Star bloat with DSLR
- maceemiller
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Star bloat with DSLR
I've a question about the cause of star bloat in my images with my equipment.
I'm not looking for an "exact" answer of why because I don't believe that exists.
I have an unmodified Canon 100d on a Sky-watcher star adventurer mini. The lens i use (shown below) has its apature fully open at F2.8.
I recently imaged Andromeda at ISO400 and 60 second exposures. Below is a stack of 60 exposures and stretched quickly to show the star bloat.
My questions are is 60 seconds too long with this lens or is the ISO too high? Maybe the lens is no good? Is being out of focus an issue..I don't know if im in focus as when I zoom in on certain stars I see spikes from the lenses iris (I think that's right!)
My next imaging session will be the Pleiades and I don't want
to make the same mistake.
Thankyou
Carl
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Re: Star bloat with DSLR
Looking at your stacked image, there is asymmetry in the bright stars, where most of the light is concentrated at the bottom and it fans out towards the top. Is there some play in the lens that could cause it to skew? The smaller stars look OK so the tracking is probably fine, I don't think it's that.
The stars look a bit un-sharp. Was there any dew on the lens? Maybe the lens quality is not good enough at
I would try to focus during the daytime, and mark it on the lens or simply put tape on the lens so it can't move. I doubt if the temperature difference between day and night makes a lot of difference regarding focus.
135 mm should definitely be doable for a commercial tracker, possibly at 5 minutes if your polar alignment is good. The risk of trails is less at 60 seconds of course. I don't think 60 second exposures are the problem.
- sdbodin
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Re: Star bloat with DSLR
Good luck,
Steve
Cameras; Atik 460ex mono, Zwo ASI1600MC-cool, QHY5L-II color and mono
- JayTee
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Re: Star bloat with DSLR
Cheers
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- maceemiller
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Re: Star bloat with DSLR
I've just had a close look at the lens mounted in the camera.SkyHiker wrote: ↑Mon Sep 13, 2021 9:54 pm
Looking at your stacked image, there is asymmetry in the bright stars, where most of the light is concentrated at the bottom and it fans out towards the top. Is there some play in the lens that could cause it to skew? The smaller stars look OK so the tracking is probably fine, I don't think it's that.
Its attatched by an M42 to EOS adapter which, ive never noticed before, does have a bit of play.....
I can move the whole lens and adapter about 1mm in any direction I want.
When the camera is in a horizontal position the whole lens and adapter moves down and settles to the bottom so It does seem like its skewing.
It was a cheap adapter but I wonder if its worth putting the camera on a table pointing up, centeralising the lens and putting some electrical tape around it to hold it in position?
The obvious is to purchase a better mount yet I suppose the tape trick may "eliminate" this as a problem?
Carl
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Re: Star bloat with DSLR
Thankyou for your advice on stopping down to F4.sdbodin wrote: ↑Mon Sep 13, 2021 11:26 pm Looks like a lens fault. I would try f4 and see if the stars are pinpoints. I have the same problems, my old Soligor 135mm of the '60s looks similar, but my new modern Rokinon is near perfect. Attached image of M110 at full scale shows it well, both with the same camera, Asi1600MC-cool, and at f2.8.
Good luck,
Steve
I've always been in denial about stopping down...."more light in must be better so open the apature wide open".....
Along with the post above regarding asymmetry (which I have a "test solution " for, ive just bit the bullet and tried your advice, albeit on a street light about 200 meters away (its currently 10am in the UK yet its pouring with rain and dark....street light has come on so I've put up a fishing umbrella and got my camera under it)
Took 2 pictures of it using the 135mm lens, 1 at 2.8, the other at 4.
Zoomed in on the camera screen and what a difference! The F4 is much sharper!
Tried it again with a prime 200mm lens....same result!
I'm a converted person now.....stopping down 1 click is amazing....just wish I'd done this before and not been so stubborn about what I thought was better (but then again I'm a Taurus)
Thankyou once again....and to everyone who's replied on this thread....been a massive help
Roll on the next clear night so I can attack The Pleiades
Carl
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Re: Star bloat with DSLR
You may want to check if there are springs that are supposed to be tight but are not. The kind that are bent pieces of metal in the adapter. If they are not tight enough take the adapter out and stick a screw driver in between and twist it to bend them apart, then tighten it again and see if it helped. This trick helped me with some adapters that were too loose.maceemiller wrote: ↑Tue Sep 14, 2021 7:17 amI've just had a close look at the lens mounted in the camera.SkyHiker wrote: ↑Mon Sep 13, 2021 9:54 pm
Looking at your stacked image, there is asymmetry in the bright stars, where most of the light is concentrated at the bottom and it fans out towards the top. Is there some play in the lens that could cause it to skew? The smaller stars look OK so the tracking is probably fine, I don't think it's that.
Its attatched by an M42 to EOS adapter which, ive never noticed before, does have a bit of play.....
I can move the whole lens and adapter about 1mm in any direction I want.
When the camera is in a horizontal position the whole lens and adapter moves down and settles to the bottom so It does seem like its skewing.
It was a cheap adapter but I wonder if its worth putting the camera on a table pointing up, centeralising the lens and putting some electrical tape around it to hold it in position?
The obvious is to purchase a better mount yet I suppose the tape trick may "eliminate" this as a problem?
Carl
- maceemiller
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Re: Star bloat with DSLR
Thankyou but this is a solid piece of cheap metal from China.....not a spring in sight!SkyHiker wrote: ↑Tue Sep 14, 2021 2:35 pmYou may want to check if there are springs that are supposed to be tight but are not. The kind that are bent pieces of metal in the adapter. If they are not tight enough take the adapter out and stick a screw driver in between and twist it to bend them apart, then tighten it again and see if it helped. This trick helped me with some adapters that were too loose.maceemiller wrote: ↑Tue Sep 14, 2021 7:17 amI've just had a close look at the lens mounted in the camera.SkyHiker wrote: ↑Mon Sep 13, 2021 9:54 pm
Looking at your stacked image, there is asymmetry in the bright stars, where most of the light is concentrated at the bottom and it fans out towards the top. Is there some play in the lens that could cause it to skew? The smaller stars look OK so the tracking is probably fine, I don't think it's that.
Its attatched by an M42 to EOS adapter which, ive never noticed before, does have a bit of play.....
I can move the whole lens and adapter about 1mm in any direction I want.
When the camera is in a horizontal position the whole lens and adapter moves down and settles to the bottom so It does seem like its skewing.
It was a cheap adapter but I wonder if its worth putting the camera on a table pointing up, centeralising the lens and putting some electrical tape around it to hold it in position?
The obvious is to purchase a better mount yet I suppose the tape trick may "eliminate" this as a problem?
Carl
Will just have to visually align and test
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