Hello everyone!

Is there an astro-imager who can give me an idea of a good focal length and focal ratio for planetary astro-photography? 

A lot depends on the camera you're going to be using, but in the main you want as much focal length as your mount will support and your aperture will support.

The best lunar and planetary images being made today by amateurs are using Celestron C-14-class scopes and imagers like the Lumenera 2-1 and webcams or DMK/DBK firewirecams. There are also some purpose-built systems like Paolo Lazarottis Cassegrain "cameras" and large-aperture dobsonians equipped with tracking platforms and webcams or DSLRs (a'la Wesley Higgins).

I have used a very fine 1,200mm-focal-length 8" apo for my best images to date. That's an F8 native system. However, most of those were made using at least a 2.5X Powermate to extend the focal length, and some of the best one were made between 4,000mm and 6,000mm focal length (extended using barlows). So, the "best" results with that particular system are achieved at F20 to F40.

If you use a DSLR, you need even more focal length to get a good image scale at the same focal length. Put another way, a DSLR will give better results on the same scope at less than optimal seeing (due to the lesser magnification).

If you use a DSLR or other large-chip camera, you need at least a 2" throughput for your imaging train. So, if you're starting from scratch with a setup to do lunar and planetary imaging, I'd suggest something like an 8" SCT with a 2" visual back as a starting point using off-the-shelf, readily-available, affordable equipment.

To get an idea of what's possible, look in the Online Reader Gallery here to see what others are doing with specific equipment setups. For a more in-depth treatment of this subject, see Robert Reeves' excellent guide (click here).

The primary set-up I'm going to purchase is a deep-sky set up. It consists of:

http://www.telescope.com/control/product/~category_id=N0059_M42_Kettell/~pcategory=ssg_nebulas/~product_id=52080

After a little more research I discovered the Orion Apex 127mm Maksutov-Cassegrain

 http://www.telescope.com/control/product/~product_id=09825 has a focal length of 1540 mm and a

f/12.1  Is this telescope a good choice (for planetary imaging) to add to the equiptment I intend to purchase?

The 127mm Mak has twice the focal length of the Orion "astrograph", so it's a better choice for lunar and planetary imaging.

Its focuser, however, is not going to be as precise as that on the 6" scope and I'm not aware of an upgrade for it. When you are using very long focal lengths (e.g., the 127mm Mak and 2X or 3X barlow) you will need very fine control of focus.

I downloaded and read the instruction manual for the camera. In my opinion it will not be a good choice for a planetary imager. It should work fine for the Moon. It is designed for imaging the brighter DSOs and for making long exposures as single frames.

What you need for the best lunar and planetary imaging is a camera that produces a video stream of hundreds or thousands of frames (like a movie) and post-processing of those frames through a program like Registax. This allows you to capture many more frames during instants of the 'best seeing' -- which is the primary requirement for good planetary imaging. A webcam like the Philips SPC900NC, or a firewirecam or Lumenera would be a better planetary camera.

I suspect you could do almost as well with the 6" scope and a better camera than with the 127mm Mak (again, primarily due to lack of fine-focus control).

Your 6" scope is a widefield scope, so not the optimum choice for planetary imaging. Unless your primary DSO targets are widefield objects, then you should consider an 8" SCT for which an after-market dual-speed focuser is available.

I also eyed the Orion StarShoot Solar System Color Imager III

http://www.telescope.com/control/product/~category_id=astroimaging_camera/~pcategory=astro-imaging/~product_id=52018

How do you think this product will fair for planetary imaging when paired with the Mak and other equiptment mentioned earlier? 

If your interests are planetary first, then DSO second, take the earlier advice and go with a small SCT or Mak-Cass (that has a good focuser). If DSOs are your principal interest, stay with a short newt (or Mak-Newt). I use a DK-Cassegrain (native f/12) w/ X3+ optical multiplication for planetary work.

Remember, Orion is not the only manufacturer of amateur equipment (there are many others that offer a range from lower priced through astronomically priced). In most csases, you'll get quality proportional to price. At least spend some time looking through Astronomy and Sky & Telescope magazines (and the web) before you pull out the plastic.

Robert