M51 2nd try, question..

[MilwaukeeLion]

Took another shot at M51 last night.  C-11, f6.3 reducer, Nikon (in DX mode 1.5x).  These are only 2 min subs with darks, bias, flats. Challenging imaging at this focal length.  For some reason my target drifted as you can see from 3rd attached image, which of course ruined my stars.  Any idea what may be causing the drifting, alignment error?  Mount is level and I did drift alignment in PHD, was spot on at least until I slewed to target apparently.  Wonder if something goofy going on with mount or have wrong setting in PHD.  Attached is a snap shot of PHD2 guide log viewer if anything jumps out at you.  Thanks for any help.

[alacant]

Excellent. To my untrained eye, the guiding looks good, star wise at least. I suppose you've lost some of the frame due to the drift, that's all. Try without the reducer to fill more of the frame perhaps? The best we have is a miserable-by-comparison 1200mm focal length:(

[vlaiv]

How do you guide? Slow differential flex can cause such drift over 2.5 hours, it might not be obvious in single frame, especially if short (2 minute) subs were used. There were 75 2-minute frames in 2.5 hour.

I estimate around 380px shift between images, that would give roughly 5px shift in each sub. Looking at star shapes in crop that sounds about right, they are elongated by 4-5px (~8-9px major axis, ~4px minor).

 

[MilwaukeeLion]

4 minutes ago, vlaiv said:

How do you guide? Slow differential flex can cause such drift over 2.5 hours, it might not be obvious in single frame, especially if short (2 minute) subs were used. There were 75 2-minute frames in 2.5 hour.

I estimate around 380px shift between images, that would give roughly 5px shift in each sub. Looking at star shapes in crop that sounds about right, they are elongated by 4-5px (~8-9px major axis, ~4px minor).

 

I guide with small 50mm scope 162mm fl with ASI 120mm-S ccd.  So OAG needed to fix drift?

[ollypenrice]

An OAG is the way to go with any long FL reflector. 

Olly

[bobro]

The guide report shows dithering - perhaps this has a direction bias?

 

[vlaiv]

11 minutes ago, MilwaukeeLion said:

I guide with small 50mm scope 162mm fl with ASI 120mm-S ccd.  So OAG needed to fix drift?

Yes, as Olly already said, OAG is going to be the right thing. You can check guide scope connection to main OTA, in normal circumstances you should be able minimize differential flex just by making sure mounting is tight and does not move, but since we are talking C11 here, there is a possibility that guide scope is well secured and that it is actually main mirror of SCT that is doing the moving. OAG fixes this because it also gets light from main mirror as imaging camera, so any motion of main mirror will be picked up by guider and reacted to it.

[MilwaukeeLion]

7 minutes ago, bobro said:

The guide report shows dithering - perhaps this has a direction bias?

 

That's a good question, didn't even know I was dithering.

[vlaiv]

10 minutes ago, bobro said:

The guide report shows dithering - perhaps this has a direction bias?

 

I don't see dithering, peaks in graph fit with Peak RA and DEC errors so I think those are normal guiding errors (wind or shake or something).

[MilwaukeeLion]

10 minutes ago, vlaiv said:

Yes, as Olly already said, OAG is going to be the right thing. You can check guide scope connection to main OTA, in normal circumstances you should be able minimize differential flex just by making sure mounting is tight and does not move, but since we are talking C11 here, there is a possibility that guide scope is well secured and that it is actually main mirror of SCT that is doing the moving. OAG fixes this because it also gets light from main mirror as imaging camera, so any motion of main mirror will be picked up by guider and reacted to it.

This makes sense because I can get 5 min perfect stars with my 1200mm refractor.  Probably has to do with SCT mirror.