.


Edited by klaussius, 01 August 2020 - 10:59 AM.


#8 klaussius

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Posted Today, 10:52 AM

Alright, last night I was out testing a theory, and indeed it was the primary mirror shifting, but not due to anything lose.

 

TL;DR, it was collimation. Collimate your OTA vertically, so the weight of the primary keeps the springs loaded.

 

Long version is, even though I collimate often, the way I did it meant that as soon as I moved the OTA it would decollimate almost instantly.

 

It seems the springs holding the primary in place aren't strong enough, and in the orientation I usually collimate (with the OTA horizontal) it doesn't fully lock once I'm done. There's still a bit of wiggle room, and as soon as I change the OTA's orientation the mirror shifts. If I do the collimation with the OTA vertical, the weight of the mirror keeps the springs loaded and after locking everything remains very tight, and no shifting happens.

 

I tried the concept last night, I collimated a few days ago with the OTA vertical (it was very uncomfortable let me tell you, I have to find a way to do that comfortably). Checked it thoroughly for a shifting mirror, no shift that I could notice. Stored it, took it out yesterday, still in good collimation. Took it outside, shot a few targets, and today I'm processing the subs and they look alright. Pinpoint stars all over, even though my guiding was not very good last night.

 

So, it clearly was the primary mirror shifting due to the way I did collimation at home. I could change the springs in the mirror cell for stronger ones, but I don't feel comfortable disassembling the OTA just yet. I did notice the primary was quite dirty when inspecting it for mirror shift, so I will probably have to bite that bullet and disassemble if only to clean it. So... some other day.

 

Now... CCD inspector is still reporting tilt. I think it's either not very accurate, or it's detecting a really minute tilt that doesn't need to be addressed.

 

I ordered a laser collimator so I'll be able to collimate more accurately and see if it still reports tilt after that. We'll see.




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