On Mon, Jan 7, 2013 at 2:49 PM, Steve Sharpe <[email protected]> wrote: > At 9:22 PM +0200 1/7/13, Boris Liberman wrote: >> >> On 1/7/2013 9:07 PM, Darren Addy wrote: >>> >>> Honestly, I think the best option is the Pentax O-GPS1 if you shoot >>> Pentax models that are compatible. What can be done with them is >>> amazing. Like any photography, wide field is easier to do than >>> telephoto (which gets increasingly more difficult with focal length). >>> But check out what this guy did with a K-5, an O-GPS1 and a 200mm f2.8 >>> lens: http://poirierstephane.free.fr/photos/index.php?/category/132 >>> AMAZING. I recently purchased an O-GPS1 to go with my K-5 and intend >>> to start with my Super-Multi-Coated Takumar 135mm f2.5 v2 along with >>> some more wide field stuff... just as soon as I get the fires in my >>> life put out. >>> : \ >> >> >> I should support year suggestion, Darren. It simply makes the most sense. >> However I wonder if 500mm lens is something that software on the camera and >> in the GPS module could work with. Mostly likely they will, but it needs to >> be double checked. > > > For an astronomical telescope, f8 is not that slow. Schmidt-Cassegrains - > the most popular off the shelf optical system - are usually f10 (though > focal reducers are available). I think what's more important is the optical > quality of the lens. A lot of those inexpensive mirror lenses are not very > good. > > As for astrophotography with the O-GPS1...I've gotta get one! > > -- > > Steve Sharpe > [email protected] > • > > http://earth.delith.com/photo_gallery.html
I know that f/8 is not unusual for a visual scope. (my 8" dob is f5.9) But f/8 is still f/8 photographically (exposure-wise). We've also switched subjects (more than a little) slightly when we are talking about using the telescope as the lens to the camera. That's called prime focus photography and it means that the projected image will need to reach the DSLR sensor (with no lens attached). I'm not wild about leaving my DSLR sensor exposed to the elements for any longer than necessary, but that's what you are doing when you use it on a reflecting telescope (as opposed to a catadioptric which is sealed like an over-sized mirror lens). You also need to have a way to really lock down that telescope focuser because it would be great (not) if the focusing tube with camera body attached worked its way right out of the focuser. And you are still back to the issue I originally raised as to How Good/Accurate the tracking unit is on the telescope you are mounting it on. Because now you are effectively using the focal length of the telescope the same as you would a lens. On my 8" dob that is 1200mm. Every step better be smooth or you are going to have a vibration. You also need to be perfectly polar-aligned or you will have issues with longer exposures. In short, prime focus photography is not done cheaply or easily. You should certainly "cut your teeth" on something easier and work your way up as you learn the various pieces of the puzzle. -- PDML Pentax-Discuss Mail List [email protected] http://pdml.net/mailman/listinfo/pdml_pdml.net to UNSUBSCRIBE from the PDML, please visit the link directly above and follow the directions.
