7 replies to this topic

[Mikeiss]

I recently purchased a used ASI1600MC-Cool and have been running into some issues. No matter what spacing configuration I tried, I ended up with some crappy stars at the edges. The middle of the frame was sharp, but as you got to outer half of the image the stars gradually get worse. I do have a set of pretty cheap calipers, but they aren't easy to read, so I resort to measuring with the calipers and then holding them next to a ruler with millimeter markings. A little crude to say the least.

Yesterday I got desperate and made a trip to Starizona to see if they could help me out. The scope in question is my Vixen ED81S with the dedicated .67 reducer, which requires 63.5mm of back focus. After a few minutes they got me squared away with the back focus and I tried again last night. I imaged the Double Cluster which was pretty close to zenith when I stared imaging, and much to my displeasure, the stars still were crap at the edges.  Pictured below is my current imaging train as well as the star field. Any suggestions would be greatly appreciated, thanks.

 

-Mike

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[Mikeiss]

I'm realizing that the compressed JPEG might not be the best representation. Here's a link to my image via drop box.

 

https://www.dropbox....t=nlfz5zmo&dl=0

[Mikeiss]

This is what Siril says in regards to tilt. I'm not sure how to read that.

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[Tapio]

I looked and I think you really have to pixel peep to call edge stars crappy.

If you are unhappy with those I would just clean them with BlurXTerminator.

Not sure you can do much better with adjusting the back spacing.

(And funny that theyd didn't try to sell you a better reducer/flattener in Starizona...).

[freestar8n]

It would be easier to assess with the actual fits file, but directly inspecting stars around the edge you can see there is coma mainly on the left and the right side is better.  The presence of coma means the spacing of the reducer may not be optimal - and that is independent of any possible sensor tilt.  Sensor tilt does not cause coma to appear - but if coma is present then tilt may make the coma worse on one side than the other - but which side is worse should change as you slightly change focus.  You just can't assess tilt from a single image - despite what various tools claim to be doing.

 

Unless the reducer is perfect - there may always be some coma evident and you won't be able to remove it completely - but you'll be able to find the best backfocus where it is minimized.

 

It may just need a collimation tweak to make the left side much better while the right side gets slightly worse.  You want any aberrations to be balanced around the edges.

 

This appears to be a single image that was debayered - and you can see chromatic effects in the edge stars where the red flares out.  Much of that should go away if you stack and align with Bayer drizzle - and separately align the individual color planes - assuming the alignment code works well with the color channels independently.

 

So things to do are:  1)  See if the best side switches when you change focus slightly - indicating sensor tilt.  2)  Change backfocus slightly and see if the edge stars all around show more or less coma.  If coma increases try going the other way.  3)  If one side is definitely worse and you see coma in the central stars - adjust collimation.

 

After doing all this and confirming there is still sensor tilt - and you can see the "good side" switch as you go through focus - then try to tweak the sensor tilt.

 

Frank

[Jim Waters]

+1 on what Frank said above.

[Mikeiss]

I've finally done some more testing. My friend lent me his generic Orion Field Flattener for short refractors to see if I had similar issues using that. I tried it on the Double cluster and  as it turns out, I had no issues at all with it. Which is pretty darn annoying. I just plopped the reducer in the back end of the the ED81S set the back focus to 55mm and yeah the stars look pretty good. I think this means it's a back focus issue, or maybe something with the Vixen reducer?.

I may have made some headway there too. After lots of trial and error I tried again the other night with the Vixen reducer. I still don't think the stars are great in the corners, but they are better than they were originally. This was at 65.4mm of back focus with a filter. There was no filter when I used the Orion field flattener. I will try again tomorrow night or maybe Monday.

 

https://www.dropbox....t=i3idyadn&dl=0

 

https://www.dropbox....t=5smb4x7o&dl=0

[39cross]

I have the same reducer and have been working under the assumption that the backspace is the standard 55mm.  The Vixen diagram for the scope to camera attachment shows the wide photo adapter and a T-ring.   The diagram is here: http://vixenamerica....067_Reducer.pdf

 

Does all of this add up to 63.5mm with a standard DSLR?