More experiments with the 80mm refractor

During the day, I tried the 294MC camera on the 80mm refractor using a 0.5x focal SVBony reducer I just purchased and different adapters (nose pieces).

I got the following results.

Focus	Nose piece	FR	F/ratio	ArcSec/px	FOV width (deg)	FOV height (deg)
Diagonal	N/A	1	5	0.76	0.87	0.60
Diagonal	9mm	0.92	4.6	0.83	0.96	0.65
Diagonal	15mm	0.57	2.85	1.34	1.54	1.05
No diagonal	24mm	0.43	2.15	1.78	2.05	1.40
No diagonal	27mm	0.39	1.95	1.96	2.26	1.54

With a lot of focal reduction, focus can only be achieved with the diagonal removed. With no or less focal reduction, focus is only achievable with the diagonal (otherwise focuser does not come out far enough - and I don't have extension rings for it).

NB: The 27mm nose piece is the one that comes with the camera.

Interesting math:

drift speed with 80mm at f/2 with 7.8x slower than the 250mm at f/5 (=160/1250) => so if I can take decent 1s shots with the 250mm, I should be able to take 8s shots of the same quality with the 80mm
80mm at f/2 requires 1.6x exposure time to capture the same amount of light (=(250/80)^2/(5/2)^2), so 8s shots should be equivalent to 5s shots with the 250mm at f/5.

=> I should be able to take single frames with the same amount of light captured than a 5s frame on the Dobsonian but the quality (drift-wise) of a 1s frame. Assuming I can get 8s in a row without jerky movements (which besides drift is the most common reason for dropped frames), I should be able to get better shots than I have done with the 250mm without focal reducer.

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Things did not go as planned ;-)

I used my photo tripod since the feet are closer together but it did not quite fit on the EQ platform. I did not have a board big enough to stack on top, so I used a board that was long enough but not wide enough and had two feet on the stacked board and one on the platform. I added weight to prevent the whole thing from toppling over.

I started with M42 and 0.43x FR and could not make sense of the picture (see below), then I went to no FR, things were better except for very pronounced vignetting. I tried going to 0.57x FR but could not reach focus, it was somewhere between using the diagonal and not using it. I guess during the day time, my focus must have been slightly off.

I took a few pictures without FR. Being F/5, it would require almost 10x the exposure time (=(250/80)^2) to gather the same amount of light at the Dobsonian. But the drift speed is only 3x slower (=1250/400). At the end, I limited myself to 2s exposures and took just a few stacks: M42/M43, NGC 2024 (Flame Nebula) and M45 Pleiades.

After that I switched to visual observations.

My other goal for the evening was to see whether my photographic tripod could work on my Death Valley trip when I won't have a chair.

It goes higher and therefore can be used standing up and also is easier to pack. However, it does not have fine adjustment knobs. It worked okay, aiming was easy, and star hopping was not too bad. I think it's the one I will pack. I observed the usual suspects but of course the views were a lot less impressive than in Sea Ranch.

Observations: