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Mad, hypothetical thought on a new setup


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I got the RASA running on a 10Micron GM2000, which is amazing. I shot some Ha with it, which is super amazing. But I have run into the usual thing with my UK setup: So little time of shooting available here and light polluted to hell.

So, I thought. With that mount, wouldn't the most fantastic thing be a triple refractor setup with mono cameras, so I could shoot Ha, O3 and S2 simultaneously at the same target? Hypothetically, what would be a good scope/camera combination in the 100-130mm refractor line?

Hypothetically...

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So, hypothetically in 100-130mm refractor line you want this:

10-12" RC + ASI6200 mono

:D

Three of those of course, or maybe Quad setup would be even better - so you can do LRGB and Ha, OIII, S2 + Quadband for luminance at the same time?

 

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1 hour ago, vlaiv said:

So, hypothetically in 100-130mm refractor line you want this:

10-12" RC + ASI6200 mono

That would hypothetically require a mount capable of 100kg payload. I don't have that. Yet.

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Multiple refractor rigs are very nice. I was in near the beginning of this some years ago with a dual Tak 106 rig and now have a dual TEC140. However, they are not without their vices. My own experience is that at low resolution they are easy. The dual Taks/11 meg Atiks just worked out of the box but at 3.5 arcsecs per pixel. It ain't so easy with the dual TEC140 and others, like Peter Goodhew, have reported issues with two high res refractors. Our dual TECS are working at 0.9 and 1.1 "PP and the slave scope (the one not carrying the guider) sometimes trails slightly. Peter found this as well and solved it with an active optics unit on the slave scope. Where does the flexure come from? Postal order for anyone who can tell me. We have a very expensive and very good Cassady T-GAD alignment device to get the slave aligned with the main. You can no longer buy these but you'd need two of whatever replacement you found (FLO do a good one).  I was offered a third Tak/11 meg camera by a person who liked what we were doing and wanted to join in, but for me two was enough. There is even a school of thought which says it is, in the end, easier to use two (or three) mounts. £££££££££. 😁

Low res widefield, the multi refractor is a winner. At high res the flexure gets difficult.

Olly

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4 hours ago, ollypenrice said:

Low res widefield, the multi refractor is a winner. At high res the flexure gets difficult.

Interesting. I can't grog why it would be any different from having just one? That's so un-intuitive to me.

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I think if you want to go a bit more mad on spending then I would commission a triple/quad all in one scope to be made. 

Say 4 matched 125mm front elements fitted to a 250mm tube with internal separators and 4 focus tubes at the back. Might even be possible to add a small 5th lens in the centre for guiding.

Camera wise I'd opt for Starlight Xpress.

You would also need to set up software to sync exposures so you can dither.

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Interesting thought. But that would be fragile in the sense that if one of them had a problem, it can't be replaced. 

I suddenly remembered the dragonfly. I don't think they are taking pretty pictures with it, but it is interesting. ten-lenses_orig.jpg

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6 hours ago, Datalord said:

Interesting. I can't grog why it would be any different from having just one? That's so un-intuitive to me.

Well, first consider a single OTA carrying a guidescope. This can still suffer from differential flexure but if the guidescope is mounted on the OTA at least some sources of potential flexure are elmiminated. (Anything arising from the dovetail or tube rings will be guided out.) Using an AOG will guide out all forms of flexure. Now add a second, or 'slave' scope. This must remain precisely parallel with the guided scope. It will also be mounted on an adjustable alignment device, which adds to the rigidity challenge. (You might be very surprised by how misaligned two scopes are when just bolted together onto a common dovetail.  They do need careful alignment.) I can think of two reasons why the twin Taks always worked and the twin TEC sometimes trail slightly on one side. Firstly the Tak OTAs are very short and, therefore, easy for the mounting hardware to control. The much longer TEC OTAs have a far greater moment with heavy lenses at both extreme ends. (The TEC flatteners are real lumps of glass.) The second, and more likely reason in my view, is that the Tak resolution of 3.5"PP can absorb small amounts of diff flexure when the TEC system at over 3.5 times that resolution, cannot.

One thing's for sure: many dual rig users experience flexure which is hard to trace. I know this because I receive quite a few PMs and emails on the matter and some of our guests run dual rigs as well. I had one such email just a few days ago.

I've thought the same as MarkAR with regard to the construction of a dedicated twin/multi scope from the ground up. A manufacturer who came up with this might steal a march in the marketplace. However, the manufacturer would still have to achieve precise alignment of the lens cells. Maybe that would be routine, maybe not. I don't know. And then, alas, the focusers have to be independent so one very real source of minor flexure would remain.

One ray of financial hope 😁: if you have a scope per filter you don't need any filterwheels.  Less to go wrong, as well.

Olly

Edited by ollypenrice
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At high resolution active optics is the only realistic solution as @ollypenrice  noted on PG's system. That's what the pros use on segmented mirror telescopes to keep the segments aligned. Mechanics have to deform under varing loads and the tolerance is extremely small.

Maybe just having multiple mounts and scopes would be more cost effective or, even more radical, don't spend capital spend revenue and move to remote hosting site with more clear  nights. 

Regards Andrew 

PS possibly the most cost effective solution is to use CMOS cameras in low read out noise configuration and take exposures short enough to avoid and sign of differential flexure. 

