For some time now during taking long exposures > 30 Secs for Deep Sky Objects , I get egg shaped stars and always wondered if this was an issue with Polar Alignment or Auto Guiding. Am using my ZWO AM5N mount with a Askar SQA 55 Refractor fitted with a 60 mm Guide Scope and ASI 120 MC as a Guide camera Last week I got a total Polar alignment error between 3-4 arc min using ASI Air All Sky Alignment method as Polaris is not directly visible to me. This was considered acceptable after checking with various Online forums. Was also able to get near perfect Auto Guiding ( as per enclosed image) and yet was not able to get perfect Stars for 30 sec and beyond exposures Do I conclude that the Polar Alignment has to improve to under 1 arc min for achieving round stars for long exposures. Any suggestions welcome  |
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Not really. Besides the accuracy of the graph which I would dispute, if your PA is around 3 arcmin guiding isn't really affected at all. Something else is at play, one possible culprit is how solid is the guidescope attached to the scope which is n.1 issue with guidescopes in general.
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The Guide scope is firmly mounted on the Main Telescope and does not have any play for any movement. ( See attached image)  |
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Simple way to test this (I don't think it's as robust as it should be):
Point the scope at the zenith and take a short exposure without guiding, say 10s, with both scopes at the same time. Then point the scope at the meridian @ 30 deg altitude and take the same shots. Using PI or something similar align the shots (use as reference the one with highest resolution/image scale) first at the zenith and copy or note down the transformation matrix and then repeat the same with the one at the meridian. If the system is truly rigid the two transformation matrices are going to be identical (or close enough within the error margin). The degree with which they diverge is an indication of the degree of flexibility of the system.
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Hi Aloke. I recomend you to check the focal lenght of the guidescope, those guiding numbers are probably not real, or you are guiding on a dead pixel. I don't like the way your telescope and guidescope are attached. It may feel that is firmly attached, but they are probably flexing. After that, harmonic mounts need shorter exposures, maybe 1 second at maximum.
Kind regards and good luck!
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Polar alignment issues will generally show up as image rotation and it will become worse the closer you are to the pole. The four things that can cause elongated stars are:
1) Mechanical flexure between the guide system and the imaging system (as Andrea has pointed out.)
2) Guiding errors that can be caused by poor guider calibration, warm pixels, drive train errors, and a bunch of other stuff.
3) Mechanical vibration, which can be caused by poor stability where the scope is set up (such as on a balcony), fan induced vibration, focus flop, and other things that cause movement of the image on the sensor.
4) Astigmatic optical errors due to optical misalignment, defocus in an astigmatic system, or field astigmatism.
The biggest mechanical problem that I immediately see in your rig are the ring supports for your guide scope. Rings are notoriously unstable and that is very likely a big part of your problem. You must have a VERY mechanically ridged connection between your guide-scope and the imaging scope so get rid of those rings and figure out how to firmly bolt that guide-scope directly to the imaging scope. There is no need for angle adjustment. The important thing is rigidity.
Fix that first and then let us know what you get. My guess is that you’ll immediately get better results. Then we’ll work through the secondary problems that you might uncover next.
John
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Thanks all for your suggestions. Am going to try again tonight and see if I can improve based on some of these suggestions. Appreciate your help !!
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I'm extremely confident that the guiding graph/statistics you're showing are not accurate. The way you have your guide scope attached also has many points of flexure. The scope supports on those types of rings flex a lot. The rings themselves are mounted with only one point of contact with the dovetail, which will flex. The dovetail is mounted to a handle in a cantilever manner which will flex. The attachment point between the handle and the scope is small and will flex/slide with the expansion/contraction of the metal and the only thing holding it being a hand tightened wingnut. The whole lot is probably flexing relative to the mount, also, since the scope is attached to the saddle in just one spot with a small shoe.
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I am 99,9% sure that the focal length and/or pixel scale of your guide scope and guide camera are not set correctly in the ASIAir software.
No mount can guide at the RMS you are showing. It is using incorrect numbers as the basis.
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I have to question your guiding graph for starters, I wonder if you've got the guide scope focal length set up correctly, but I can definately guarantee that your mount is not capable of 0.07" RMS guiding numbers.
If your guide scope focal length is set up incorrectly, you will get RMS correction values that are incorrect, and you could indeed have considerably worse guiding than you are being lead to believe.
Secondly, if you're getting star trailing in long (lets say, 2~10 minute exposures) with a sub 5 arcminute polar alignment, then something else is going horribly wrong… Especially with a 264mm focal length… Honestly with a 264mm focal length and a sub arcminute polar alignment, a 3 minute unguided sub should be perfectly fine or very close to.
I would 100% be aiming for a 1 arcmin or better polar alignment, but I can guarantee than anything under 5 arcmin is perfectly good enough for a setup like yours. Any star trailing you're seeing is going to be on account of flexture in the system somewhere. This could be in the guide scopes focuser, the guide scopes adjustment rings, the guide scopes mount to the imaging scope and so on.
Go through all the fixutre screws and make sure they are tight..
Another thing that CAN be an issue, is if your guide scope's axis is not relatively parallel to the imaging scopes axis, that can certainly negatively affect your results. Where possible, you want to be guiding on a star inside the imaging scopes field of view, or just outside of it.
Essentially, what you're experiencing here is why so many people move to an off-axis guider, and never look back. Guide scope differential flexture was the bane of my existence 15 years ago, I moved to an OAG and now I use them on EVERY telescope I use from 300mm focal length to 2840mm focal length. The only time I would use a separate guide scope now is with a camera lens, (135~200mm focal lengths) and that's only because the backfocus limitation would prevent me using an OAG.
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I second what Alex says.
I'd try just switching off the guider completely - based on your PA numbers you should be getting perfectly good stars at only 30s exposures.
Also, one thing nobody else has mentioned: you are sure you have correct backfocus and no tilt? Perhaps post a couple of subs here if you are unsure….
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Thanks Alex and all for the feedback. Yesterday night I did some more imaging and got a PA alignment of around 2.5 arc min. The average Guiding error was between 0'5 - 0.8". The Stars were fairly round for 30 Sec exposures. Will be posting some images after stacking and processing
However I take the feedback. I am also contemplating to shift for OAG in near term as have got good feedback on multiple forums on the same. On the other issues raised
1. The SQA55 Petzval does not have any back focus issues and have ensured the right back focus distance as per specs. The Guide scope is also well focused and Stars are round and clear.
2. The Flex issue could be a major one as I hear from above. Checked up the specs from Askar SQA55 and they have recommended the same mounting for a Guide scope. Am looking at replacing the same with a normal dovetail assembly which I have for my other Newtonian scope
3. Once again rechecked the settings of scope , camera etc and all was fine
Thanks again for a good discussion and suggestions
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