View Full Version : Finding focal point

Terence Baz
April 14, 2008, 02:58:22
I recently bought a DMK 21AU04.AS camera. I have an old Super C8 and a Newtonian f/5 scope.

I have used the C8 in the past with Konica ASA 3200 film to take images of sun spots using a Thousand Oaks Type 3 solar filter. I took both prime focus & eyepiece projection photos (using a tele-extender) & I was pretty happy with the results.

Last weekend, I tried to use the digital camera doing the same thing but I could not find the focal point for the camera. Eventually, I used an Orion Steady Pix adapter with the camera but had to bring the camera as close as possible to the visual back of the Celestron but even then, I could not get the camera close enough to make the image sharp.

I thought I would have better luck with the Newtonian trying to image Saturn the following night but the same thing happened. In both cases, I even had to take off the camera's eyepiece holder to get it closer but still the image was not sharp enough.

Both scopes are well collimated with excellent optics & I can obtain very sharp visual images with both of them. What am I doing wrong? Any suggestions? I thought that finding the focal point would be the least of my problems.

April 15, 2008, 07:15:42
Try to do prime focus, no need to do eyepiece projection or afocal with a DMK.

Terence Baz
April 15, 2008, 19:28:55
Thanks very much for taking the trouble to reply. Yes, I used prime focus with both scopes for the camera initially. For some reason, the Sony chip seems to need to be closer to the secondary mirror for both scopes. I don't know why because, when I use eypeices for visual use, the focussers on each scope are able to find the focal point without any problem. The skies will be clear here again tonight & I hope to have the chance to try again.

April 17, 2008, 03:33:06
I suggest you to try during day time.

Actually, the focus position of my DMK is very close to one of my eyepieces (a Japanese made 20mm Widescan Type III), so it's not particularly demanding.

Terence Baz
April 17, 2008, 22:26:24
Thanks again. I went back to basics this afternoon & directed my 16" f/5 on a neighbor's house across the road. It is about 75m away. The camera focussed on this just fine. Will have to see how this compares to infinity. I moved the focusser about 1.75" or 55mm out from its base so, hopefully there will also be enough tolerance for it to reach infinity.

June 24, 2008, 06:54:17
I am having the same issue. I can't get to a good focus. Please see the attached JPG. This is my best effort so far. I've opened a problem with the manufacturer and I am going to call the store (OPT in Oceanside CA) to see if they can help me errrr us.

If you learn anything about this, please write to me CAPTMGIK@san.rr.com
Jim in San Diego