Size of objects and Exposures

[Legin (Nigel)]

Hello,

I have an Orion 12inch XTG dobsonian telescope. The scope is great for visual work. Just for the record my scope has a 1500mm focal length and I use a DMK41 which has a 8mm diameter I also use a D3200 DSLR which has a 27.8mm diagonal measurement on the sensor.

I know the basics 1500/8=187.5x (CCD) and approx. 54x for the DSLR. Plus I use a 3x Barlow at times. As we all know this adds a degree of difficulty. While the magnification becomes quite high it is near impossible to focus at 562.5x. This is expected as it is really pushing my scope. The DSLR works at approx. 162x but the DSL seems to have a lot of noise.

Just for the record the DMK wins 90% of the time. It has a much smaller chip than the DSLR but the resolution is very high and the noise is low. I am starting to like the look of black and white. It also helps you focus on details rather than pretty colour.

I have mostly been using the avi approach. I tryi the occasional single shot but it is hit and miss.

I am wondering how long I could or "should" go for various objects. I have a rough idea but is there a way to actually work it out.

My basic idea is do not exceed 60sec for Jupiter or the Sun (filtered of course). Saturn I might be able to go for as long as 2min though I would still probably only go a minute or so. Mars, Neptune and Uranus can go many minutes as they do not rotate so much. With the Moon you can go as long as you like.

So like I said earlier I "get" the basics.

Is there any way to calculate the maximum time I can film an avi for in order to avoid rotational problems. Mostly the Sun, Jupiter, Saturn and to some extent Mars (be it much less of a problem).

Also I have been having problems with getting my setting right with the dmk. I am gradually improving as I mess about with the settings. But it seems near impossible to get the gain, gamma and exposure correct to get a decent histogram. It always seems pushed to the left meaning it is clipping.

For example if you look at my attached images while they are good enough they look a bit clipped and fuzzy. Any pointers here would be greatly appreciated.

Thanks in advance.

[Camelopardalis (Dunk)]

Sounds like you've got a good grip on it Nigel I'm only dipping my toe in the water myself and reckon I need a better camera

For planetary/lunar, some of the stacking software can derotate, but I use a GEM so I've not had to do it. If you find that function, you should be able to record the avi as long as you can keep the object on the chip. Also many say to use f/20 or higher on planets, my scope is f/10 by default. I failed at the second hurdle last week when I couldn't get Jupiter on the chip with the Barlow

For focussing, you might want to try a bahtinov mask ... you point the scope at a bright star and adjust the pattern accordingly and then just don't touch the focuser again.