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Old 04-09-2011, 08:51 PM
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DavidTrap (David)
Really just a beginner

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Join Date: Jun 2009
Location: Brisbane
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Mini Review - Focal Reducer for Meade ACF 8inch

I am reporting my initial results with an Astro-Physics CCD Telecompressor (CCDT67)

As many will be aware - reducing the focal length of a corrected SCT is not entirely successful with a standard 0.63x focal reducer/flattener as the image may become "over-corrected". There is a distinct lack of advice on the net about what does work. There has been some success reported with the AP Telecompressor - advertised as a reducer only, not a flattener.

The amount of reduction and the size of the illuminated circle are dependant on the distance between the reducer and the imaging plane. There is a calculation on the AP website. I worked out a distance that would give me about 0.75x reduction and an illuminated circle ~44mm in diameter. I needed a circle this large to illuminate the OAG prism on my QSI583 - even then it is not fully illuminated. I did not order the recommended AP16T nosepiece as it would have given more reduction, but a smaller illuminated circle - I had a custom adapter made by Precise Parts which is much shorter than the AP nosepiece.

I finally deforked my LX-90 and put it onto my equatorial mount. After much waiting for the clouds to clear last night, I was able to shoot some images (just random star fields - that was all the sucker holes would allow - tonight, of course is much clearer but it's a school night and I have a long day at work tomorrow.) I've attached some images and field curvature plots from CCD inspector which show a few things:

The native focal length as reported by plate solving is 2222mm, with the reducer in place it is 1740mm. This is a 0.78x reduction, which is pretty close to what I calculated using the formula on the AP website.

The CCD Inspector plots (although with limited stars and through horrible seeing with last nights rain in SEQld) are remarkably similar for both the native and reduced focal lengths. I surmise this means the lens is indeed a reducer only and not a flattener. The central 80% of these plots are reasonably flat (see 3D plot), and I suspect the turned up corners might be due to some vignetting. There is a definite central brightness to the images, which will hopefully improve with flats. These images have only had dark frame calibration and a light stretch. I'm fairly happy with the flatness of the field from the ACF scope - although I will hold my judgement on that until I've had a chance to calibrate the images with some flats.

Guiding wasn't the best - I'm thinking seriously about a Lodestar guider to get something more appropriate for OAG. I had to do 8 second exposures at bin 2x2 with my QHY5 to get a smudgy star on which Maxim could guide...

Overall, I'm happy with my first efforts. This reducer is only appropriate for a small CCD (e.g. KAF-8300). There are larger reducers available from AP that screw onto the larger backs of 10 & 12 inch SCTs, which should yield a larger illuminated circle. As far as AP bits go, the CCD67T is reasonably priced too at USD$148.

Comments are greatly appreciated,

DT


The images are:
Native focal length, 60 secs, bin 2x2
CCD inspector plot

Reduced focal length, 60 secs unguided, bin 2x2
CCD inspector plot

Native focal length, 300 secs guided, bin 1x1

3D plot of Reduced focal length, 60 secs unguided bin 2x2
Attached Thumbnails
Click for full-size image (2222mm bin 2.jpg)
136.2 KB65 views
Click for full-size image (2222 bin 2 plot.jpg)
91.4 KB57 views
Click for full-size image (1740mm bin 2.jpg)
117.5 KB64 views
Click for full-size image (1740 bin 2 plot.jpg)
130.9 KB52 views
Click for full-size image (300sec bin 1.jpg)
147.4 KB61 views
Click for full-size image (3D plot.jpg)
88.1 KB51 views

Last edited by DavidTrap; 05-09-2011 at 01:07 PM. Reason: added a few bits
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