|
Hi Pete,
How is he planning on autoguiding then? With a guidescope? Differential flexure will be the major issue. Not impossible to overcome but when I used my guidescope I got terrible flexure. But then again my guidescope isn't the absolute best.
Per Planewave PDF on the reducer it only works with ST cameras and CFW or STL with no external CFW.
It does not work with Apogee or FLI cameras and CFW although I was told in an email it would work with my Proline and CFW but not through the MMOAG.
Conflicting data.
I imagine that it does not work with a Proline and CFW as that would be 30.23mm + 20mm = 50.23mm.
Per their chart the reducer can be spaced on a CDK17/20 (a different reducer for the 17/20 from the 12.5 inch CDK) from 43mm to 52mm.
Ideal spacing behind the reducer is 47mm. There, spot sizes are 7 micron stars and F4.51.
At 51mm you get 8.7 micron stars and F4.45. So you could whack one in front of the Proline and CFW4/5 (not CFW 4/7 which is thicker - I think its more like 25mm thick versus about 19 or 20mm for the CFW4/5). If you tried using your PL and CFW 4/7 it won't come to focus per their PDF.
So Proline + CFW 4/5 gives 50.23mm or perhaps 49.23 (have to measure it, CFW 4/5 is either 19 or 20mm thick, its probably on the FLI website).
But now you have no autoguider and have to use a guide scope. But I suppose you are now back down to 1952mm focal length so a longer focal length very rigid guide scope would work. Perhaps one of those AP 900mm focal length jobbies. I also have a Vixen 95LMC with about 1m focal length. Getting a guide star those seems hard work at that focal length.
On the other hand an STL11 with no external filter wheel or ST series cameras with a CFW work. But you wouldn't be able to use them with an AO device which is a shame as that seems to be the main drawcard for using an SBIG camera.
QSI with built offaxis guider would work I am pretty sure. A series 600 QSI is a bit more like a FLI camera now with a built in filter wheel and decent cooling and an offaxis guider. You are limited in chip choices though (smallish chips).
Perhaps SX cameras and their AO unit may be slim enough.
Basically its got to be slimmer than 51.8mm and ideally only 47mm thick.
I can email you the 2 pdfs I have on this.
I have pretty much ended the thought of using one. Unless I came up with a good autoguiding solution to 1952mm focal length. Its not totally dead as perhaps a better guidescope than what I have been using may work. I should chuck on my Vixen and see if that does it. Also use the SX Lodestar instead of the ST402 to reduce weight and flex.
Of course at 1952mm focal length and F4.45 a Proline 16803 may be in the ideal zone for pixel size and seeing. At 3 arc second seeing average you would be perfect with .96 arc secs/pixel with that setup (going for 3X sampling ie. 3 x .96 = 2.88 which is close to your seeing of 3 arc seconds). So would an STL11, both have 9 micron pixels.
So I guess the issue is you need an STL11 to match the optics with the focal reducer (smaller pixels are a slight mismatch) and self guiding or a slim line camera with an AO (Starlight Express?? or QSI but only small chips).
If you end up using a smaller chip like an 11002 or 8300 or 3200 etc then what is the point? You got a wider FOV but back to less than what a PL16803 covers without a reducer! Only gain is what you are imaging is at a fast F ratio. But then I find the CDK17 exposes images very quickly anyway because of the large aperture.
Conclusion - if you are using an 11002 or smaller chip the reducer is of little use and does meet the current needs of imagers and is too limited and the same or better results would be achieved by using a larger chip anyway. Backfocus is too limited. Probably a consequence of the CDK design with its corrector.
However, if you did have a reliable guidescope setup and you were using a large chip like the 16803 then F4.5 and a super widefield may be still appealling. Perhaps you may have to resort to 5 minute subs to prevent tracking errors from differential flex.
I am not sure how the SBIG STX fits into this. But I imagine it is also too wide and requires an external CFW as well.
Greg.
|