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Dual Imaging Scope Alignment Mechanism


Gina
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From the experience of other members, it seems that the chances of the images in the cameras on my two ED80s lining up is virtually zero. This can be due to several causes :-

  1. The optical system in the two scopes may not be exactly parallel with the bodies.
  2. There may be inaccuracies in the alignment of the focusers.
  3. There could be misalignment between any of the many other links in the optical chain - OAG, filter wheels, etc.

Some of these could vary with focussing or orientation so any misalignment might not be solved with a fixed correction system (like slotted holes and shims). It therefore seems that an alignment system that is easily variable in both RA and Dec will be required. This will be what this thread is about.

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Would the SW Guidescope Mount not do the job , or the ADM one for twice the price ... ?

http://www.firstligh...ount_290712.pdf

http://admaccessorie...Max_Guider.htm?

From Steve's report of the SW unit, it looks like it should work. He does have lots of flexure elsewhere though. I'm hoping to get my flexure well below that. We shall see :D

I'm basing my alignment device on similar principles except mine will be built directly onto my 10mm mounting plate that connects the two scopes. This is in turn mounted onto the NEQ6 with a Losmandy style dovetail bar. The alignment plate(s) will be bigger than the SW one with the scope rings mounted directly.

Now the scope rings present a definite flexure problem in themselves and my previous ST80 + ED80 setup had a connecting plate on the top mounting points of the rings, providing much greater rigidity. There really needs to be something to stop the sideways movement. It makes me wonder if a totally different approach is required - not using conventional scope rings (or maybe modifying them in some way).

Edited by Gina
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Thinking about the scope rings further makes me realise that the SW or ADM type of alignment device isn't going to work. It still leaves the two scopes attached by a single point on each ring with lots of possible flexure at those locations.

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What is really needed is a rigid connection between the guide rings at one end and alignment in two orthogonal directions at the other end. I'm thinking in terms of a thick plate with two holes in for the scope bodies one end and two plates that slide on one another for alignment at the other. Of course, this would not be practical but it's that sort of connection that's needed between the two scopes to stop relative flexure. Let's say, for argument, that we have the focuser end fixed and make the other end the variable one.

OK - so how do we duplicate the function of this holes in plates system with something practical? The two hole plate at the focuser end may be replaced by scope rings connected top and bottom. This still leaves the possibility of flexure in a trapezium manner but that doesn't affect the relative angles so doesn't matter too much. This then leaves the variable end. One difference from the guide scope requirements is that the relative adjustment required is very much smaller. I think the initial alignment should be within a degree or so and we want to adjust to arcminutes.

With the two rings at the objective end connected top and bottom with fixed plates there can be no sideways movement but flexure in the mountings would be sufficient to allow up/down movement if the bottom fixing of one scope ring were moved up/down. To allow variation both sides of zero, the fixed ring could have a spacer plate. So we can produce a small up/down alignment with a screw thread adjustment on the bottom ring mounting by relying on the bending of the connecting plate on the top of the rings.

So we now have a vertical adjustment while retaining a rigid connection between the two scopes. One down, one to go :D

Edited by Gina
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I've just checked how rigid the ED80 mountings are with their standard rings on the 10mm ali plate and with nothing on the top. I find I can produce a movement of 0.1mm without straining myself. Converting to angle with the rings say 200mm apart (practical maximum) that's 1 in 2000 or 0.0005 radians. Pi radians = 180 degrees. So the flexure is 0.0005 x 180 / Pi = 0.029 degrees or 1.7 acrmins which is over 100 arcsecs, equating to around 50 pixels with the cameras I'll be using. This puts figures on the tenuous flexure business. OK so the amount of differential movement is likely to be much less than this but it does give an idea of scale and what seems a very strong mounting doesn't seem so strong and rigid when we're talking about arcseconds. Connecting the tops of the rings should reduce this possible flexure be an order of magnitude at least.

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OK so we'll now look at how much adjustment we will want. Let's assume we can get the alignment within a degree to start with.

One degree is Pi / 180 radians and if we take the rings as 200mm apart the adjustment will be Pi x 200 / 180 = 3.5mm

Edited by Gina
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In an ideal world (not our one then!) perfect alignment would be the aim. However, suggest that you don't get too hung up on this because the next issue will be exact camera orientation!

My recommendation would be to get he best alignment you can easily achieve and concentrate on the differential flexure issue as any reasonable misalignment can be easily resolved with your software alignment routine followed by a minor crop.

Sent from my iPhone from somewhere dark .....

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Yes, I was thinking in terms of within 5% of frame size. A 5% crop is not unreasonable. And yes the camera orientation error will also contribute to the need to crop. I found with my widefield dual imaging rig I could get well within this limit. Mind you, camera orientation with the 314L+ is quite easy with the large back and well spaced connectors to go by. The 460 will rely more on image alignment.

5% is around 2-3 arcmins. With an M6 thread the pitch is 1mm so 3.5mm represents 3.5 turns so either way amounts to 7 turns and 2-3am equates to a 30th to a 20th of a turn. This seems quite reasonable.

Edited by Gina
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I think I'd abandon traditional "rings" altogether and use a split aluminium clamp block that bolts together around the tube. This could then bolt to the baseplate with a pair of bolts. I suggested to Olly the other day that alt adjustment relative to the other scope could be provided using a collimation style push/pull bolt arrangement, so the bolts pulling the clamp down onto the plate actually hold the clamp against bolts (with locknuts if required) passing through the plate.

