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Old 07-10-2018, 08:05 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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What's up with my optics?

Amongst all the overcast weather we've had lately, I've had brief opportunity to try and solve a couple of issues I've been having. One was focus related and in doing this I've noticed some odd results.

First is what I believe to be astigmatism. The star shape is elongated one side of focus, and the elongation switches direction (90 degrees) on the other side of focus. I'd seen what appeared to be off-axis astigmatism on this in the past, and is one reason why I'm shooting with the QHY178M -- tiny sensor meant the problem was outside the FOV. This is different and covers the frame.

The second is some asymmetry in the size of the CO inside and outside of focus. After analysing the same star in an image either side of focus, I determined the % of the star shape (length, not area) that represented the CO:

Outside of focus:
X-axis: 45%
Y-axis: 39%

Inside of focus:
X-axis: 54%
Y-axis: 54%

I've attached a couple of gifs, one demonstrating what I believe to be astigmatism. The other I believe indicates under correction.

So I guess I'm after three main things in writing this post:

1. Confirmation of my interpretation -- am I seeing what I think I'm seeing?
2. Thoughts on the significance of these issues? No mirror is perfect and neither is the environment they're in. Are these tests sufficiently poor to cause concern?
3. Thoughts on causes / corrective measures

I've read that a mirror that is hot and cooling to ambient can cause over correction. I appear to have the opposite problem. My ROR isn't well insulated and I recently noticed that condensation was forming on the primary some mornings, so now I leave the primary mirror fan running 24/7 which has effectively resolved that issue. Maybe my mirror's actually colder than ambient due to this, thus giving me under correction rather than the typical over correction?

I've also recently noticed collimation shift over different altitudes. After acquiring a laser I see this stays perfectly in place as I move the scope (relative to the center spot on my primary), but viewing through a cheshire I can see it move by what appears to be a couple of mm between zenith and horizon. Seems to be due to primary moving around... can't win! If you restrict the movement, astigmatism, if you give it a tiny amount of space, collimation shift. Anyway, my reason for mentioning that is... is it possible the collimation shift interacts with the coma corrector (paracorr type 1) to do weird stuff, like the issues I've presented here? Or must this problem be in the mirror or its housing?
Attached Thumbnails
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Click for full-size image (correction.gif)
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Last edited by codemonkey; 07-10-2018 at 08:17 AM.
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Old 07-10-2018, 08:26 AM
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Atmos (Colin)
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From what Clive was mentioning in the other tread, it looks like astigmatism in your first one.

From what I’ve read, outside of simple doublet refractors, it’s not unusual to have different infra and extra focal images. The more complicated the optical design, the less likely that they’ll be similar.
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Old 07-10-2018, 08:58 AM
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peter_4059 (Peter)
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Have you tried the inside/outside focus test (second set of images) without the paracorr?
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Old 07-10-2018, 09:13 AM
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Quote:
Originally Posted by Atmos View Post
From what Clive was mentioning in the other tread, it looks like astigmatism in your first one.

From what I’ve read, outside of simple doublet refractors, it’s not unusual to have different infra and extra focal images. The more complicated the optical design, the less likely that they’ll be similar.
Cheers Colin. Yeah, I'm of the understanding that these things are never perfect, but I'm not sure what falls within the boundaries of acceptable and normal.

Right now the astigmatism means that unless my focus is dead on, I get elongated stars. I actually thought this was to do with the backlash-not-backlash dec problem I'm having until I captured these (5 second, 2xbinned) images.

I'm also suspicious that the CO issue is having a very slight impact on SGP's AF routine, causing it to get it ever so slightly wrong, thus leading to subs with elongated stars.

Even if AF was perfect so I saw no elongation, my understanding is that astigmatism causes softness in the images. I've been getting *lots* of really bad FWHM subs lately that I was putting down to bad seeing. Basically 3" + has become the norm, whilst it used to be typical for me to get some around 1.8" on the Esprit 120 most nights, with the average probably more like 2.2" . I'm basically wondering how much these optical issues are impacting my in-focus images. 10% increase in FWHM? 20%? 50%?

Quote:
Originally Posted by peter_4059 View Post
Have you tried the inside/outside focus test (second set of images) without the paracorr?
Cheers Peter. I haven't yet, no. If I did try that and it became more symmetrical, what would that suggest? Incorrect spacing? Poorly behaved paracorr?
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Old 07-10-2018, 09:59 AM
clive milne
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Lee,
the way to move forward is as follows:

Remove the coma corrector from the system.
The collimation is now more or less irrelevant as long as the optical
axis (the sweet spot) falls somewhere on the chip.

Do the test again (without the coma corrector) looking at how the stars behave through focus at the centre of the sweetspot.

If you still see astigmatism in the images, rotate the primary 90 degrees.
If the astigmatism flips its orientation then you have an issue with the primary. If it stays the same, then the issue lies with the secondary.

The above test assumes that the primary mirror cell is not inducing stress on the mirror blank.

btw) The test is more sensitive if done visually through an eyepiece (the CCD will integrate seeing/tube currents)
which the brain is pretty good at rejecting.
You don't need that high a magnification either. A 12mm eyepiece will do just fine.

best
~c

Last edited by clive milne; 07-10-2018 at 10:12 AM.
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Old 07-10-2018, 10:34 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Thanks Clive :-)

I'm pretty suspicious of the mirror cell to be honest; I've been thinking about replacing it. It's a pretty crappy 9-point system with cork pads and 120 degree edge support. They also glued the mirror clips so I can't adjust them vertically and one of them was touching the primary. I've since added some shims to increase the clearance to avoid that. I've also noticed at least once that one group of the rear mirror supports seemed to be putting a fair bit of pressure on the mirror, whilst the other two seemed to be flopping around and adding little to no support.

