Ballard Loc: Grass Valley, California

Early this week I tried an experiment where I put the .7x focal reducer I had for my refractor onto my Meade F10 Schmitt Cassegrain telescope to see if it would work. This reduced the focal length from ~4000mm to ~2800mm. The biggest effect was the extremely strong vignette at the corners of the field for which the flat frames didn't completely compensate correctly for. The other thing I found is that if a very bright star is just outside the field of view it can cause annoying internal reflections in the image. The first image of horsehead shows these effects in a stacked HA image taken with 10 frames at 900 seconds each (the image was stacked and stretched and no other processing). I believe the bright star Zeta Orionis caused the streaks on the lower right side of the image and the bright corners are due to the overcompensation of the flat frames for the vignette. The problem was worse with a luminance image, so I found a different target. I choose the barred spiral galaxy NGC 2903 in the constellation of Leo for the second image. In this image all the bright stars were within the image so no internal reflects occurred, there was still some bright corners from the flat frames overcompensating for the vignette which I could handle with the appropriate masking and just a bit of cropping.

The image below of the Galaxy NGC 2903 was constructed with the following subs.
15 frames at 500 seconds each using a Green filter.
15 frames at 500 seconds each using a Red filter.
15 frames at 500 seconds each using a Blue filter.
10 frames at 240 seconds each using a Luminance filter.
10 frames at 900 seconds each using a 5nm Hydrogen Alpha filter.

All images were calibrated with Darks, Flats and Bias frames.

All images were shot using a ASI 6800 monochrome pro camera with the Gain at 100 and the temperature set to -20C. The telescope used was a Meade 16 inch LX200 with the .7x focal reducer mentioned above.
The noise is still a bit higher than I like in the image of NGC 2903 and I expect a lot more subs are needed (particularly in Luminance) to clean this up. The red and Ha were added together in a ratio of .7x for Red and .3X from the HA. Note: The HA helps bring out the locations of nebula and star formation within the galaxy (Note: the red knots in the galaxy).

All questions, comments and suggestions are welcome.

Horsehead showing effects of the focal reducer.

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NGC 2903 in the constellation of Leo at 20 million years distant. (One close inspection you will find many more distant galaxies in the image)

(Download)

Michael1079 Loc: Indiana

Beautiful job on these. Thanks for sharing the Filters and types that you used. That is good information!

Ballard Loc: Grass Valley, California

Hi Michael1079
Thanks for checking out the image of NGC 2903 and the horsehead and for the comment. I've got a lot more experimentation to do before I get this Focal reducer to work correctly. I've been looking for one made for the ACF optics used on the newer Meades, but haven't found one yet, but it's fun to try new things even if they don't work out completely as hoped for.

alberio Loc: Casa Grande AZ

If you hadn't mentioned them, I wouldn't have noticed. That is one nice horse you have, looks Arabian.

Ballard Loc: Grass Valley, California

Hi alberio
Thanks for checking out the images of the horsehead, NGC 2903 and for the comment. I have some more testing to do with the reducer to see if I can block the extraneous light flares from bright stars just outside the field of view (maybe some masking just before the reducer or maybe rotate the offaxis guider in case the light is glinting off the pickoff prism). I want to use the reducer for some objects larger objects that don't fit well at prime focus.