Question on dithering using PHD2

[John Natale]

Hi all,

I imaged the fireworks galaxy last night, and tried dithering using sharpcap pro software and PHD2. I am not sure if dithering worked since I didn't see any annotations in the PHD2 graph saying if dithering occurred and didn't see the image shift at all during capture. Can anyone take a look at the phd log below and let me know if it was successful? Also can you give me an opinion if the guiding was successful with the total error, etc? As you can see I am new to PHD2 guiding.

There were some egg-shaped stars in the final image and I am also curious how I can get rid of them in the future. The processed image is below so you can see what I mean. I was careful with my polar alignment so I don't think that was the problem, but I could be wrong.

PHD2 log:

https://drive.google.com/file/d/1oWgG0i0wHOeksO2PLr99D5qvMaiu5RUA/view?usp=sharing

Final image with egg-shaped stars:

https://drive.google.com/file/d/1q3ciIf21eFf4108jcnFPoTcXM2ay35bI/view?usp=sharing

Thanks in advance for any input; I appreciate it!

[Brian Valente]

Hi John

I can see one dither near the beginning. otherwise in your 5+ hour run there is no other dither command registered in the log

Your guiding result indicate a total RMS of 0.49" over the course of the 5 hour run with RA and Dec very close. You didn't say what was your imaging resolution but that would have produced nice round stars

At first glance it appears guiding was excellent, no issues with polar alignment

In fact, it's so good, it seems a little suspicious. 

I don't see anything obvious that sticks out as a smoking gun to indicate it a problem, but for example i'm looking at your starmass over the entire 5 hour run and it seems exceptionally steady. (starmass is the yellow line)

Again, no obvious signs of issues like hot pixel, maybe you can really good seeing conditions?

the image scale also indicates you were guiding at accuracies of 0.06-0.08 pixels. I'd seen similar before but wow you are right at the limit of pixel resolution

You also imaged for 5 continuous hours, it appears you imaged two hours past the meridian, is that correct?

Regarding your resulting image, it doesn't really look egg shaped to me as in star elongation. there seems to be focus issues and perhaps some backspacing. the stars mostly look out of focus at different wavelengths. Can you describe how you acquired the data in sharpcap? are these long exposures that you later stacked, is this eaa, what kind of camera and filters were you using? how did you stack the resulting images?

So again overall these rms values look very good, I just don't know if I entirely believe it

As someone being new to the hobby i'd say you are doing well, regardless

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[John Natale]

Thanks Brian. I was using sharpcap pro, and they were long exposures (4 minutes) later stacked with deep sky stacker. I have a Celestron C8 scope (recently collimated),  EQ6 mount, ZWO asi 294 camera and a 0.7 reducer. I didn't use a filter. Also I made sure to focus properly with a bahtinov mask before imaging. You asked what my resolution was... how do I find that out?

Anyway, do you have any idea why it registered just one dither in the PHD2 log? Why would it stop dithering after only one time? Also- do you have any other ideas on what caused the egg-shaped stars if my guiding was good? Could something else (collimation?) have caused this? I doubt it was collimation because I collimated it very well in advance. Not sure what the problem could be. If you have any other ideas please let me know.

Thanks again,

John

[Brian Valente]

why do you think your stars are egg shaped (i.e., elongated)? To me they look out of focus and misregistered across the colors

I appreciate your sense of focus with a bhatinov, but it's obvious to me it isn't in focus. Or at least something happened in the processing or acquisition to cause focus to go out

if you have a one shot color camera, these are not normal looking stars, the colors are different sizes. almost like a thousand mini planetary nebulae!

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[George Shoup]

And we know with the high temps in the USA that a refocus will be needed as your scope cools at night, particularly if the OTA is aluminum tube.

Sent from my iPhone

On Jul 20, 2023, at 10:27 AM, Brian Valente <bval...@gmail.com> wrote:



why do you think your stars are egg shaped (i.e., elongated)? To me they look out of focus and misregistered across the colors

I appreciate your sense of focus with a bhatinov, but it's obvious to me it isn't in focus. Or at least something happened in the processing or acquisition to cause focus to go out

if you have a one shot color camera, these are not normal looking stars, the colors are different sizes. almost like a thousand mini planetary nebulae!

<image.png>

To view this discussion on the web visit https://groups.google.com/d/msgid/open-phd-guiding/CAJa45i6oB_rC6CqGpjUTp%2BKh8XJJhNu_cDPGjAsxFbKso1%2B2Wg%40mail.gmail.com.

[John Natale]

I got really good focus with the bahtinov mask before imaging; but as you pointed out the stars may in fact be out of focus due to temperature differences. But it may have also been the result of poor processing on my part. The software I use (startools) has many different modules and I am not exactly an expert at using it... I may have overprocessed the stars or used the wrong modules thus giving them this "small nebula" look.

However, I looked at my raw stacked image (right out of dss before processing), and upon close inspection, the stars in that are slightly egg-shaped, especially on the top part of the image. I attached the autosave (stacked) file below, and if you look closely (or enlarge it) you can see it. Single light frame images also show it. Maybe I am being too picky but I can tell. Not sure what has caused this problem but it was likely during capture...  one person said my scope (C8 Edge) may be out of collimation, but I checked and it looks good to me. Anyway I am not going to chase this issue down any longer, but if you have any more ideas please let me know.

Thanks!

Link to dss autosave file:

https://drive.google.com/file/d/1VZEhiJEMf6nYnZNnzFl_xuF5oulWWTR9/view?usp=sharing

[Brian Valente]

Thanks for posting that

I'd say yes, you definitely have collimation issues and probably some tilt. Star elongation (with heavy chromatic aberration)  is most pronounced in the upper left and shows comatic elongation. the comatic direction changes with different sensor positions so that's probably some combination of backfocus and tilt.

look at your stars in the lower right and you'll see they are round.  FYI the corrections in blurxt can really improve things here as a short term fix

I still think focus is not quite there. The stars seem bloated to my eye. It could be an artifact of the coma, but I really think you many issues going on here.

 Celestron SCTs can be really easy to collimate, and really challenging to collimate well.

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