Precision about Astro-Physics CCDT67 focal reductor

Equipment Reducer Ritchey-Chrétien
6 replies•473 views
Astrarno avatar
Astrarno avatar
Hello dear Astrobin members,

I think about the Astro-Physics CCDT67 focal reductor for my Ritchey-Chretien 8" telescop.
On the Astro-Physics documentation I can read that the " Image Circle Diameter" is about 29 mm at the nominal 0.67 reduction factor which seems OK for the 22mm diagonal of my camera sensor (ASI1600). But the term " Image Circle Diameter" is not clear . Is it only the diameter with a good geometrical image with round stars or is it also the full light circle diameter without vignetting?
Thanks a lot for giving me this precision.
Clear sky.
Astrarno
Robert Gillette avatar
Robert Gillette avatar
Astrano,

For an authoritative answer, you should contact Astro-Physics directly.  They're very responsive.

I have that focal reducer and find its optics superb. See my recent images of M 51 and M 92, both from an f/13 Mak-Cass reduced to f/8.7

CS, Bob
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Rod Kennedy avatar
Rod Kennedy avatar
OK I'll bite.  I have an RC8, use the Astro-Physics CCDT67, ASI1600, and I have poured over the documentation from AP on this compressor and understand the equations. The image circle in the case is where the stars are good. Vignetting kicks in earlier and you can easily deal with the flats.  For the compressor settings I'll describe in a moment there is a 25% drop-off in the corners relative to the centre.

The compressor was designed for a higher F ratio telescopes, something like F/9 and above.  The 0.67 compression factor for the RC at F/8 is not optimum and not necessary for excellent performance.  If you search around you'll find everyone reducing the compression factor since 0.67 is a bit too severe and the stars will suffer (here 0.67 requires 88mm spacing from the compressor shoulder to sensor for your system that will yield 0.73 arcsec/pixe).  Better to go for 0.75 compression with 63mm spacing (anything around that will work) and that'll give 0.65 arcsec/pixel.  The spacing needs not be exact.  It's a great compressor and works very well over a wide range, but I'd stay away from 0.67.  When I compare theory with practice I get very close with 0.64 arcsec/pixel measured.  It all works great.

Rod
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Astrarno avatar
Astrarno avatar
Thanks Bob and Rod for answering me so quickly.

Of course Bob, I tried to contact AP but up to now I got no answer.

I understood Rod that you explain me that the Image Circle in the AP document is not the full light Circle. I note also your advice to use a less severe compression factor. I note also to use flat field but , however, as you have exactly the same se up than me, do you have an evaluation of the full light circle diameter versus compression factor?
Clear sky!
Astrarno
Robert Gillette avatar
Robert Gillette avatar
Try AP's support contact email.  Let me know if that doesn't work for you. 

Bob
Bob Lockwood avatar
Bob Lockwood avatar
Keep in mind that AP flatteners and reducers are made for AP scopes. Using them on other scopes is just try and see if it works. Call them, they will most likely know it it will work. As for getting with them, I have yet ever not had them answer the phone when I call them.
Brian Boyle avatar
Brian Boyle avatar
Hi Astrarno

I used this reducer with my RC8 and  ZWO ASI1600MM. 

The corrector was fine, but the main problem I found was that the reduction exacerbated the non-flatness of the RC field. Instead, I went on to purchase a 1.0x flattener [not reducer].  Unfortunately I wasn't able to find any flattener/reducers for the RC8.

CS Brian