Re: What do I need for astrophotography?

[Herb Chong]

i realized from some private email that this description of the Pentax 
PF-CA35 adapter is a bit misleading. the objective focal length of the 
PF-100ED is 650mm, but with the PF-CA35 attached, Pentax rates it as a 
1250/12.5 lens. unlike the older Pentax telescope adapter, the PF-CA35 has a 
lens in it. that lens, in my opinion, accounts for much of the flare and CA 
that i see. for high contrast subjects like birds or branches against a sky, 
it's pretty nasty. for lower contrast scenes where there is no sky in the 
background, the flare and CA are much less objectionable.

Herb....
----- Original Message ----- 
From: "Herb Chong" <[EMAIL PROTECTED]>
To: <pentax-discuss@pdml.net>
Sent: Tuesday, February 01, 2005 6:38 AM
Subject: Re: What do I need for astrophotography?


telescopes have a much lower f-stop than you might think. the Pentax 
PF-80ED is a 600mm/12.5 with the PF-CA35 adapter. quality might also be 
lower than you might expect too. my PF-100ED is a 650/12.5 with lots of CA 
and flare. i get a much more light out of my A* 400/2.8 with the 1.7X 
extender and much less flare and CA. i use the PF-100ED for birding some 
of the time, but i have never gotten anything publishable with the camera 
attached. it is an excellent scope though. if your friend's telescope has 
a tracking mount for long exposures, the small aperture is less of an 
issue.