I am collimating my rcos 16 truss. I have a 70mm ffc installed with the 2 inch backspacer inside my the rotator with a stl-11000. This was from a 1x1 binned image

The attached file is what I get with ccdinspector.

The stars look good binned 2x2 but there are problems on the left of the image on 1x1 binning.

Is something out of line or is this about the best I can expect. It is much better than without the ffc.

perfect can be the enemy of good

thanks,jeff

screen_print.JPG (78 KB, 21 downloads) ccdinspector

Hi Jeff,

Looks like there may be a slight tilt to the image plane. It appears to be diagonal from top left to bottom right.

The newer version of CCDInspector, v2, tells you the exact direction and amount of tilt. Version 1 showed only the tilt in X and in Y directions, which would not show a diagonal tilt like the one you have.

A 20% curvature is not bad, and should be barely noticeable, but I would try to eliminate the tilt, if possible.

With proper flat field corrector adjustment and spacing, I've seen the larger RCOS scopes produce less than 8% curvature. This gets the scope into competition with the likes of TAK FSQ106 in terms of flatness of field: FS106 so far holds the record for the lowest curvature I've seen of any scope, measured at about 5%.

Regards,

-Paul

Hi Jeff,

I've been doing some measurements on my 16RC with the 82 mm FFC. Good FFC performance requires excellent collimation. I get pretty close using CCDInspector's defocused star collimation tool.

I measure FWHM's iin Mira Pro individually near the centers and corners. It looks like the worst case for my KAF16803 (Apogee U16M) is around 23%. If I scale the image down to KAI11002 size, the worst case spot growth is almost immeasurable - maybe 2%. However, using the curvature measurement in CCDInspector, I get a larger indicated curvature. See attached screen capture.

So there is some discrepancy. Perhaps Paul could comment.

KAI11002_curvature.gif (32 KB, 19 downloads)

Paul,

Here is a link to the FITS file in question.

Hi John,

CCDInspector extracts stars from the whole field of view to figure out curvature. Curvature is measured from minimum to maximum FWHM determined in the whole frame.

The best way to measure this cleanly is to image a short exposure star field, with no background objects (and extra-galactic star fields <g> ) mixing in to make PSF determination more difficult.

In your crop, the stars towards the bottom left corner appear to be in the range of 2.15 - 2.20 arcseconds, while stars in top right corner are 2.0 arcseconds or less. This may have nothing to do with the optical properties of your system, but rather caused tracking/guiding and seeing variations during the long exposure.

Regards,

-Paul

Thanks,

So I understand that my collimation is probably good at 1.7" with 1.7fwhm stars.I guess should leave my mirror adjustments alone.

Could part of this be due to a misalignment of my optics in relationship to the axis of my scope ?

I will experiment with different readings with the chip at various degree's of rotation using my rcos rotator.

To be honest, I am not sure that I even need to image with a 1x1 binning using my scope/camera combination.The images are quite large. At 2x2 binning the stars in the corners look quite good.

thanks,
jeff

Jeff,

Of course, if the stars look fine to you, there's no need to adjust anything! Just because CCDInspector can measure some minute difference, doesn't mean that it needs to be corrected

Regards,

-Paul