Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Need help diagnosing round but slightly large stars

  • Please log in to reply
9 replies to this topic

#1 shitijb

shitijb

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 12 Aug 2019
  • Loc: Seattle, WA

Posted 24 May 2020 - 07:16 PM

First time using a telescope and an astro camera, got 30 min of data on M81 last Friday. I was assuming I would fail miserably since it is my first time with a completely new setup, but I guess I did better than I expected

 

Autosave 2

 

However I think the size of stars looks a little big (compared to lets say: https://www.astrobin.com/brcrj2/0/). And it shows even more when I crop in. My setup is this:

  1. Orion astrograph 8" with Baader mpcc mark 3 coma corrector
  2. ASI 183mm pro with EFW filter wheel and L3 Astronomic filter
  3. Guiding with SSAG Pro, 60mm guidescope, PHD2
  4. Orion Atlas EQ-G
  5. Used a bahtinov mask to focus with APT bahtinov aid
  6. Processing: stacked in DSS with default settings and played with some sliders in Adobe lightroom

My guiding RMS according to PHD2 was 1.16" (RA: 0.62", DEC: 0.97") with polar alignment error at 0.0'. I think the DEC RMS was higher because I didn't balance my newtonion axially. Both guidescope and camera + filter wheel are hanging on one side, I'm trying to fix this.

 

How do I make the stars tighter? Did I miss focus? Or is there a collimation issue? Or is the higher RMS in guiding causing this?

 

 



#2 Kevin Ross

Kevin Ross

    Viking 1

  • *****
  • Posts: 930
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 24 May 2020 - 07:59 PM

A number of factors can contribute to bloated stars. It could be guiding. It could be focus. Or it could be your processing. Can you post the unstretched (but stacked) autosave.tif from DSS to a file sharing site like Dropbox? And maybe a single unprocessed sub, too. Also, your PHD2 guiding (not debug) log would be helpful.

 

But honestly, with the setup you have, your guiding RMS numbers aren't bad. But RMS doesn't tell the whole story.



#3 ChrisWhite

ChrisWhite

    Fly Me to the Moon

  • *****
  • Posts: 5,838
  • Joined: 28 Feb 2015
  • Loc: Colchester, VT

Posted 24 May 2020 - 08:27 PM

Difficult to tell much with the small example you posted on CN.  Stars look clipped, so you might be overexposed.  If you expose too long stars look bigger.  Also your guiding might not be good enough for the image scale you are at.  As kevin mentioned, the RMS isn't the whole story, but you are at a pretty fine image scale with the 183. When I was imaging with an 8in newt I was at or below 0.6" total RMS.  If I was at 1.16" it was very bad. 

 

I spent a year imaging with an 8in Newt on an Orion Atlas and in my experience it's too much scope for that mount.  I'm not saying you have to buy a better mount, but when you overload a mount you really need to dial everything in to make it work better.  So, get your balance just right and make sure the mount is meshed correctly (if you are willing to adjust meshing the gears).  Make sure you get really good polar alignment and block the wind and do everything you can to stack the deck in your favor.  The focuser should be pointing down towards the ground (at the counterweight bar) not off to the side. 

 

I'm also guessing that you have the two counterweights that came with the mount down towards the bottom of the bar. If so, that's asking for trouble.  Get another weight so you can slide the weights as close to the top of the bar as possible.  More weight closer to the mount is MUCH better than less weight at the end of the bar. 

 

If you have some budget and want to chase better images, ditch the guidescope and install an OAG.  That will give you a nice jump in performance.  Those guidescopes are supposedly good to 1500mm or whatever, but I don't think those specs are considering people using 2.4um pixels on their main imaging camera. 


  • rockstarbill and TelescopeGreg like this

#4 WadeH237

WadeH237

    Fly Me to the Moon

  • *****
  • Posts: 5,768
  • Joined: 24 Feb 2007
  • Loc: Snohomish, WA

Posted 24 May 2020 - 08:48 PM

Hmm.

 

To my eye, the image posted here has sharper stars than the Astrobin link.  There is a nice double star just off to the side of the galaxy.  I always look for this double in any image of M81.  And your image above shows it split better than the Astrobin image.

 

I think that they look bloated because they are white clipped, so that the star cores (and galaxy core) are pure white disks.  I am guessing that this is a result of processing, because the core of the galaxy is clipped in the image, but should not be that bright in the data.



