Bloated Stars

[Petesandie]

Can anyone help with this Why are my stars looking like this. Equipment using Explore Scientific 102mm triplet scope Altair 0.8 Reducer. Atik 414ex ccd color I've tried every combination of spacer for back focus  from 10mm to 50mm.  Bahtinov mask to get focus on bright star [spot on]. Exposure time from 1sec to 2min  Just cannot get stars pin point Thanks in advance Pete 

[RayD]

Most of the stars look like donuts, which would typically point to focus.  I appreciate you said you did it with a Bahtinov mask, but that's my initial thoughts.  I would also check the spacing as you note you have tried 10mm to 50mm, but that reducer needs 55mm, and maybe even a little bit more at f/7.

[Ben the Ignorant]

You focused well and then focuser crept because of heavy load, or not enough friction?

[michael8554]

5 hours ago, Petesandie said:

Bahtinov mask to get focus on bright star [spot on].

Maybe a dimmer star would give you more critical focus.

But Ben the Ignorant may be mis-named ..........

Michael 

[don4l]

Is that a processed image?  If so, do the individual frames have the same "doughnut" effect?

The smaller stars look like they are in focus.  I've seen similar effects with some noise reduction algorithms.

 

 

[Petesandie]

Individual frames show donut stars. Processed 5 frames to show outcome. No darks or flats 

[Uranium235]

May be a creeping focuser, depending on the load and how high up its pointed.

You may need to tweak the hex screws under the focuser and see if that improves the situation. 

[Davey-T]

If the flattener needs 55mm and the camera has 13mm back focus spacer should be around 42mm.

Wrong spacing doesn't give you uniform faults across the frame so as said it looks like the focus is moving after removing the B'Mask.

Dave

[RayD]

Yes, sorry, I should have made my point clearer.  I was referring to spacing rather than the actual spacer required to get the right spacing.  

As noted this isn't the issue here and is just a secondary comment as it was mentioned in the OP. 

 

[Thalestris24]

Spacing is probably not an issue as it's such a small chip. But focussing could change with temperature or simply as a consequence of tightening the focus knob... Certainly my ts triplet can take a long time to temperature stabilise and I've fitted an electronic motor focuser which is a boon

You can also get dots in stars from dss stacking with hot pixel detection and removal but if individual subs have the problem then it must be a focus issue.

The sensor is also very sensitive so would probably benefit from shorter exposures though the op says has tried very short exposures.

Louise