CCD Inspector

[martin_h]

Could someone with more brain cells than me tell me what I'm looking at here. I'm assuming its telling me that the field is not quite flat(FF/FR needs tweaking) but overall there is not much tilt in the system??

Right, wrong, close?

[Tinker1947]

Will watch with interest....)

[Oddsocks]

You are close....

What the image map tells you is:

Tilt:

The total tilt reported is low at only 1%, which should be very good but strangely I've never seen very high total tilt scores on any of the tests I've run so I tend to ignore that score and focus on the individual tilt in X and tilt in Y scores.

The colour map shows the tilt is skewing the FWHM across the image unsymmetrically, the focus will be good in one corner and getting progressively worse as you reach the diagonally opposite corner of the image.

Tilt should be addressed by adjusting the camera chip orthogonal to the optical path to bring the colour map back into symmetry, try to get tilt in X and Y as small as possible.

I ran a CCD Inspector test for Tinker1947 yesterday, if you look at his tilt readings they were only 0.1 arcseconds which is quite remarkable for a SCT: http://stargazerslounge.com/topic/205755-c11-betelgeuse/

If you can correct the tilt then when you focus on a star in the centre of the field the defocus due to curvature will be symmetrical about the image centre.

Curvature:

Your curvature measurement is 17.5% which is the percentage difference between the lowest and highest FWHM star values measured across the image map.

By increasing or decreasing the distance between the flattener and the CCD you may be able to reduce the curvature more, some flatteners are better matched to one make of telescope over another so it is a matter of tweaking the distance a little a time, so that the maximum and minimum FWHM values get closer together and as they do so then the percentage score will fall, but watch the map shape for distortions if you go too far away from the optimum distance, the 3D map is best for this and will show a hump or well in the centre when the spacing distance is overcooked.

If the optimum flattener to CCD distance is already met then the difference between the maximum and minimum FWHM values and the percentage score will only increase whichever way you change the distance.

My own experience with CCD Inspector has been that sorting out the tilt in the system first leads to a reduction in the curvature percentage as well, I guess that would be logical since the curvature score is derived from the FWHM values in the image and if the chip is tilted even slightly then FWHM would be quite different on one side of the chip against the other.

Collimation:

Strange reading that one, at 16.5 arcseconds reported I would have expected the central cross hair to be displaced to one side, I think this may resolve itself once the tilt is improved.

So I would say, sort out the tilt first and take another test image before making changes to the spacing, it may well be that the curvature score will drop anyway and hopefully the odd collimation result would sort itself out at the same time.

CCD Inspector definitions:

Min FWHM = Lowest star FWHM measured on the image map

Max FWHM = Highest star FWHM measured on the image map

Curvature = Percentage difference between lowest and highest star FWHMs on the image map

Tilt in X, Y, = defocus from left to right and top to bottom expressed in arcseconds or pixels

Total Tilt = The absolute amount of measured tilt in the image described as a percentage and its direction shown in degrees and by the pointing arrow

Collimation = The distance between the physical center of the image and the optical center of the image shown in arc seconds or pixels

Map Colours:

Black = Lowest FWHM

Blue = Slight Defocus

Green = More Defocus

Red = Highest Defocus

[martin_h]

Hummmm it seems that sag in the system is the main problem and I dont quite know how I'm going to sort that one out. I have the WO ZS71 and the reccomended FF/FR,BUT it doesnt fit properly into the focuser..ie it doesnt go in far enough so that when he pressure screws are tightnend the brass compression ring sits half over the groove and tilts the whole lot. I might have to find someone with a lathe and get them to shave of a mm or two off the back end to allow it to go in further.