Alt-Az mount for newtonian astrophotography

[Carl Johan N]

Hi! I'm interested in starting a longer project to create a rather large but still portable mount. My goal is to maximize performance for weight, so consider this a mostly theoretical discussion, I know some of these subjects are challenging. The project is for my Bachelor Degree in Mechanical Engineering.

What I would like to find out is if someone has experience with using large-ish newtonians on an alt-az mount for astrophotography? Most modern very large telescopes are laid out in this fashion, for structural reasons (making a huge equatiorial mount is at best impractical).

An example of a question I had is what on earth happens with the diffraction spikes? Can they be removed with something like sigma clipping or would this pose an issue? (Ie, with field rotation I can do long exposures, but diffraction spikes will be rotating during the exposure)

Hope someone has tried this!

-Carl Johan 

[happy-kat]

Alt az mounts track in tiny left right up down movements and do not account for the earth's rotation even if they keep the object in the field of view though this is not apparent visually it does show on imaging. The exposure length is limited by the inability to track field rotation and the longer the focal length the quicker field rotation becomes apparent. Whilst you can image within the confines of an alt az mount the edges of the image need to be cropped off even when staying within the limits of the mount. On an altaz mount the telescope does not rotate only the object the telescope is pointing at.

Are you looking at taking lots of short exposures over a long period of time? I guess this then could present the issue of the defraction spikes issue you mention.

Edited November 25, 2018 by happy-kat

[happy-kat]

I'm sure you'll get more replies and good luck with your project.

[Thalestris24]

1 hour ago, Carl Johan N said:

Hi! I'm interested in starting a longer project to create a rather large but still portable mount. My goal is to maximize performance for weight, so consider this a mostly theoretical discussion, I know some of these subjects are challenging. The project is for my Bachelor Degree in Mechanical Engineering.

What I would like to find out is if someone has experience with using large-ish newtonians on an alt-az mount for astrophotography? Most modern very large telescopes are laid out in this fashion, for structural reasons (making a huge equatiorial mount is at best impractical).

An example of a question I had is what on earth happens with the diffraction spikes? Can they be removed with something like sigma clipping or would this pose an issue? (Ie, with field rotation I can do long exposures, but diffraction spikes will be rotating during the exposure)

Hope someone has tried this!

-Carl Johan 

Hi

I doubt if anyone here has tried what you describe although it depends what you mean by 'largish'? If it's quite big and heavy but you happened to want to image with it then you probably wouldn't put it on a purely alt-az mount in the first place (though some people will try anything...). It is possible to make use of a wedge. Meade fork-mounted (sct) scopes can do that, I believe.

Louise

[Carl Johan N]

Hi, I had something like say a 12"-16" inch scope in mind. The setup would of course be accompanied by a derotator. I'm not talking about an entry-level setup here

-Carl Johan

[Thalestris24]

19 minutes ago, Carl Johan N said:

Hi, I had something like say a 12"-16" inch scope in mind. The setup would of course be accompanied by a derotator. I'm not talking about an entry-level setup here

-Carl Johan

Yeah, that's big and heavy.... There are people (so I've heard...) who've done some short exposure AP with big Dobs. If you keep exposures short enough then you can escape the rotation. Have a look here

Louise

[tonyowens_uk]

It's a well researched subject Carl, with some modern twists:

1. The criterion of mounting adequacy is lowest natural frequency of the OTA with the axis clutches (if any) closed. Aim for better than 15 Hz. and better than 25 Hz if high resolution planetary imaging is a science objective.

2. start with the axis modules and make them identical. Refined worm drive or traction or direct drive torque motor transmissions - take your pick. Each has pro's and cons and there is no clear winner

3. Get rid of bending moments in your structural design i.e all unloaded metal. Minimise use of counterweights. Decide at the outset whether this is to be portable, transportable or fixed installation as the design approaches differ for each.

4. Use stressed skin structures as far as possible - they are light and stiff. Don't ignore 3D printed massive parts as a way to do this on the cheap. Replace mechanical fasteners with bonded construction. Incorporate structural damping.

5. Ignore conventional thinking in mount design and look hard at the LBT on Mt Graham USA.

6. Get the controls equipment off of the mount and put it in a nearby humidity-controlled controls enclosure (except for motors and encoders obviously). Use quality high flex robot cable and IP67 multi-pole connectors.

7. If you go 'whole hog' you need a cost effective >= 24 bit absolute encoder on each axis. The only one I know of is Dave Rowe's as used by 10Micron which  is brilliant and proprietary.  If there are others out there I'd like to know....

8. You need mount modelling, for decent GOTO's and tracking accuracy, ideally a flavour of Tpoint.

Have fun! This kind of work is directly applicable to robot and machine tool design so worth a bit of effort if you are an early career design engineer!

Edited December 27, 2018 by tonyowens_uk

[Carl Johan N]

On 26/12/2018 at 22:56, tonyowens_uk said:

It's a well researched subject Carl, with some modern twists:

1. The criterion of mounting adequacy is lowest natural frequency of the OTA with the axis clutches (if any) closed. Aim for better than 15 Hz. and better than 25 Hz if high resolution planetary imaging is a science objective.

2. start with the axis modules and make them identical. Refined worm drive or traction or direct drive torque motor transmissions - take your pick. Each has pro's and cons and there is no clear winner

3. Get rid of bending moments in your structural design i.e all unloaded metal. Minimise use of counterweights. Decide at the outset whether this is to be portable, transportable or fixed installation as the design approaches differ for each.

4. Use stressed skin structures as far as possible - they are light and stiff. Don't ignore 3D printed massive parts as a way to do this on the cheap. Replace mechanical fasteners with bonded construction. Incorporate structural damping.

5. Ignore conventional thinking in mount design and look hard at the LBT on Mt Graham USA.

6. Get the controls equipment off of the mount and put it in a nearby humidity-controlled controls enclosure (except for motors and encoders obviously). Use quality high flex robot cable and IP67 multi-pole connectors.

7. If you go 'whole hog' you need a cost effective >= 24 bit absolute encoder on each axis. The only one I know of is Dave Rowe's as used by 10Micron which  is brilliant and proprietary.  If there are others out there I'd like to know....

8. You need mount modelling, for decent GOTO's and tracking accuracy, ideally a flavour of Tpoint.

Have fun! This kind of work is directly applicable to robot and machine tool design so worth a bit of effort if you are an early career design engineer!

Thanks for your thoughtful reply Tony. I failed to keep tabs on this threads so sorry for missing it. I ended up limited the thesis to the drive only, and investigated whether it would be feasible to make a friction drive with a cheaper low accuracy servo for a reasonable cost. I will send you the projcet if you're interested :) Where's your telescope design experience from, if I might enquire?

-Carl Johan N