Observing Notes 3rd May 2016 - Piggybacking and Neodymium Filter

I spent some time trying to improve the collimation on my scope and then turned it to Jupiter. The seeing was slightly better than that I've been experiencing lately and I could see a bit of detail visually in the x261 eyepiece. So I stuck the D3200 on and took 2 3-minute videos, one with a single Barlow and one with a double Barlow arrangement. Both videos were shot at 50 FPS; the double Barlow video was definitely dark (underexposed).

These were the results I got by stacking all 9000 frames from the single (x2) barlow video. I don't yet know what I'm doing with the Registax software:

After several fairly disastrous nights attempting prime focus photography, I decided to concentrate tonight on piggyback photography using my Nikon D3200 on the back of the Celestron 9.25" alt-az mount.

After retrieving the piggyback bracket from my accessories case, I went on a hunt for the appropriate screws to mount it in the 2 screw holes on the top of the rear mirror assembly. The holes had the 2 short screws in, I needed the slightly longer ones to attach the piggyback bracket.

NB If I take the bracket off again, I must remember to replace the two original short screws in the holes; the longer ones will screw in further without the bracket in place and could chip the primary mirror.

So the plan for tonight was to (1) try my hand again at DSLR piggyback photography on a driven mount (last time I did this was with a film camera) (2) practice my focussing technique and (3) experiment with the Hoya red intensifier (Neodymium) filter effects on the images produced with the 55m focal length lens on my Nikon D3200.

I decided to image (1) Cassiopeia and (2) Lyra as they're both fairly compact constellations and should both fit nicely in the field-of-view provided by the 55mm lens. I would also image each both with the Hoya red intensifier and without and compare the results.

Next to focussing. I made some progress on this. Rather than trying to adjust focus in the live view screen on the back of the camera (using the zoom-in magnification), I found the following technique much more successful. Set the camera lens on infinity. Take a 5-second exposure. Then view the image in live view and zoom-in and inspect the focus. Turn the lens focus ring a tiny anti-clockwise turn and repeat the process. Repeat until the focus for the 5-second image appears as good as you can get it. Now set the camera to the desired exposure time (e.g. 30 seconds, a minute, whatever) and start imaging in earnest.

I eyeballed the camera in place on the bracket on top of the scope and did a goto to Navi (SAO 11482), the central star sort-of in Cassiopeia. BTW, this star was renamed after Gus Grissom, the brave astronaut who died in the Apollo 1 fire. His first name was Ivan. I've been to his grave near the top of the hill at Arlington Cemetery (note to self, post photos I took on this blog). Using the goto to his star worked well, Cassiopeia filled the field of view nicely. Here's a stack of ten 15 second images using the Neodymium filter:

And the results from Lyra, same parameters but this without the Neodymium filter:

To summarise, the results were interesting. Basically, the filter appears to reduce the reddish-yellow skyglow, but at the expense of losing fainter stars. But the benefits were more apparent in objects closer to the horizon (where the skyglow is stronger). This is rather subjective, but I felt the images of Cassiopeia (close to the horizon in the north) were better WITH the filter, but the images of Lyra (much higher in the eastern sky) were better WITHOUT the filter.

You can remove a lot of the skyglow in post processing using Photoshop, so it's a bit of a toss-up whether to use the filter or not. I think my rule in future will be to use the Neodymium filter if the subject is close to (less than 30 degrees above) the horizon; otherwise, image without the filter and adjust any skyglow in Photoshop. You can do something about the skyglow in post-processing, but you can't invent stars that aren't there in the original image!

Or to put it more simply, use Mk 1 eyeball. If the sky looks reddish-yellow where your imaging target is, use a Neodymium filter. Otherwise leave it off and adjust in post-processing.

I had a go at imaging Delphinus, taking ten 15 second exposures without the filter. It was very low down in the sky and the camera lens had started to dew by now, but here's the result:

To finish off the evening I had some lovely views of M57 in Lyra and the double cluster in Perseus, both with the focal reducer in. I'm thinking that, if it wasn't for the focussing problems I experience when photographing, I would leave the focal reducer in all the time.