Mount for Astrophotography

[skfboiler]

I heard a rule of thumb about the mount payload capacity of 50% if you are doing astrophotography. What are the pitfalls in imaging or the performance of the mount itself if your OTA plus camera setup is close to 80% of the rated mount load capacity?

[jjongmans]

An overloaded mount won't track well, resulting in elongated stars. The rule of 50% isn't always true, some manufactures are very conservative about their load figures. What mount are you going to use?

[earth titan]

I'm currently at about 60% with no problems at all.

As well as load, it is vital to remember two other factors.

Firstly, the bigger the payload, the more importantly balance is. This can be tricky but well worth spending time on getting it right.

Secondly leverage. Try to keep all the scopes down low and avoid flimsy brackets. If I remember school physics correctly, moments of leverage relate to distance away from the pivot x mass. Therefore keep everything as low as possible.

Typed by me on my fone, using fumms... Excuse eny speling errurs.

[pete_l]

It's a numbers game. The lower the loading on your mount, the higher the percentage of subimages that will be good.

The weight "limit" that manufacturers quote is pretty arbitrary, even for visual use. So it's not an absolute limit, as in: 24.999kg being OK but 25.001kg causing the whole thing to burst into flames - it just doesn't happen.

What does happen is that the greater the load on your mount's bearings and the worse the balance (a scope has to be balanced in all 3 axes), the greater the chance that the rough bits in the mount's drive will cause "bumps" in the tracking as they rotate into critical positions while tracking and the more pronounced those deviations will become. Greater loads will also put more strain on the motors and their ability to run smoothly. That doesn't matter too much when you're slewing (so long as the greater speeds & higher loads don't burn the motors out) , but it's an issue when running at tracking speeds - which is most of the time.

So, if you're careful with your balancing and lucky with the individual build quality of the drive train in the mount (and/or have tweaked it) you can track reasonably smoothly with a higher load. However if you just slap the rig together, aren't painstaking about balancing it AND catch a rough bit of the gear / worm then you may never get tracking good enough to image with.

Generally as the load increases, the proportion of good sub-images will go down. Generally the shorter each sub is, the less chance that something will be wrong with it. Generally the closer your counterweights are to the body of the mount, the easier that tracking will be. Generally the shorter the focal length of your scope, the more tolerant it will be of poor tracking. Generally the better your scope is shielded from gusts of wind or other vibrations/shock the better your subs will be.

These are all generalities and there are many individual exceptions, but these are the things that differentiate good imaging systems from poor ones.

[skfboiler]

An overloaded mount won't track well, resulting in elongated stars. The rule of 50% isn't always true, some manufactures are very conservative about their load figures. What mount are you going to use?

I was considering the Orion Atlas rated for 40 lbs (18 kg) payload and the Celestron CGEM DX rated for 50 lbs (23 kg). As far as the OTA, I was considering the Orion 10 inch F/3.9 Astrograph and the Celestron 11 inch Edge HD. The weight of the Orion Astrograph is 25 lbs (11.3 kg), I think this is without the tube rings and finders scope. The Celestron weight is 32 lbs (14.5 kg). When you add the weight of the camera not to mention guide scope (which I don't think I'll be using), the Atlas mount may be out of the question. However, after researching, folks with the Orion 11 inch Astrograph and the Atlas report no problems. Right now I'm leaning towards the Celestron CGEM DX.

[ollypenrice]

I'm always surprised, in these conversations, how rarely the matter of focal length comes up. It is, for me, pretty much the be-all and end-all. You can buy a mid-range mount, perfectly capable of carrying the OTA in question in terms of wieght but, if that OTA has a focal length of 2.5 metres, then a mid range mount is probably going to need an awful lot of fine tuning and coaxing to guide succesfully. If the OTA is an F10 SCT then you are going to need very longs subs, which compounds the problem. You are also, I suspect, going to need some luck in terms of how good an example your mid-range mount happens to be. They vary considerably.

If, on the other hand, you push the same mount a little harder in terms of weight with a fancy F2.8 astrograph of FL 500mm then you will probably get perfect results very easily.

Personally I kept well away from long FL imaging until a premium mount came along, in this case Yves' Mesu Mount 200. However, some folks do get great results from mid range mounts at long FL but not everyone does.

Olly

[Gliese581c]

i have a cgem dx its a way better mount that the orion atlas

much better software all-star alingment too.

[swag72]

I have had issues with my mount, I think it was overloaded and didn't like the weight I had on it. Now I have sorted out my imaging rig and have a small refractor on the HEQ5 and it will buzz around all night like a dream.

My immediate thought is that is we need to ask about payload and how much kit we can put onto a certain mount then we are probably going to struggle with it a little. As the mount is the key part of the imaging setup, if we are trying to find the odd kilo here or there and work out payload maximums, then really, get a different mount.

I think if you have to ask about the payload, you are under mounted.

[Velvet]

I think 50% may be a little low on mount Loading, but the less you have i suppose the easier it is for the mount to work and track. Quite a few have high loading

issues and there is not a lot you can do bout them. It is possible to set up and then remove all non required items before starting to track and image

but it is not recomended.

My advice get the best mount you can afford, its the most important part of the kit for imaging. Then look at the scope tnhat will fit the mount well and not overload

velvet

[skfboiler]

Thanks for clearing this up for me. When I'm ready I will get the CGEM DX or something comparable at that time.

[NickK]

If you've loaded heavy then balance becomes exponentially more important in my view.

[ncjunk]

I like Ollys comment of "if you find a good example of the mount". Someone with an eq6 and c11 may have no problems as they are lucky to have a good example of the mount which tracks well. Someone else may have the same mount with slightly worse tolerances that just cant seem to get decent results without a lot of tinkering.

You get what you pay for in this game, eq6 and 20kg plus loading = lots of tinkering.

Mesu 200 and 20kg plus loading = balance and go.

If you have the time to adjust and adjust again but not the money then go eq6 or wait for the new eq6.

If you are loaded then go mesu or a similar high end mount.

If you just want general astrophotography a small load, ed80, on an heq5.

In the main, though, i'd go with half the load capacity for eq6 or 5s.