this is where disable_dfs.zip comes from:
https://www.pentaxforums.com/forums/pentax-dslr-discussion/112734-k20d-dfs-startrails-3.html
basically download the zip, unzip it in your SD card, then power on the camera either with the card door open (K20D) or while pressing the Av button (K-x).
However.... someone on the Singastro forum reports the DFS disable
does not work on the K-x (you're limited to 30 seconds). But it does work on the K20D which is good for me.
sjwaldron: Astromart. However you need to pay to become a member and see the prices. I wasn't comparing the Pentax SDUF to the TOA130 or AP130GT, but rather to the FSQ-106.
For deep sky wide-field photography, the diameter of the lens is less important than flatness of field or focal ratio (the faster and the flatter, the better). while a bigger lens means more resolution, there's not much difference between 100mm and 130mm. People who needs lots of resolution (normally for tiny deep sky objects) don't use refractors, but large (10" to 12") Ritchey-Chretien astrographs.
this is a very important consideration: diameter of the front lens is very important for VISUAL. but for photography, not so much. See, 130mm vs 100mm is almost twice the area (and thus light-gathering power). Important for our eyes. But a camera... just double the exposure time and you get twice the light.
Worse, the AP130 is f/6 while the Pentax is f/4 -- that's more than one f-stop faster so basically a 30 second exposure on the Pentax would gather as much light as >60 seconds on the AP130.
A lot of people use small telescopes (e.g. the FSQ-85 "Baby Q") that have more elements (e.g. the FSQ-85 / FSQ-106 have four elements compared to three on the AP130 and TOA130) for photography. The fourth element is a field flattener. This is where the Pentax does well, the field is flat over a 6x7cm frame (the TOA130 and AP130 can do that too actually but you need to buy a dedicated field flattener).
Actually for pictures of things like galaxies, any old lens will do. Here's an old photo of M45, the Pleiades (otherwise known as "Subaru") that I took with a 100mm D-FA macro lens:
and this one is M31, the Andromeda Galaxy (very noisy, 3 shots only at 30 seconds each) with a William Optics Zenithstar 70ED (70mm diameter, f/6.2). Notice that the stars become egg-shaped as you get far from the center. That's field curvature. At the time I didn't have a field flattener, so I got egg stars (the W-O Zenithstar only has 2 elements).
A lens with a flat field would not suffer egg stars (notice the Macro lens shows no egg stars, it has lots of elements). The D-FA Macro has some chromatic aberration however, that a real ED telescope wouldn't have.
A very popular lens for astrophotography is the Nikon 180mm f/2.8 ED Ai-S or later. Flat field, ED glass. Pentax doesn't have an equivalent (the 300/4 A* ED IF and 400/4 A* ED IF cost way more). I'm thinking of getting that Nikon, since it mounts on Pentax bodies.