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Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 4:42 pm
by JosephS
Good Afternoon all.
I'm relatively new to the SCT world and have noticed I'm having an issue with collimation. No problem with the center of my field...just the periphery. Anybody having a similar experience with Celestron SCTs?
Joe
Re: Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 4:49 pm
by Graeme1858
Hello Joe
Welcome to the forum.
That looks more like a back focus issue than a collimation issue to me. What shape are the stars in the periphery when in focus?
Could you give us some more information on your camera, if there's a focal reducer and your current back focus please.
Regards
Graeme
Re: Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 6:20 pm
by yobbo89
I think it's somewhat normal to have some sort of correction problems when not in focus towards the edge, but it looks like the error is all going one way but the center looks collimated. maybe camera tilt is part of the problem , i would try to get a larger defocused star for ther center and check both inside and outside focus and then do a long exposure in focus , what kind of gear do you have in your image train, filters,reducer crayford focuser etc?
Re: Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 10:40 pm
by JayTee
That image is perfectly normal for an SCT. As you get further away from image center, the secondary mirror also gets further away from the center of the stellar image. Also, notice that the effect is symmetric. The displacement is relatively equal at the right and left edges of the image with the displacement on opposite sides of the stellar image. SCT collimation should always be done with your star of interest as close to image center as you can get it.
BTW, your scope is very well collimated based on the center star image.
Cheers,
Re: Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 11:10 pm
by JosephS
Thank you all for your responses...I should have been clearer about the image posted.
This was a totally de-focused image with intent on collimation.
I'm using a Celestron 9.25 HD EDGE 0.7 reducer/OAG/Moonlite Focuser/Filter wheel (in this image L filter)/and ZWO ASI2600.
I am getting round stars when focused and capturing some nice images so not a back focus problem, but I was concerned about the radial degradation of collimation as I went out to the periphery. Do other folks see the same aberration with their collimation? I was under the impression this was less likely to be seen with Celestron HD Edge model compared to Celestron Advanced VX models
Again thanks again for everyone's thoughts,
Joe
Re: Celestron 9.25 HD EDGE
Posted: Sun Oct 29, 2023 11:35 pm
by AstroBee
The question is, Do the sharply focused stars in the corner look good? Are they pinpoint stars or do the appear egg-shaped like in this image.
It does appear to be a backfocus issue from just that one image. What is the distance from the back of the 0.7x reducer and your sensor plane?
Re: Celestron 9.25 HD EDGE
Posted: Mon Oct 30, 2023 10:45 am
by yobbo89
JayTee wrote: ↑Sun Oct 29, 2023 10:40 pm
That image is perfectly normal for an SCT. As you get further away from image center, the secondary mirror also gets further away from the center of the stellar image. Also, notice that the effect is symmetric.
Ahh yes, now I look at it again, it is indeed symmetric through out the image.
Re: Celestron 9.25 HD EDGE
Posted: Tue Oct 31, 2023 10:53 pm
by Bigzmey
JosephS wrote: ↑Sun Oct 29, 2023 11:10 pm
Thank you all for your responses...I should have been clearer about the image posted.
This was a totally de-focused image with intent on collimation.
I'm using a Celestron 9.25 HD EDGE 0.7 reducer/OAG/Moonlite Focuser/Filter wheel (in this image L filter)/and ZWO ASI2600.
I am getting round stars when focused and capturing some nice images so not a back focus problem, but I was concerned about the radial degradation of collimation as I went out to the periphery. Do other folks see the same aberration with their collimation? I was under the impression this was less likely to be seen with Celestron HD Edge model compared to Celestron Advanced VX models
Again thanks again for everyone's thoughts,
Joe
Typically, optical systems are designed to correct aberrations in focus. Since you are getting nice images I would not be concerned how it looks out of focus. Your collimation appears spot on.
Re: Celestron 9.25 HD EDGE
Posted: Tue Oct 31, 2023 11:05 pm
by messier 111
welcome to tss ,
you will surely get a response from members who are more experienced than me with this type of telescope.