#5 Kevin Ross

Kevin Ross

    Viking 1

  • *****
  • Posts: 930
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 24 May 2020 - 09:35 PM

Yes, I also suspect the underlying data is good, that it's a result of processing.



#6 shitijb

shitijb

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 12 Aug 2019
  • Loc: Seattle, WA

Posted 24 May 2020 - 09:38 PM

A number of factors can contribute to bloated stars. It could be guiding. It could be focus. Or it could be your processing. Can you post the unstretched (but stacked) autosave.tif from DSS to a file sharing site like Dropbox? And maybe a single unprocessed sub, too. Also, your PHD2 guiding (not debug) log would be helpful.

 

But honestly, with the setup you have, your guiding RMS numbers aren't bad. But RMS doesn't tell the whole story.

 

Thank you for the replies, here is a link with the following:

https://drive.google...6Fenc8jcaOO5C9F

  1. PHD2 log file
    The image is from log section 4. Log section 1-2 I was still messing with focus so it looks terrible, and Log section 5 has color data (only G and some R because I ran out of time :( ), but I haven't integrated it yet
  2. Autosave.tiff from DSS
  3. One light frame FITS file

 

Difficult to tell much with the small example you posted on CN.  Stars look clipped, so you might be overexposed.  If you expose too long stars look bigger.  Also your guiding might not be good enough for the image scale you are at.  As kevin mentioned, the RMS isn't the whole story, but you are at a pretty fine image scale with the 183. When I was imaging with an 8in newt I was at or below 0.6" total RMS.  If I was at 1.16" it was very bad. 

 

I spent a year imaging with an 8in Newt on an Orion Atlas and in my experience it's too much scope for that mount.  I'm not saying you have to buy a better mount, but when you overload a mount you really need to dial everything in to make it work better.  So, get your balance just right and make sure the mount is meshed correctly (if you are willing to adjust meshing the gears).  Make sure you get really good polar alignment and block the wind and do everything you can to stack the deck in your favor.  The focuser should be pointing down towards the ground (at the counterweight bar) not off to the side. 

 

I'm also guessing that you have the two counterweights that came with the mount down towards the bottom of the bar. If so, that's asking for trouble.  Get another weight so you can slide the weights as close to the top of the bar as possible.  More weight closer to the mount is MUCH better than less weight at the end of the bar. 

 

If you have some budget and want to chase better images, ditch the guidescope and install an OAG.  That will give you a nice jump in performance.  Those guidescopes are supposedly good to 1500mm or whatever, but I don't think those specs are considering people using 2.4um pixels on their main imaging camera. 

Thank you for the tips, I'll get another counterweight. I am using 2 counter-weights with the shaft extension right now like you mentioned. For the axial balance I am going to put the camera down like you mentioned and mount my guidescope on top instead of the finder shoe. Also getting some magnets to dial in the balance axially even more. I'm not handy enough to mess with the internals like gears, but I might have the courage consider it later :) I did consider an OAG as well, but it felt like a more complicated setup (I am a beginner after all, this is my first attempt with a telescope and not a lens)

 

 

 

Hmm.

 

To my eye, the image posted here has sharper stars than the Astrobin link.  There is a nice double star just off to the side of the galaxy.  I always look for this double in any image of M81.  And your image above shows it split better than the Astrobin image.

 

I think that they look bloated because they are white clipped, so that the star cores (and galaxy core) are pure white disks.  I am guessing that this is a result of processing, because the core of the galaxy is clipped in the image, but should not be that bright in the data.

Interesting, I thought the double star was a little distracting personally :p, and yes I only have experience in lightroom for daytime photography so it is possible that processing has made things worse



#7 Kevin Ross

Kevin Ross

    Viking 1

  • *****
  • Posts: 930
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 24 May 2020 - 10:21 PM

I think the data looks pretty reasonable to me. I think you got a great amount of detail in only 30 minutes! Gotta love that light bucket that is a f/3.9 Newtonian! :)

 

I think you clipped too many whites (and blacks) in the processing.

 

Here's my quick try at processing. Now I admit, I am not the best person on this forum at processing. I'm sure others could do better. 

 

Autosave-2.jpg


Edited by Kevin Ross, 24 May 2020 - 10:23 PM.


#8 Kevin Ross

Kevin Ross

    Viking 1

  • *****
  • Posts: 930
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 24 May 2020 - 11:26 PM

I looked at your PHD2 log. I'm sure the true experts will eventually chime in, but in the meantime, you get to listen to me. smile.gif

 

I think you are doing quite well with that setup. I have that same setup (different camera). So I know first hand how difficult that Orion Newtonian can be on an Atlas mount. I think you've done a good job taming the beast.

