I have a SkyWatcher 150p (150mm aperture, 1200mm focal length, F8) Dobsonian-mounted telescope that I use for visual astronomy. Does anyone know if there's a problem with buying a set of mounting rings (assuming I can find them) and putting this telescope on a Goto mount? (OTA diameter is 7.125")
Right now, I use an Apertura AT60ED for my astrophotography, but it's limited to wide-field images. I was thinking about using the 150P (although it's a rather slow F8) for astrophotography as a start for a longer focal length scope if I can attach it to a Goto mount. I'm actually shopping for a Goto mount and I'm trying to decide what payload capacity I might need for the future, which got me thinking about using the 150P for the time being until I get a larger scope for astrophotography.
Thanks!
Hi Greg, you could certainly capture AP images with that 150mm dob providing you can reach focus with your camera/filter setup. You should check that you can reach focus with whatever camera you might be using and your focuser. I've not really done dobsonian AP before, so I'm no expert and you could reach out to someone like Paul Walker who has. Also, can you insert a focal reducer?
The thing to consider when picking a mount like a long tube dobsonian isn't necessarily the total weight but the moment arm of the setup. So while it may seem like you could use a smaller mount, i'd pick a beefier mount. I think the smallest I'd go with is a Orion Atlas on the inexpensive side. It can be tuned pretty darn good. I've had 2 of those and they served me well. They do require guiding and you'll loose some frames, but they do the job and the support community is huge. Ioptron makes some mounts with encoders now, and I know they have a following but I've never used one. I've always thought their spin on the standard German Equatorial mount is clever. I use an AP Mach2 now and have a 1100 on order because i have a tec 180 coming in September (long moment arm). The good thing about AP stuff, is that you can always sell and move up.. The cheaper stuff has a markdown but not terrible and the expensive stuff can be sold for as much as you bought it for.
In any case, some random feedback/thoughts. Good luck
Terri
@terri Great, thanks so much, Terri. I appreciate the advice and insight! There are so many things to consider, and unknowns to factor in, it's mindboggling! 😲
Thanks again!
I concur with what Terri says. I use up to a 10" f/4 (1000mm f.l.) Newtonian on my Atlas EQ-G (40 lb payload spec). This is pushing this mount to the limit. Joe Comeau has used a similar setup successfully as well. But that is above the weight spec for the mount. My scope with camera & guiding setup weighs about 43 lb. Because of my oversized guidescope (see below) is on “top” of the main scope I need 40.5 lbs of counterwieght (required 58 lbs. until I extended the counterweight shaft). I can only do up to about 4 minute exposures before flexure between the guide scope and main scope causes out of round stars (would be less of a issue with a standard autoguiding setup). I use a Celestron NexGuide stand alone autoguider in conjunction with a 90mm f/13.8 Cassigrain for guiding. Much heavier than needed if using a computer with a standard autoguider.
I have never heard of focal reducers for Newtonians, never looked either. At f/8 at least you won’t need a coma corrector. In case you or someone reading this doesn’t know the math relating how changing the f/ratio affects the exposure time- I will assume you are using a 0.8x focal reducer/field flattener on you Apertura AT60ED f/6 scope (I am also assuming it is f/6). This produces an f/ratio of f/4.8. Compared to the f/8 of the Newtonian the difference is 8x8 / 4.8x4.8 = 2.7. So to get similar results in image brightness and noise you will either need expose each frame 2.7x longer or your total time will need to be 2.7x longer.
I would say get a bigger mount than the Atlas EG-G if you can swing it depending on what the biggest scope you think you will use on it.
Assuming the camera can go to very short exposure times, probably the easiest way to check focus on the Dobsonian is to point it at the Moon.
I concur with what Terri says. I use up to a 10" f/4 (1000mm f.l.) Newtonian on my Atlas EQ-G (40 lb payload spec). This is pushing this mount to the limit. Joe Comeau has used a similar setup successfully as well. But that is above the weight spec for the mount. My scope with camera & guiding setup weighs about 43 lb. Because of my oversized guidescope (see below) is on “top” of the main scope I need 40.5 lbs of counterwieght (required 58 lbs. until I extended the counterweight shaft). I can only do up to about 4 minute exposures before flexure between the guide scope and main scope causes out of round stars (would be less of a issue with a standard autoguiding setup). I use a Celestron NexGuide stand alone autoguider in conjunction with a 90mm f/13.8 Cassigrain for guiding. Much heavier than needed if using a computer with a standard autoguider.
I have never heard of focal reducers for Newtonians, never looked either. At f/8 at least you won’t need a coma corrector. In case you or someone reading this doesn’t know the math relating how changing the f/ratio affects the exposure time- I will assume you are using a 0.8x focal reducer/field flattener on you Apertura AT60ED f/6 scope (I am also assuming it is f/6). This produces an f/ratio of f/4.8. Compared to the f/8 of the Newtonian the difference is 8x8 / 4.8x4.8 = 2.7. So to get similar results in image brightness and noise you will either need expose each frame 2.7x longer or your total time will need to be 2.7x longer.
I would say get a bigger mount than the Atlas EG-G if you can swing it depending on what the biggest scope you think you will use on it.
Assuming the camera can go to very short exposure times, probably the easiest way to check focus on the Dobsonian is to point it at the Moon.
Thanks so much, Paul (and Terri). Correct, the AT60ED is an f/6 doublet refractor and I'm currently using a field flattener, but not a reducer, with it.
It sounds like a beefy mount is critical for larger and heavier scopes, or those with a larger moment arm (as Terri pointed out). I think I have a LOT more research to do to find a class/type of telescope that (when I get one eventually down the road) would be midland in aperture/fl and would allow me to do somewhat higher power astrophotography than what I can do with my AT60ED, but not be so heavy as to require a mount that cannot be moved easily.
Once I find a suitable "future" type/class of scope and know the approximate total weight I expect to use with all the other equipment, I can decide on a somewhat portable mount. Right now, portability is important since I don't think I am going to set up anything permanent in my backyard and will ultimately travel with my rig sometimes. An intermediate-payload mount will certainly limit me in terms of the weight (and aperture/fl) I can put on it, but it won't break my back (or the bank!) when I transport it.
I was looking at the newer iOptron CEM/GEM mounts which look reasonably portable and have what I want in them, e.g., connectivity/compatibility for my ASIAir, accurate Goto functionality, PPEC, etc. Those iOptron mounts are newer, though, and not really proven and certainly not debugged yet. There are some older mounts that have been around a while and have quite a bit of user history, but some don't have the connectivity or more up-to-date features I'm looking for.
What do you think about harmonic mounts? I'm not sure if they are as good as a GEM-type mount, but I have no experience at all with them.
Thanks again for the reply and advice!