I also don't think the astigmatism has always been present, but I couldn't rule it out. I've definitely seen off axis sagittal astigmatism in the past but this looks different.

I'm also highly suspicious of possible thermal issues caused by the fan being on 24/7 I changed that recently and I'm pretty sure since then I've seen a lot of bloated stars. I'm just not sure if that's a coincidence or if it's causative. I don't really want to stop that as I'll then have condensation forming on the mirror. I did do out of focus star tests with the fan off and on in order to determine that there was no vibration issues and I couldn't detect a difference so I don't think there's vibration issues.

Unfortunately I don't do visual and don't have an eyepiece. I've had half a mind to buy one recently but now I could be looking at having to fork out for a new cell, or even a new mirror so I'm disinclined to splash out on an eyepiece at the moment.
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Old 07-10-2018, 10:37 AM
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Lee "Wormsy" Borsboom

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This is the primary cell, for reference: https://www.teleskop-express.de/shop...n-Spiegel.html
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Old 07-10-2018, 11:25 AM
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Shiraz (Ray)
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agree with Peter and Clive - take out the CC before you test anything. CCs introduce severe internal aberrations that can influence the image if the CC is not exactly on and aligned with the optical axis and with the right spacing.

If you still find astigmatism with the CC out, then the main optics are suspect (mainly the secondary mounting ?).

However, if the optics are OK, suggest putting the CC in and then laser collimating through it to get as close as possible to the proper CC alignment.

Re the fan, FWIW, I find that having a fan running all the time helps a lot with clearing the layer of warm air from the mirror surface and getting rid of tube currents. Used to do planetary imaging and it was fascinating to watch the resolution improve within a couple of seconds of switching on the fans. In rare perfect conditions, the fans seemed to introduce enough turbulence to cut the finest detail, but most of the time the resolution was definitely better with fans on. For DSO imaging it always seems better.

Cheers Ray

From https://www.telescope-optics.net/sub...htm#correction

"For complete correction of a paraboloid, more complex correctors are required. In general, they need to have three (or more) single lenses, or two or more achromatized doublets. The lenses are more widely separated, creating more degrees of freedom, so that combined aberrations can be brought to a negligible minimum (for instance, the Paracorr-like corrector corrects coma with the front achromat without inducing spherical aberration, but it does come at a cost of inducing enormous astigmatism, then corrected with the rear achromat - something that cannot be done without widening lens separation). Examples of this advanced corrector type are TeleVue's Paracorr and Wynne triplet."

Last edited by Shiraz; 07-10-2018 at 12:08 PM.
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Old 07-10-2018, 04:29 PM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Thanks for the info Ray!

I'll definitely try without the CC and in fact, collimation now sounds like the most likely culprit. Hopefully that's the case as it should be a relatively simple fix.

My main concern with the fan is that I leave it on day and night which isn't the norm... that and the fact that most of my recent troubles seem to have appeared since adding it.
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Old 18-10-2018, 07:44 PM
gb44 (Glenn)
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About the fan and condensation on the primary -- the alternative i used was a low wattage light close to the mirror cell set to be on during the risk period and that kept my 18inch mirror dew free.
Just that little heat solved the issue.

The fans I installed for cooling purposes were too effective - dropping the mirror temp and enabling dewing.

GlennB
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Old 26-10-2018, 09:33 PM
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Lee "Wormsy" Borsboom

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Thanks Glenn, that's good advice. I do want to get to the point of full automation which is one reason why I chose to go with a fan at the rear rather than a lightbulb as I could in theory leave the fan running all the time as it would help with cool down / boundary layer as well as keeping the primary dew free in the mornings.

Ok so I finally got a clear night tonight so I was able to do another quick test. I fully intended to do what all the smart people above suggested, but then I thought I'd try something quickly and then I got fixated on that instead.

I theorised that if the astigmatism was caused by misalignment of the CC / collimation, it would stay in the same orientation relative to the image if I rotated the camera. If it were the mirrors it would rotate with respect to the image.

I have a Moonlite Litecrawler with built-in motorised rotator so I can rotate very accurately while keeping everything else the same and I like tests where I can easily and accurately change just one variable.

I performed the rotation test close to zenith and closer to the horizon. The results were the same. When I rotated the camera, the astigmatism changed it's orientation with respect to the image, which makes me think it is one of my mirrors. Am I on the right track here?
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Old 31-10-2018, 07:37 AM
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codemonkey (Lee)
Lee "Wormsy" Borsboom

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Ok, found the problem.

First off I was starting to think I had bad spherical aberration based on the star test. Turns out I don't have any significant levels of SA, I just hadn't defocused enough. Apparently you should do that test at about 10 waves of defocus and mine was at about 4. Tests at an appropriate level of defocus show no signs of SA.

Aaaand I think I fixed the astigmatism. I tried to turn the primary in its cell and found it was jammed up on the rear supports. After releasing that and recollimating my stars are round and small. Still not entirely sure what happened there... maybe humidity and the cork pads? Good to have something working again anyway, was starting to lose what little hair I have left!

To top it all off, looks like we actually have some clear skies ahead. Even snuck in about 2.5hrs of data last night before the clouds rolled in.
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Old 31-10-2018, 09:15 AM
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Atmos (Colin)
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Glad you’re making some headway Lee
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