Re: [APML] First halfway decent cassigrain image

["Bradley" <bhope@thorngarden.net>]

Morning, Ray..and thanks for the comments;

Since it was the first time I'd really used the STI, and with only average
seeing and using Rigel to focus on which is even lower in the sky..I suspect
it was me being off a bit in guessing where center was in the apparant
'depth of field' presented by the focuser. I wanted to stay fairly close to
my target instead of having to slew a distance after setting focus..but I'm
going to try using one of the belt stars even though a bit dimmer..or even
higher, Betelgeuse or Bellatrix, which being brighter might be easier to
use. I offset the star about a third of the way towards the edge from center
of fov..but someone suggested I might need to go a bit further, closer to
halfway out. I could try changing the placement of the reducer but with the
way the off-axis guider is set up I'm not sure I'd have enough adjustment to
be able to focus the reticle at f/10 and get the camera focused at f/6.3 at
the same time. Also, the guider is about the same length as a standard
T-adapter, and I've always seen that setup with focal reducer - adapter -
camera...so the way I'm using it now 'should' be ok...I think. My
understanding is that a real problem with vignetting and field curvature
would result if I tried to use focal reducer - microfocuser - guider -
camera.

Next time I get clear weather (somehow managed to dodge some of the new
equipment/bad weather bugaboo, at least initially) I'm going to bracket the
focus on either side of where I 'think' center of focus is and we'll see. I
probably should have gotten the 180 line/inch ronchi screen when I bought
the STI since my understanding is the 300 line/inch is more susceptable to
poor seeing than the 180...but that was my error and I'll have to live with
it for a little while. Once I find the 'sweet spot' then I'll worry about
field curvature and vignetting issues..though it should be noted the stars
towards the edges are a lot dimmer which would make for sharper appearing
stars. If you'll note the smaller stars around the center, they also appear
'sharper' (to me, anyway).

There are a few ronchi/knife edge focusers out there but the STI is the only
one I know off which is set up for right angle viewing with diagonal and
eyepiece (which can be rotated to different placements) making it much more
convenient to use and gives you an enlarged view of the grating/knife edge.
Being able to change out adapters for different cameras is an advantage over
the long haul as well as being able to switch from ronchi to true knife edge
and back if you want. Since my LX90 died and went to scope 'hell'o just
after I bought the STI I hadn't had a chance to use it till now...but, so
far I like it and will probably like it even better when I get the knife
edge adapter. The STI seems well made and it's very easy to swap out
adapters and screens. I have no problem, at this point, in recommending it
to anyone.

Clear skies

Bradley
48°07'12.72" N 123°27'29.16" W
http://astronomy.thorngarden.net


----- Original Message ----- 
From: "Ray Butler" <ray.butler@nuigalway.ie>
To: "Discussion of Film Astrophotography" <astro-photo@seds.org>
Sent: Friday, January 07, 2005 3:07 AM
Subject: Re: [APML] First halfway decent cassigrain image



Nice shot, Bradley. I would never have the patience to manually guide
such a long focal length!

I wonder if the reason why the focus on M42 is a bit soft, is because of
field curvature? I get the impression that the stars around the
perimeter are sharper (but that could be an artefact related to
processing with vignetting present). If they are sharper away from the
center, it could be because the 0.63x reducer-corrector may not be
operating at the optimal distance from the focal plane...your off-axis
guider between the reducer and the camera might be the culprit. Not sure
what you could do about that, though, except to position your focus star
in the same part of the field that the nebula etc. target will be.

How do you find the STI Stiletto IV? I've been wondering about getting
one, sometime.

Cheers,
Ray


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