View Single Post
  #9  
Old 27-05-2011, 03:02 PM
samirkharusi's Avatar
samirkharusi (Samir)
Registered User

samirkharusi is offline
 
Join Date: May 2011
Location: Oman
Posts: 3
None. Use tiny pixels.

You did not say what kind of imaging you are after. If it is deep sky or birding, using Barlows or tele-extenders have had their day. Technology has moved on. If you are after planetary imaging you ought to go with a Powermate that takes you to f25 to f35, or thereabouts (depends on the pixel pitch of your camera). Let me explain.

For DSOs and for birding we need the captured images to be sharp (i.e. not waaaay over-sampled). Most refractors, even mucho $ premium APOs deliver stars that are no smaller than, say, 8 to 15 microns in diameter, worse off-axis. So if you have a camera sensor that has pixels in the 4 to 5 micron range, you are already at Nyquist, for that OTA. So, to eke the most resolution out of your lens/scope for a few to several hundred $, it is far more effective to upgrade your camera to one of the latest, small pixel DSLRs, eg Canon 600D, 60D, etc. A side bonus is that you do not lose any f-stop in doing so. For older DSLRs with 6 to 7 micron pixels it was still attractive to use 1.4x extenders, but not 2x. It's only with the earliest DSLRs with 9 micron sensors (and film) that 2x tele-extenders were useful. Ditto for 2x Barlows.

Now, for planetary imaging the point to start is with video capture, not stills. So one needs a webcam or a DSLR with LiveView. Next we use only the on-axis performance of the OTA or camera lens. On-axis performance can be MUCH better than even slightly off-axis. The cheaper semiAPOs have too much field curvature and if you use both a field flattener and a Barlow you are entering uncharted waters, presumably quite murky. Hence for planetary we are chasing the diffraction-limited theoretical resolution of the OTA on-axis and we do not mind if the captured videos are all bloated and fuzzy. The sharpening comes in post processing a stack of several hundred frames. So for planetary capture with 5 micron pixels we would need to be at f20 or higher magnification (basically > 4x the pixel pitch in microns). Hence 2x Barlows are not sufficient. We would need 3x to 5x Power Mates for such use. More on all this and other stuff on my website:
http://samirkharusi.net/
Direct links to testing tele-extenders:
http://www.pbase.com/samirkharusi/te...tele_extenders
Magnification required for planetary webcamming:
http://samirkharusi.net/sampling_saturn.html
Deploy that itching credit card on a camera upgrade, modded of course, rather than on a 2x. Besides, you'll get reinvigorated to take more pics of the family while awaiting nightfall
Reply With Quote