I appear to be having major issues with my 8" Edge HD scope. Attached is a screenshot of an attempt last night to see what I could do. Clearly the "donuts are pretty bad, but it appears that the position of the secondary shadow is in a completely different position for stars in different parts of the field.
Complete setup consists of the scope, a 0.7 Reducer, Moonlite litecrawler focuser, ZWO OAG and Filter wheel and a ZWO ASI1600MM Pro camera.I have double checked the spacing and the reducer requires backfocus of 105mm and with a number of spacers I have put in, it currently sits at 104.75mm.
I tried collimating a single star last night, and while I could get that star round, other stars in the field looked like "comets". Also, once a star was technically "round", I couldn't get anywhere near reasonable focus.
Any thoughts anyone?
I've got an older C8 which I use with a f6.3 reducer corrector.
I don't think it's a collimation issue, I think its backfocus.
When the backfocus is incorrect, I see the same type of out of focus donuts as in your image. If you focus you'll see coma increasing towards the edge of field.
I'd check the backfocus measurements and remember that filters affect backfocus distance, so you'll need to correct for that. Try increasing backfocus and see if the donuts change.
I’m concerned about your final statement that you can not reach focus on a star? The only way this would be possible is if there’s something obstructing the mirror movement or if backfocus is waaay off.
Check that nothing is obstructing or preventing mirror movement during focus such as faulty mirror locks, faulty focus rod, or screws used to anchor accessories that are too long.
Have you ever removed the secondary mirror or corrector plate? Might be worth checking that the secondary central screw is tight and holding things securely. Be very careful- if this lets go your secondary could drop.
Maybe send us a pic of your image train just to confirm back focus?
The EDGE series Back Focus for the C9.25 is 146mm according to the Celestron Data Sheet. Is 105mm correct for the C8 EDGE, as I recall the older Celestron SCT's were 105mm?
However, the 146mm measurement is from the very end of the threads of the x0.7 Reducer, which is hidden when you assemble all the components.
This is how my Edge C9.25 is set up but with all the poor weather, I have yet to test out this configuration.
I would advise you to collimate without the reducer attached, I have slight tilt in my reducer which will add to your problems. This link should help you collamator https://www.celestron.com/blogs/know...limation-guide
Does the Edge HD still exhibit some coma at the edges (As in is it expected to?)
If it does, you could probably collimate it the same way I used to do my Vanilla C925. Focus on a centered star and then collimate to have all the "Comet tails" pointed directly away from the center of the frame. So edge of field stars in the middle of whichever edge of the image they are located should have the coma tails pointed directly left, right, up or down, depending on where they are in the image, corner stars with the coma pointed dirtectly in to the corners etc.
I found it easiest to align the camera so that the long side matched the "Flat" made by two of the collimation screws, to make it easier to work out which screws to adjust. I think I would do as Simon suggested and collimate first with the reducer removed.
One thing that is interesting to me is that the coma tails are opposite to what I always saw with my C925, with the shadow of the secondary to the outside of the defocused star, not the inside, perhaps an effect of the Edge HD reducer?
Hi Dennis
Yes the back focus is 105mm from the reducer. The Moonlight focuser adds 1.25" or 31.75mm so that gives a surface that is easy to measure from.
Malcolm
Quote:
Originally Posted by Dennis
Hi Malcolm
The EDGE series Back Focus for the C9.25 is 146mm according to the Celestron Data Sheet. Is 105mm correct for the C8 EDGE, as I recall the older Celestron SCT's were 105mm?
However, the 146mm measurement is from the very end of the threads of the x0.7 Reducer, which is hidden when you assemble all the components.
This is how my Edge C9.25 is set up but with all the poor weather, I have yet to test out this configuration.
Thanks for all the replies everyone. I think that fact the stars were close together but displaying different collimation errors. I will play a bit more with the suggestions when the sky clears again
Hi Dennis
Yes the back focus is 105mm from the reducer. The Moonlight focuser adds 1.25" or 31.75mm so that gives a surface that is easy to measure from.
Malcolm
Thanks Malcolm, I appreciate the update.
Have a look at this website where the Author has tested a C11 and it appears under average seeing conditions of 2 arcsec, the backfocus tolerance can be off by +-50mm for an APS-C sensor and +-20mm for a full frame chip.
Thanks for all the help. Decided to just attack the problem one step at a time. Collimate by eye with just an eyepeice, then add reducer and focuser while confirming collimation hadn't altered significantly.
Then removed everything and attached cam without any accessories and confirmed that collimation was in ball park then progressively added bits, reducer, focuser and then OAG and filter wheel. Once done I did a very careful collimation and it appears to have fixed the issue.
My guess is I made an error in my spcing but cannot for the life of me see what it was!!
Thanks for all the help. Decided to just attack the problem one step at a time. Collimate by eye with just an eyepeice, then add reducer and focuser while confirming collimation hadn't altered significantly.
Then removed everything and attached cam without any accessories and confirmed that collimation was in ball park then progressively added bits, reducer, focuser and then OAG and filter wheel. Once done I did a very careful collimation and it appears to have fixed the issue.
My guess is I made an error in my spcing but cannot for the life of me see what it was!!
Cheers
Malcolm
That's great. It's nice to know it all worked out. I'm looking forward to seeing images.