Observing Notes 2nd May 2016 - Focus Experiments With Barlow and Focal Reducer

Had a look at Eta Cas, beautiful double in x72 eyepiece. Also Alphard (Alpha Hya), a bit too bright to be used for collimation!

Next I wanted to test the various focussing possibilities of my prime focus astrophotography setup as I was getting confused about what worked and didn't.

I have a Celestron 9.25" GPS SCT, using in alt-azimuth mode. Into this I like to screw in first a Celestron f6.3 focal reducer, then a 1.25" visual back, then a standard 1.25" mirror diagonal, and then eyepieces or photo gear.

On the camera side, I have a Nikon D3200 DSLR connected using a Celestron x2 1.25" Barlow and a T-Ring Adaptor for the camera. I also have a second x2 Barlow tube.

So, the results (when I say Barlow I mean Barlow and tube) are:

Visual back - diagonal - Barlow x2 - camera: focus OK
Visual back - diagonal - double Barlow x2 + x2 - camera: focus OK
Focal reducer - diagonal - Barlow x2 - camera: focus OK
Focal reducer - diagonal - Barlow tube only (no lens) - not enough back focus. Even removing the 1"-ish steel ring from the Barlow tube doesn't help, the focal plane of the camera still needs to be closer to the scope.

So basically I can't get enough back-focus to focus the camera with the focal reducer AND diagonal in place without a Barlow lens in the chain. So with the focal reducer I'm increasing the field of view, but with the Barlow lens I'm then reducing it again, which is not what I want for DSOs. The only way with my current setup to take pictures with the focal reducer in place without a Barlow lens is to remove the diagonal to reduce the length of the chain. It's then hard to see what I'm doing without lying on the ground!

Took some stills with one x2 Barlow and the focal reducer in place. Did at least get an image, focussing could have been improved:

Imaged the area around 16 Cyg; I was hoping to get some decent Milky Way stars in here. Single 30s exposure, again need to work on focussing:

I had another go at imaging M94 without success.

I tried imaging the double cluster in Perseus. This is one of the most beautiful sights in the sky, but both clusters are too much for the field of view with the DSLR attached via a tube (no Barlow lens) and without the focal reducer. So I tried to image the two clusters NGC 869 and NGC 884 separately. The results were mixed. Again focussing is the big problem.

NB Deep Sky Stacker wouldn't stack the frames unless I resized them from the original 4512 x 3000 pixels to 3008 x 2000 pixels in Photoshop first (use batch resizing option and create maximum quality JPG files). Do not set star detection threshold below 5% in DSS, you get loads of spurious noise.

Anyway, focussing was so out (and I had mucked about with collimation earlier) that all the stars look like footballs, so I won't include any images here.