Re: [APML] Knife edge focusing question

["bert katzung" <katzung1@home.com>]

Hi Howard:
Thanks for the input; looks like this focusing thread is of general
interest. I have tried a ronchi in the form of the Stellar Tech II unit from
Richard Shell. I was surprised to find that I couldn't zero in on the best
focus nearly as well with it as I can with the B&K Astrofocuser that I have
been using. Instead of snapping from inside focus to right-on to outside, I
saw a gradual change of the band pattern from many to one to many over a
huge focusing range. My unit also suffers from a very tight Nikon bayonet:
so tight that removing the focuser to put the camera on almost always moves
the scope. (For sale: one ronchi focuser <G>.)
Thanks again....
Bert

Bert Katzung
katzung1@home.com
www.astronomy-images.com

----- Original Message -----
From: "Howard Edin" <hedin@grapevine.net>
To: <astro-photo@seds.org>
Sent: Thursday, 02 August, 2001 6:45 PM
Subject: RE: [APML] Knife edge focusing question


> Bert,
>
> I find it easier to use a ronchi screen for focusing. Admittedly I have a
> very poor laser printed screen, which is mounted ala Rockett Crawford's
> knife edge design ( see http://web2.airmail.net/capella/myknife.htm ). I
> find this easier to master initially and is the same principle as a single
> knife edge - it just helps you see where focus is both inside and outside
of
> it (I believe you saw Chuck's excellent article on focusing where he shows
a
> ronchi screen in use). I'd recommend you try it if you can't find focus
with
> a plain knife edge.
>
> The funny thing is that even a poorly cut or printed line will work as a
> focusing mechanism it's just not always as obvious as a perfect knife
edge.
> I'm using an old Televue Genesis (non-SDF) at f/5 so I find the minute
> amount of focus adjustment required to get it perfect using the little
focus
> knobs is the main problem (I'm currently trying to find a good big - say 3
> inch or more knob to use). While it's not the greatest my last M8-20
shot(s)
> turned out pretty good IMHO  ( see
> http://www.grapevine.net/~hedin/m8-20.html ohh, and comments are welcome
> 8-) )
>
>
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