AP Mount Considerations

After 3 years in this hobby (I hate calling it a hobby… addiction, passion, endeavor, pursuit…) I’ve formed some opinions about what makes a mount suitable for astrophotography. Choosing a mount for your telescope is a very individual thing. It’s very much dependent on your goals, your current and future telescopes and cameras, whether it’s to be permanently mounted, or will it be used at star parties, and other factors.

So, …. I’m not going to tell you what mount to buy. I’m just going to list some of the things I think are important to consider when selecting a mount for AP:

Rigidity, Stability, Mass: It’s hard to have a good AP mount if these elements are weak. An exception can be mass, but it’s often related to the other two elements. The mass of the mount itself can be reduced if the mount is built with better precision and can transfer imposed forces from scope imbalance, wind, etc, to the tripod or pier. The bottom line is that a mount that’s good for AP should be exceptionally stable and rigid, and typically it will be heavy.

Weight Capacity: The rule of thumb is a mount’s weight capacity for AP is half the rated weight capacity. I think that’s not a bad rule, but I’d call it a guideline. When selecting a mount try to find out as much as possible about its construction. A mount’s ability to carry load relates to how rigid/stable it is, and strength of it’s drive system. However, in a properly balanced setup the force needed to move the mount should be very low, basically the force needed to overcome friction. As such, hardened or larger diameter axis shafts supported by ball or roller bearings is a good thing because of the reduced friction and increased rigidity. Wo when thinking of weight capacity, also review the constructions details of the mount, the precision of machining, the types of bearings used, what other AP’rs are supporting with the mount and their results.

Periodic Error & Drive Vibration: Getting a mount with low periodic is important, but it’s not all important. In my opinion just as important is how much jitter is induced but the higher frequency (faster moving) parts of the drive gears. This discussion can get very technical and I’ll probably discuss what I know about it in another post, but for now I’ll say it’s not just about how much PE there is, but how smooth the PE curve is. The smoother it is, the easier it is to correct, either with PEC (periodic error correction) routines, or by guiding. There is almost no published information or specifications on this subject, so the best source I know of is talking to folks who own and have measured/analyzed a mounts performance. The general question to ask is how well does this mount guide?, or can I see a graph of a recent guide log?. When looking at a guide log, look at the raw data and check the total peak-to-peak (p-p) variation measured in arcseconds. There is always variation due to seeing, but values over 4 arcsecs p-p will give you fuzzy oblong stars.

Intended use: Do you want to use your mount at star parties, short trips to remote sites, of will you normally leave it permanently mounted. My simple example is that I bought my CGE as my upgrade from my (wobbly) CG-5. I normally keep my CGE on a permanent pier but thought I’d press it into service when I went remote as well. I was wrong. For me the CGE package with it’s big tripod was just too much to pack up, transport and get setup, was just too much. Too much weight and work for me. As a result, I ended up buying a smaller and lighter mount for when I travel.

Future Plans: Plan for you future and keep your options open. As in the previous point, you may not be able foresee your next steps.

Budget: I don’t know what to say about budget. That’s your business. Just remember the most importanat part of your imaging system is the mount. No question. Get the gest you can afford.

Any of this make sense?

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