ASI678MC Star bloating
ASI678MC Star bloating
I am very new to astrophotography. I am using a ZWO ASI678MC camera with an SVBONY 48p or Celestron DX 102 optical tube and Sharpcap pro. Generally things are working but I have been having some issues with star bloating with long exposures or high gain. I have attached two samples of my captures. The M13 is pretty good, But the M82 rather bloated. M13 is a much brighter object than M82 and this may be the underlying issue. I don't think it is a focus issue - seems to have something to do with my use of the camera (this is not a cooled camera and some nights it can be 90 deg where I live). Any suggestions on the cause and what to do?
- Attachments
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- M82 Bloated
- M82 30 stacked 1 sec raw16 SV48p ZWO.png (137.83 KiB) Viewed 3872 times
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- M13 OK
- M13 1sec 100 stacked.png (573.38 KiB) Viewed 3872 times
Re: ASI678MC Star bloating
Hello and welcome
First: it looks like the tracking/guiding is somewhat off. Especially M13 is showing elongated stars. I don't know the scopes and the mounts they are on but if you are exposing too long and the tracking/guiding is somewhat off, it will show in the capture.
The camera really is made for planetary use. It's made with planets, lunar or solar capturing in mind.
Because of this, it has a very small pixelsize of 2μm.
Your telescopes have a focal length of f/5.5 and f/6.5. This gives a moderate field of view.
What happens in that focal length, is that with longer exposures, the light is starting to "bleed" into other pixels of the sensor. This is called oversampling. Simply put: light of a star is hitting a pixel but at a certain point it will get "saturated" and will start to bleed into neighboring pixels.
When you have a short exposure time, this is no problem. There is no bleeding and the star is not blurry.
Also, the camera is very sensitive in the infrared range. Using an IR/UV Cut filter can prevent IR to be too much present in the captures. Too much IR can also cause bloated stars.
Menno
First: it looks like the tracking/guiding is somewhat off. Especially M13 is showing elongated stars. I don't know the scopes and the mounts they are on but if you are exposing too long and the tracking/guiding is somewhat off, it will show in the capture.
The camera really is made for planetary use. It's made with planets, lunar or solar capturing in mind.
Because of this, it has a very small pixelsize of 2μm.
Your telescopes have a focal length of f/5.5 and f/6.5. This gives a moderate field of view.
What happens in that focal length, is that with longer exposures, the light is starting to "bleed" into other pixels of the sensor. This is called oversampling. Simply put: light of a star is hitting a pixel but at a certain point it will get "saturated" and will start to bleed into neighboring pixels.
When you have a short exposure time, this is no problem. There is no bleeding and the star is not blurry.
Also, the camera is very sensitive in the infrared range. Using an IR/UV Cut filter can prevent IR to be too much present in the captures. Too much IR can also cause bloated stars.
Menno
Meade LX200 8" f/10 ACF OTA / Ioptron CEM25EC / Zwo ASI071MC Pro / Zwo ASI482MC
Re: ASI678MC Star bloating
Thanks for the reply. Would it help to take a larger number of photos at shorter exposures?
Re: ASI678MC Star bloating
Yes, it might be less when doing more shorter exposures.
Also, see what happens when you set the Gain value at 182 (or higher). At this value the camera's HGC Mode kicks in (lower read noise with little loss in dynamic range). Don't know if (or how) this effects the bloated stars, but you could try.
Menno
Also, see what happens when you set the Gain value at 182 (or higher). At this value the camera's HGC Mode kicks in (lower read noise with little loss in dynamic range). Don't know if (or how) this effects the bloated stars, but you could try.
Menno
Meade LX200 8" f/10 ACF OTA / Ioptron CEM25EC / Zwo ASI071MC Pro / Zwo ASI482MC
Re: ASI678MC Star bloating
OK - thanks - I will give it a try.
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Re: ASI678MC Star bloating
Hi,
is it possible that we are seeing chromatic aberration here? I don't think that the Celestron DX102 is an apochromat, so it will tend to bring red and blue light to focus as slightly different distances from the lens. That means if you focus on the red light you will get a fuzzy patch from the slightly out of focus blue and the other way around...
As Menno suggests, a UV/IR cut filter will help somewhat as it limits the wavelengths reaching the sensor (looks like the 678MC probably has a clear glass window over the sensor as it allows the IR sensitivity of the camera to be used when appropriate). A narrower filter (perhaps a simple green or red filter) might help more as it would narrow the wavelengths down further and is relatively inexpensive to buy and test.
cheers,
Robin
is it possible that we are seeing chromatic aberration here? I don't think that the Celestron DX102 is an apochromat, so it will tend to bring red and blue light to focus as slightly different distances from the lens. That means if you focus on the red light you will get a fuzzy patch from the slightly out of focus blue and the other way around...
As Menno suggests, a UV/IR cut filter will help somewhat as it limits the wavelengths reaching the sensor (looks like the 678MC probably has a clear glass window over the sensor as it allows the IR sensitivity of the camera to be used when appropriate). A narrower filter (perhaps a simple green or red filter) might help more as it would narrow the wavelengths down further and is relatively inexpensive to buy and test.
cheers,
Robin