Obviously there are far more expensive ways to do this, both DIY and off-the-shelf :)

James

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Yes, I was wondering about clam shell type "rings". I think normal rings will work with top connection as well as bottom, though. Of course, the proof of the pudding is in the eating and we seem to get very little of that! :( I'll just have to make up what my intuition says should work and then wait... and wait. I have another imaging project to finish before I put the ED80s back though.

I think an adjustable plate/strap between the tops of the objective end rings should provide adequate sideways adjustment. I may be wrong but trying things out on the scopes and an error of as much as a degree seems very unlikely. I should soon have my DIY FW finished and then I can put an Ha or SII filter in the light path and check this out in daylight on a distant object. I need to attenuate the light greatly even for the shortest exposure time of 1ms in daylight.

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a dual bar and set of good guide rings can be used to solve alignment, this is pretty easy to do, however rotation is the biggy,

Rotators add optical length and are also heavy, once you add cameras filter wheels, flatterers they is some serious weight hanging off the end of the scope.

Focusers can start to suffer and then the extra weight on the mount starts to become an issue.

A unified solution to this issue would be very good.

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For sideways adjustment, how about a pair of threaded rods joined using a long nut in the middle? The rods could either be fixed at the OTA ends meaning that turning the nut would draw them together or push them apart, or one OTA end could be free to turn (perhaps more difficult to arrange) and then turning the nut would draw the other one towards it or push it away.

James

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If I was going to make a dual rig first thing would be to have a automated rotators direct onto the OTA, FLI style focusers next, then other gubbins hanging off that. (getting focus spacing will be critical as you would want it around half way of the electronic focuser)

Id then go for a large plate to attach everything to with fine adjustment to sort alignment, this would probably mean custom OTA rings with some form of alt az adjustment.

This will also need some counterweight lens end as all the stuff of the back of the scopes really makes them sit very high on the mount.

Once that is sorted, software that will control two cameras and dither in sync with both sets of subs.

Edited by Earl
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How about top clamp fitted with a t-slot and using t-nuts to get side-to -side alignment (up/down in dec) and rear mount using screw mount for central pivot and shimming for height? (small variance in RA)

Mike

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The device of choice is the Cassady one which can carry a 7 inch Mak from direct experience, but it is expensive and heavy.

CASSADY%20PAN-L.jpg

However, some of the thinking might be possible to adapt, especially for an ED80 rig. Aligning the scopes in RA is pretty easy. Mount both on separate plates bolted to a common plate at the back and connected by a track rod at the front. I could make that work. Dec was the bit that defeated me. The Cassady device works like this: a moving top plate rides on a fixed bottom one via two large ballbearings housed in conical recesses in each plate. The top plate rocks on the balls and is adusted by bolts holding it down front and back. I think that system would be fairly easy to make. You don't really need the recesses to be conical. They just need to be the right size to stop the balls escaping. In fact I've heard of people making SCT polar alignment systems in which the whole mount rides on a central ballbearing and is pulled down by three vertical bolts whose relative length provides the necessary tilt.

As for camera alignment, I see that as fairly easy thanks to Dennis over on PAIG. He shouts at me if I'm not orthogonal and North Up so I daren't deviate from that!! :grin: I align by slewing in one axis while exposing and I just use the resulting trail to tell me how far off I am.

I don't like guide rings myself and never trust them. Wen I tried them on a dual rig I found they varied a lot from night to night, so I guess they would vary during a run as well. They were good ones, too.

Olly

Edited by ollypenrice
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I think the Cassady would be overkill for this purpose TBH. It provides far more adjustment than we need but the tilting plate idea is interesting. Rather more secure than a single threaded rod in the scope ring bottom mounting hole to adjust the height but it requires two bolts to be adjusted at the same time rather than one.

One of my initial ideas was the second plate on top of the main one with rack or threaded rod to to slide one end while the other pivots on a bolt.

As for camera rotation, the EFW2 has this built in and my DIY FW also has camera rotation provided. Both allow the camera to be rotated the full 360 degrees and set at any angle. Both are purely manual though.

post-13131-0-34525900-1372667523_thumb.j

Edited by Gina
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Actually, for me personally I shall be using different cameras - 460EX and 314L+ and the best match I could get would be to have the 460 without FR and 314 with - a ratio of 1.2 in FL (1:0.85) rather than 1.4 or 1.5. So quite a large margin round the 460 image will have nothing to match up with. In fact this could mean the alignment is good enough already without adjustment.

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Regarding Cassady...

Due to changes at the machine shop with my cousin's retirement, I will be selling off existing stock, but not replacing it. If you want something, get it now before it is gone.

Wolf nipple chips. Get 'em while they're hot. They're lovely. Dromedary pretzels, only half a denar.

Cheers

Ian

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I've been thinking about my ED80 dual rig and the two different cameras in conjunction with alignment adjustment. If I were to orientate the 314L+ at right angles to the 460EX I would get a better match by using a 2 panel mosaic with the 314L+ to the single 460EX image. I'm not very happy using an ED80 at f7.5 - so slooooooowwww! How easy or difficult that will be remains to be seen. The scope would want moving a little under 45 arcmins. That would produce a FOV of about 80 x 60 am for the 314L+ and 83 x 67 for the 460EX (If my calculations are correct). It would be easiest if the movement were in just one direction, of course.

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