CCDInspector Trial - Help please with analysis

[Adreneline]

Hi Everyone.

I've downloaded a 30-day trial of CCDInspector and I am using it to assess my Samyang 135mm + ASI1600MM-Pro imaging train.

I took forty 180s Ha exposures last night and put them through CCDInspector and it produced this graphic summary.

Where am I on a scale of 0 (rubbish) to 5 (excellent)?

My imaging scale is 5.8 arcsec/pixel.

Is Curvature purely a function of the optics?

I am assuming unless told otherwise that my Tilt figures are pretty good (less than half a pixel) - or have I got it all screwed up?

The collimation figure of 24.3" would seem to imply a misalignment of ~4 pixels - or have I got that wrong as well? Does 4 pixels matter?

Any help and advice would be much appreciated. I am trying not to get obsessive about this - not sure though if I am succeeding!

Many thanks.

Adrian

[symmetal]

Hi Adrian,

That would rate about a 4 on your scale.   It's pretty good. CCDI assumes higher fwhm figures are due to focus errors and gives tilt and curvature figures on this basis. However using standard lenses there could be other effects like coma giving a higher fwhm figure so it's better to check the star shapes in the corners to assess the reason for the higher figures before you blame it on tilt or curvature.

Dark blue indicates lowest fwhm (assumed best focus) while medium blue is still very good. Ligher blue it's worse but not too bad. When it starts going pink is where you need to check what the problem is and whether anything can be done.

The 3D curvature plot (Ctrl-3 I think) can give a more useful indication but as I mention below CCDI may interpret tilt as curvature so take it just as an indication and not fact.

If you take similar images using LRG or B filters you will probably get quite a variation in CCDI results too so need to see if there is a common 'error' on them all which could indicate a tilt error. On your Ha image it's only very slight (assuming it is due to focus) but it could be worse on blue at the other end of the spectrum.

CCDI doesn't know if an out of focus star is in front or behind the plane of focus so a progressive worsening of focus across the image is assumed to be tilt while focus that is better in the centre is assumed to be curvature, though it could still be tilt with stars on one side in front of the focal plane while thouse on the other side are behind.

As for collimation error I'm not sure what method CCDI uses to calculate it for refractors, or whether it really means anything. I did ask here some time ago what this actually indicated but no-one had an answer at the time.

I've generally found that just looking at the star shapes across the image gives a better assessment than just using the CCDI figures. CCDI assumes all errors are due to focus which in many cases is not the full story.

Alan

Edited March 25, 2020 by symmetal

[han59]

You better solve the tilt in X (between left and right). That reduces the quality. Most likely the stars are on one side a little larger.  Assuming the average FWHM is about 10, the difference is 2.9/10 is 29% Sometimes just remove and replace the camera could help. Or turn it 180 degrees and look to the difference.

Han