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Trying to understand autoguiding parameters for my setup

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#1 AnakChan

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Posted 23 February 2021 - 10:26 AM

I’m embarrassed to say that despite of years (decades!) of having a scope I’m still a noob with certain aspects in astrophotography which I’m trying to comprehend. My only excuse is that due to my laziness in the past I’ve never scrutinised my acquired data as I do now and realized there’s so much more I still need to learn.

 

So in the past 4 months, I’ve moved to a more conventional PC-styled setup with N.I.N.A., PHD2, ASCOM, etc. and I’m trying to understand my setup a little more. I’m using a Takahashi µ250CRS (2500mm/F10), on a Vixen AXD mount, with the ASI6200MM as my main camera, and an ASI174MM Mini as the OAG guide camera.

With this scope’s focal length and main camera, the resolution is 0.31”/pixel. I would say that the area I’m living in has OK seeing (2-4” FWHM). I have a bunch of questions but I’d like to start by asking for my scope with such a long focal length, a camera with 3.76 micron pxel size, and an environment with “OK seeing”, what would be considered to be an acceptable PHD2 RMS Total Error? I’ve read numerous posts and talked to various folks who say RMS Total Errors of even 0.8” is fine, but if I start to scrutinize my light subs in PixInsight, I find that once I go beyond 0.55” or even 0.5”, I feel the stars (though still round) seem a little bloated.

 

Here are some stars in Gum 15.and I got a range of RMS Total Error of 0.35” to 0.55”. I autofocus every 5th frame and where I”m at, the temperature’s not changing much at night.

 

The stars on the top left look a lot tighter with clear separation of the surrounding stars around the main star, but in the bottom left the main star’s almost swallowed the surrounding stars. I’m attributing that to poor guiding rather than focus.

 

336E31B5-434C-4240-8720-AF50253A5408.jpeg

 

To paraphrase my question again, are there guidelines to what RMS error levels are considered acceptable for a particular scope & camera configuration? Does the total RMS errors correspond to 0.31”/pixel resolution of my scope/camera configuration? Or would that not matter for the 2”-4” FWHM OK seeing of my area?



#2 Lead_Weight

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Posted 23 February 2021 - 10:52 AM

I would likely bin 2x2 with that setup. That will give you .62"/pixel. Much more manageable and likely closer to your seeing conditions.

 

I have an Edge HD 11 that I bin 2x2 at 1960mm focal length F/7, and that puts me at .79"/pixel. If your pixel scale is .62, you need to guide with less than .62 RMS to avoid seeing any issues from guiding in your captures. You always want to guide better than your pixel scale. If you're at .31, you are likely seeing limited, and possibly guiding limited in your final captures. You can see in the bottom right two images the differences in the upper right hand stars, the left one is sharper than the right one, and this is likely seeing changes between frames. Binning 2x2 will make that difference invisible, and net you sharper images overall.


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#3 AstroVagabond

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Posted 23 February 2021 - 10:55 AM

I'm a beginner, don't know the answer but right now I'm looking to keep the total RMS error less than my pixel scale. Thank you for asking the question because I would like to sharpen my knowledge here.

 

Bill



#4 AnakChan

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Posted 23 February 2021 - 11:11 AM

Thank you Andrew. It’s making more sense now and that’s a great suggestion binning as I didn’t consider that before!



#5 drd715

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Posted 23 February 2021 - 11:23 AM

Since you already have some data at 1X1 you can experiment with binning in the software and see what the difference could be.

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#6 AnakChan

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Posted 23 February 2021 - 06:39 PM

If I bin post acquisition, whatever detail I’ve lost can’t be recovered. Im gonna paraphrase Andrew’s post to see if I understand correctly.

With binning, I lose resolution but gain additional tolerance for guiding. The results may end up looking more like the bottom right where the separation of stars are less distinctive however overall acquisition across multiple subs will look more consistent.
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