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Sky 90, p2z, Mamiya 645 : Now what else do I need?

Started by snaggs, 11/15/2007 05:11PM
Posted 11/15/2007 05:11PM Opening Post
I'm just ordering all the bibs and bobs I need to do Astrophotography with my Sky 90. I've ordered the field flatener, adaptors etc. I have some questions;

1. I've been told I need a ronchi or something to focus? Where do I get one suitable?
2. Will I have a nice round circle in the middle of the 645 film or will it cover the whole film?
3. I have rolls of E100GX in the fridge. Will that be ok?
4. I do not have a guide scope at the moment, but the p2z has a periodic error 8 (+-4). This review ;

http://www.astrosurf.com/carreira/comentario_tak_p2z_eng.html

here suggests I could do exposures unguided for a few minutes. Will this be enough at f/4.5 to take a photo? How long do I need the exposures to be? I realise if you get into the magnitude counting game its hours.

Daniel.

Posted 11/16/2007 04:37PM #1
Hi Daniel,

The official number of Sky90's image circle (when used with the reducer/flattener) is 45mm diameter. This is not enough to fully cover a 645 frame. My guess is that you will get a circular illuminated area of slightly larger than 50mm. Personally I like such kind of pictures, but I do know that some people don't like this.

It is not too easy to get a Ronchi or knife-edge focuser for Mamiya 645, I guess. I usually focus my Mamiya 645 with Takahashi FM60 microscope. The result is always perfect but it is not easy to operate. (It is especially difficult on Mamiya 645.) If your camera is the newer Mamiya 645 with removable finder, an easy way to focus is to use a high magnification loupe (at least 10x, more than 20x is preferred), and directly focus on the focusing screen of Mamiya 645. Sometimes I do this, and I don't remember I ever got a bad focus.

I saw some pictures taken with E100GX, they are decent. However, to get the most red nebulosity, E200 is a better film to use. It is faster too.

The biggest problem I see in your setup is tracking. To get decent images with Sky90, just a few minutes of exposure is definitely not enough. You will need to go for at least 15 or 20 minutes. P2z is a great mount, the best one can dream of in its class. However, I don't think it can guarantee pinpoint stars with the focal length of your telescope and with exposures of 10+ minutes. On the other hand, Sky90+Mamiya645 and a guide scope are probably pushing the limit too much on P2z. It may work, if you can find a light-weight guide scope.

My suggestion is, go out and shoot anyway with whatever you have now. Try E200 (one or two stops of push processing) and expose for anywhere between 10 and 20 minutes, and see what the result is. Then you can consider how to improve your setup based on the result.

For your reference, this is a homepage of a Sky90 user:
http://paulimer.sinastarz.com/
He uses Mamiya 645, Sky90 with reducer, a guide scope, and a small mount (Mizar New AR) that is not quite as good as P2z. He managed to get many excellent photos. I am not sure if the images were taken with Mamiya 645. Even if yes, the images on the website must have been cropped. He was facing the same problem -- the telescope and the guide scope are too heavy for the mount. Later he moved to TAK EM11 but most (if not all) of the images on the website were taken with the small mount. As long as you can focus well and track well (for 30+ minutes), you can expect to get similar photos.

Hope these help.

Cheers,
Wei-Hao

Wei-Hao Wang
http://www.aoc.nrao.edu/~whwang
Posted 11/24/2007 06:58AM #2
How about a 12 megapixel point and shoot camera?