Edited by andrew s
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I can believe with a TEC140 it is almost impossible to keep them fully aligned. Would be interesting to see if things would work better with a modern CMOS mono camera which would facilitate shorter subs then than the CCDs and hence certainly mean less movement over the course of a sub exposure. I do like the sounds of the active optics solution though.

Adam

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On 27/06/2020 at 22:26, Datalord said:

That would hypothetically require a mount capable of 100kg payload. I don't have that. Yet.

DDM 100

Go on, you know you want one :evil4: :evil4:

And so do I, but it's more than BOTH my DDMs put together :eek:.

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1 hour ago, Star101 said:

see this 

Wow. Doesn't even look that expensive. And getting 35 hours of data is suddenly just a matter of 2 nights. Brilliant result.

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28 minutes ago, ollypenrice said:

With that budget you'd be far better off with three mounts!

Nah, the DDMs are a different league completely. I would happily have one of these over three others...

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2 hours ago, Datalord said:

Nah, the DDMs are a different league completely. I would happily have one of these over three others...

That's not my point. What I'm saying is that you are likely to experience diff flexure on any multiple high resolution rig so that such flexure will defeat any mount, however good. Since lots of high spec mounts can deliver 0.3"RMS, which will be effectively perfect down to any image scale supportable by the seeing, you would probably get more accurate guiding by having three such mounts than by having an 'absolute' mount carrying three scopes, two of them mounted on alignment devices. (I think it very probable that the alignment devices are the source of most slave scope flexure but I can't be dead certain of that.

Good as it is, my Cassady T-GAD may well be the source of the occasional flexure I experience. It looks like this:

CASSADY%20PAN-600x346.jpg

 

CASSADY%20TILT-600x266.jpg

With dual rigs now popular it might be the time for an entirely new approach to alignment devices. I wonder if 'multiple tube rings' might be the answer? That's to say a set of parallel tube rings accurately CNC machined from a single billet. Would this give well aligned images? That would depend on the alignment of the lens cells and on the focusers. It would be worth a try. I think it might work but I can't say I'd be too surprised to find a misalignment of hundreds of pixels.

The dual rig will not come of age until the perfect mounting hardware is designed.

Olly

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If you could get hold of 3 DDM60 that would be a cost effective solution, but the chances are infinitesimally small.

Regarding multiple tube rings, Gina has done something similar with 3D printing for her multiple wide-field rig, but only using camera lenses.

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32 minutes ago, DaveS said:

Regarding multiple tube rings, Gina has done something similar with 3D printing for her multiple wide-field rig, but only using camera lenses.

I've seen people attempt this using aluminium billet too.

James

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29 minutes ago, JamesF said:

I've seen people attempt this using aluminium billet too.

James

What we don't know without testing is how well aligned two scopes would be if placed in perfectly parallel tube rings. Making perfectly parallel tube rings in the CNC age wouldn't be difficult - but would it be a solution?

Olly

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9 minutes ago, ollypenrice said:

What we don't know without testing is how well aligned two scopes would be if placed in perfectly parallel tube rings. Making perfectly parallel tube rings in the CNC age wouldn't be difficult - but would it be a solution?

Quite.  It strikes me for example that if there must be some accommodation in the lens cell to allow for expansion and contraction of the glass, can that have a noticeable effect on alignment?

James

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The residual flexure might not be due to the saddle.  The two telescopes could be at different temperatures, the mirror cells with slightly  different tensions (as @JamesF said above), variation in tubes thickness and any number of similar small differences could contribute at this level.

 Miracle mount or not you can't beat the laws of physics.

Regards Andrew 

Edited by andrew s
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3 hours ago, JamesF said:

Quite.  It strikes me for example that if there must be some accommodation in the lens cell to allow for expansion and contraction of the glass, can that have a noticeable effect on alignment?

James

 

2 hours ago, andrew s said:

The residual flexure might not be due to the saddle.  The two telescopes could be at different temperatures, the mirror cells with slightly  different tensions (as @JamesF said above), variation in tubes thickness and any number of similar small differences could contribute at this level.

 Miracle mount or not you can't beat the laws of physics.

Regards Andrew 

As a simple soul I'd first want to see what happened if someone tried perfectly parallel tube rings machined out of one lump. I think we could all accept a slight misalignment but would it be slight?

Olly

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12 minutes ago, ollypenrice said:

As a simple soul I'd first want to see what happened if someone tried perfectly parallel tube rings machined out of one lump. I think we could all accept a slight misalignment but would it be slight?

As I have posted in another thread I'm likely to have rather a lot of time on my hands come the end of September, so I might give this a try.

James

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A lot of the OTA tube rings I have seen have some sort of cushioning material on the inside, I guess to protect the paint finish on the scope. They would have to go for a start. Also they have some form of tightening arrangement to grip the tube. How do you get this distortion the same on the two assemblies? It might be Ok on an 80 mm set up,  but there are some serious forces at work when you double the aperture.

For me, a supremely well engineered adjustable saddle would be the way to go. I’ll see if I can persuade @Tomatobro to fire up the Bridgeport.....😊

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37 minutes ago, ollypenrice said:

 

As a simple soul I'd first want to see what happened if someone tried perfectly parallel tube rings machined out of one lump. I think we could all accept a slight misalignment but would it be slight?

Olly

A rough calculation for you Tec 140 shows they need to be help within 0.7 micron to keep any shift to 0.3 arc secs, assuming  the rings spaced at 50% the focal length.

Not an issue for field matching more of a challenge for guiding.

Regards Andrew 

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