 

Your stars aren't quite as tight as they could be, but they aren't bad either. I wasn't sure if it was focus or guiding, but looking at the PHD2 log, I think it's guiding. I like looking at the scatter plot instead of the graph. The scatter plot will give you a better idea of what the final shape of your stars will be. And you can see on the scatter plot, that the points are scattered out pretty far. This will manifest itself as bloated stars.

 

There seems to be a lot of movement in the Dec axis. If your polar alignment is as good as PHD2 thinks it is, you shouldn't see much movement at all (and I usually don't see much movement in Dec when my polar alignment is good). So you might have some imbalance of your scope, or maybe some wind.

 

It could be seeing related, too. 1 second exposures might be too short for your seeing conditions. If you lengthen your exposures to 2 or 3 seconds, and all of a sudden the Dec guiding smooths out, that's most likely because of seeing conditions, and you can't go any faster than that.

 

 

(click image if you want to be able to read anything)
Capture.PNG



#9 shitijb

shitijb

    Lift Off

  • -----
  • topic starter
  • Posts: 20
  • Joined: 12 Aug 2019
  • Loc: Seattle, WA

Posted 25 May 2020 - 01:31 AM

I think the data looks pretty reasonable to me. I think you got a great amount of detail in only 30 minutes! Gotta love that light bucket that is a f/3.9 Newtonian! smile.gif

 

I think you clipped too many whites (and blacks) in the processing.

 

Here's my quick try at processing. Now I admit, I am not the best person on this forum at processing. I'm sure others could do better. 

 

attachicon.gifAutosave-2.jpg

That's relieving, I thought I was missing something bigger in my imaging technique. Yes, I bought a fast reflector (and decided to struggle with collimation and balance issues) because I live in Seattle and must make as much of the limited imaging time as possible :)

 

How did you process it? :O I mainly tried to make the background black and pushed up contrast and clarity. Does using something specialized like pixinsight make these things easier?

 

 

I looked at your PHD2 log. I'm sure the true experts will eventually chime in, but in the meantime, you get to listen to me. smile.gif

 

I think you are doing quite well with that setup. I have that same setup (different camera). So I know first hand how difficult that Orion Newtonian can be on an Atlas mount. I think you've done a good job taming the beast.

 

Your stars aren't quite as tight as they could be, but they aren't bad either. I wasn't sure if it was focus or guiding, but looking at the PHD2 log, I think it's guiding. I like looking at the scatter plot instead of the graph. The scatter plot will give you a better idea of what the final shape of your stars will be. And you can see on the scatter plot, that the points are scattered out pretty far. This will manifest itself as bloated stars.

 

There seems to be a lot of movement in the Dec axis. If your polar alignment is as good as PHD2 thinks it is, you shouldn't see much movement at all (and I usually don't see much movement in Dec when my polar alignment is good). So you might have some imbalance of your scope, or maybe some wind.

 

It could be seeing related, too. 1 second exposures might be too short for your seeing conditions. If you lengthen your exposures to 2 or 3 seconds, and all of a sudden the Dec guiding smooths out, that's most likely because of seeing conditions, and you can't go any faster than that.

 

 

(click image if you want to be able to read anything)
attachicon.gifCapture.PNG

Thank you for the tips, the scatter plot helps a lot. I was thinking if the guiding was bad then stars should have trailed, but now it makes more sense why that did not happen.

 

And yes, I did notice balance issues axially, but thought it'll be ok (I guess I was wrong). I'll work on fixing that. I do wonder though maybe another advantage of going off axis like ChrisWhite mentioned is reduced weight, maybe that will help? Im using this guidescope: https://optcorp.com/...NxoCa2AQAvD_BwE, which itself is 2lbs. 2lbs sounds too insignifcant though?

 

Thank you for that tip as well, I'll try increasing the exposure time next time to see if that helps in bringing RMS down.



#10 Kevin Ross

Kevin Ross

    Viking 1

  • *****
  • Posts: 930
  • Joined: 28 Mar 2018
  • Loc: Traverse City, MI

Posted 25 May 2020 - 03:10 AM

If you want to stay with DSS and Photoshop, nothing wrong with that. Trevor Jones from Astro Backyard makes beautiful images with DSS and PS. Here is one (of many) of his Photoshop tutorial videos.

 

https://www.youtube....h?v=5GgruZ-1zQ0




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics