Lee
30-03-2013, 09:35 AM
I picked up a 200mm f/4 "imaging-optimised" Newt from Bintel this week, the carbon fibre version. Only had some minimal first light last night, in short lived sucker holes, but did notice some concerns that may affect some people.....
If you buy one of these to use your DSLR for prime focus work, I suspect there still may not be enough in-focus. My little Atik-420 CCD reached focus no problem - but its sensor is 13mm inside the unit. It reached focus with the focuser racked out at 10mm or so, add the depth of the 1.25" adaptor (10-12mm or so), maybe a couple of mm for the 1.25mm nose piece flange on the camera - infinity focus is likely falling approximately 35mm or so outside the fully racked in focuser tube....
From what I've measured/read, DSLRs tend to have their sensors 40-odd mm inside.....
Low-profile focusers will likely still be needed.... unless my measurements are off/wrong..... I'll have to add a moonlite to get enough back-focus to use my filter wheel.....
So - what actually defines an "astrograph" newtonian??
To add - my first impressions are favorable - the scope has a nice finish, the focuser feels very sturdy, without noticeable slop, and coma on my small chip not particularly noticeable - once I get some decent longer exposures I may revise this though!
If you buy one of these to use your DSLR for prime focus work, I suspect there still may not be enough in-focus. My little Atik-420 CCD reached focus no problem - but its sensor is 13mm inside the unit. It reached focus with the focuser racked out at 10mm or so, add the depth of the 1.25" adaptor (10-12mm or so), maybe a couple of mm for the 1.25mm nose piece flange on the camera - infinity focus is likely falling approximately 35mm or so outside the fully racked in focuser tube....
From what I've measured/read, DSLRs tend to have their sensors 40-odd mm inside.....
Low-profile focusers will likely still be needed.... unless my measurements are off/wrong..... I'll have to add a moonlite to get enough back-focus to use my filter wheel.....
So - what actually defines an "astrograph" newtonian??
To add - my first impressions are favorable - the scope has a nice finish, the focuser feels very sturdy, without noticeable slop, and coma on my small chip not particularly noticeable - once I get some decent longer exposures I may revise this though!