From: Milton Aupperle <milton@outcastsoft.com>

Date: May 19, 2010 11:00:35 AM MDT

To: Astro_IIDC@yahoogroups.com

Subject: Re: [Astro_IIDC] Tarantula nebula..NGC 6744.. Jupiter

Mark;

The DSLR will very likely  have a much further back distance to the CCD than a C-Mount IIDC Camera will, depending on how many adapters you have on the end of the OAG to attach the DSLR. So you will likely need to increase the distance from the OAG guide camera by adding extenders. You can physically measure this with a tape measure to get reasonably close (within 5 mm) expected lengths for the start of the OAG to the CCD focal plane.  If the distances to the CCD's are different, you need to add / remove extenders rings to make them parfocal. I already sent you my preliminary unfinished "Guide to using a OAG" which discussed this step by step too.

Lastly did you ever hook up the DSLR and the OAG and then try imaging a bright object (like the moon) or even a distant terrestrial object (trees etc.) as I suggested several times in the past? You need to plan your imaging sessions, not just slap stuff on the scope and expect it to work. 90% of DSO imaging is trial and error, it's usually unique to your hardware, your set up and and observing conditions.

Milton Aupperle

On 18-May-10, at 7:16 PM, Mark Gaffney wrote:

Earlier in the night I`d managed a satisfactory unguided image of the
Tarantula Nebula with my Nikon DSLR. PA was such that with the large
4310 x 2868 pixel chip I simply slewed to it with the HC & there it
was in Live view! I had the camera set at Bulb & ISO 3200 & took a 44
second exposure. Focusing was done on star Acrux previously. I
understand that if I`d set my CG-5 mount to Sidereal tracking it would
have been better still.
Anyway, here`s the Astro IIDC part of it. Later on, early in the
morning I decided to try guiding an image with the DSLR of NGC 6744
just after it`d passed the meridian here. This is a quite difficult
southern hemisphere galaxy. So I set up my CRG OAG with the Scorpion
20SOM in the guide port & tried both short & long size nosepieces. The
focus was right from earlier in the night for the DSLR but try as I
might (even with binning the Scorpion 4 x 4) turning the arm,
adjusting the prism angle & sliding the camera in & out, I couldn`t
find any stars to guide on. I find getting the right settings for the
exposure is like shooting fish in a barrel anyway! There is a real
sparsity of stars about this object so I decided to consign this
project to the too hard basket. At least with the OAG!? (I have a Kwiq
guider in the pipeline hopefully..)
I then slewed back & forth trying to find another interesting object &
nearly settled on the Eagle nebula as a subject (perhaps the best
choice available at this time) but it was slipping behind trees so I
decided to try Jupiter with my Flea 2 colour 640 x 480 pixel CCD. I
was a good half an hour until I found an out of focus blob on the
screen, etc, etc..
I`m attaching a snapshot of the best I eventually got after taking a
number of movies at differing settings (after basic stacking &
aligning in Astro IIDC). I think my collimation is definitely doubtful
for objects at around this FL..
My Sun images from a couple of months ago exhibited a similar problem
with focus..(I will have to seriously look into this..!)
I`ll also attach a version of the DSLR Tarantula so you`re not
entirely disappointed after reading of my post...

Mark.