400mm Focal Length Triple Scope Imaging System

[Gina]

Since I have changed my multiple telescope imaging system from four scopes to three I've decided to start a new topic for this as it is quite different from the previous multi-scope incarnation.  So many things have changed that a new thread is indicated to avoid confusion.  This does follow on from other threads but I decided it best to start again.

The rig consists of an Esprit 80ED Pro in the middle and two ST80 scopes either side.  All these scopes have a focal length of 400mm and a focal ratio of f5.  Of course the ST80s are nowhere near the quality of the Esprit but when used for narrow band imaging they are adequate for use with the OIII and SII filters as these contribute less to an NB image than the Ha.  The very narrow bandwidth reduces the chromatic aberration that occurs in normal use, to an insignificat amount.

These scopes with three 460EX CCD cameras and all their controls are mounted on a very substantial aluminium plate of 500mm x 250mm x 10mm thick.  This in turn is securely screwed to an 11" ADM dovetail for attachment to my EQ8 mount on conctete pier in my observatory.

For guiding I will be using an SLR telephoto lens with LodeStar X2 guide camera mounted underneath the aluminium plate.  I decided that use of an OAG would not be suitable.

[DIYASTRO]

Hi Gina,

Glad to see that you are back on track with the multi imaging system.

Pleased to hear that the oag is off the menu!

Look forward to seeing your construction for the revamp from your original version.

Kind regards

Boyd

[Gina]

Thank you Boyd

[Thalestris24]

Hi Gina

That setup sounds good! Hope you get the weather to use it! May I ask why the telephoto lens for guiding? Just curious

Louise

[Gina]

Thank you Louise   Because I have a couple of lenses available, though I might use a finderscope - I have a few of those spare too.  I'll see which works best.

[Gina]

Here's a photo showing the three scopes mounted on the heavy aluminium plate with the ST80s mounted on smaller sub-plates to allow for alignment with the Esprit 80ED.  Only two cameras are shown as the other 460EX is currently on the MN190 with FW in the observatory, set up for imaging (if we ever get a clear night!).  Alignment in RA is by threaded rods and hand wheels and Dec (if needed) by screws in threaded holes in the thick aluminium plate.  Once set the alignment will be clamped by 4 screws in each ST80 mounting sub-plate.

The remote focussing for the three scopes can be seen in the photo but I'll show more detail later.  The Esprit uses a stepper motor driving the slo-mo focus knob through a pair of spur gears.  The ST80s have a large quadrant spur gear and small pinion on each stepper motor.  The stepper motors are driven from an Arduino through ULN2003A drivers.  The Arduino sketch is used in conjunction with a VB computer app to provide remote course and fine focussing.  I shall probably implement auto-focussing via ASCOM later.

[Ags]

Hi Gina, I've done some shooting with an ST80 and one thing I noted was the field curvature - do you have flatteners for the ST80s?

[Gina]

Hi Agnes   No I don't have flatteners but the image sensor of the 460EX is not large -12.49mm Horizontal x 9.99mm Vertical (15.97mm diagonal) and I'm not seeing any noticable sign of field curvature   Here's a test image from last year with the older configuration.  The red lines across the stars were due to the SII filter being skew-whiff - now fixed.  This was only about an hour of imaging.  Narrow band with 5nm Ha, 3nm OIII and 5nm SII Astrodons.  Pacman SHO Hubble Palette 2014-12-08.

[Gina]

Here's a few more photos.

ST80 RA alignment adjuster.  M5 threaded rod held in a ball bearing with hand wheel.  Drive lever has an M5 nut which runs on the M5 rod.

ST80 Dec alignment adjuster screw.  Other end of screw presses on ST80 mounting sub-plate to raise or lower nearest edge - pivots on far edge.

ST80 focus drive.

Esprit focus drive.  (Note I have only one screw attaching bracket to ali plate ATM.  There will be another M4 screw into threaded hole on the near side of the bracket.)

[PNJ]

Very impressed setup.   Good luck.

Do you mind if i ask where you got the threaded rod/hand wheels/cogs etc.

thanks

Paul J

[Gina]

Thanks paul   Stainless steel M5 threaded rod, nuts, bolts, washers etc. from ebay and the handwheels and gears plus brackets etc. were 3D printed out of ABS plastic.  I have an UP! Plus 2 3D printer and various colours of ABS filament.

[davedownsouth]

Hi Gina, very impressive set-up  .  Hope you don't mind a probably very simple question, but how do you attach this set-up to your mount?  Do you just have a dovetail bolted to the underneath of the metal plate shown in post 6 

[Gina]

Thanks Dave   Yes, just that - an 11" ADM dovetail screwd on with Allen headed M6 screws into threaded holes in the aluminium plate.

Quick snap...

[Ags]

Gina, I've been away for a while but you have not stood still - EQ8, Esprit 80ED, multiple CCDs - it is a pretty awesome setup. I do have another question for you - is there sample variation in the scope - are they all of 400.0mm focal length, or are there a couple of millieters variation in there? I presume this is corrected in software?

[Gina]

There is bound to be some variation from the 400mm designed focal length - nothing is perfect and there are manufacturing tolerances.  Plus, of course, my alignment won't be perfect either but all these variations are taken care of by using RegiStar to align the stars so that the images fit exactly on top of each other.  Variations in size, position and angle are all taken care of in RegiStar.  IMV this is a virtually essential piece of software for multi-scope or even any multi-filter imaging.  Other software can do this but I find RegiStar very good indeed.  It can be used to align panes of a mosaic too

[bogdog]

nice setup..is it my eyes or is your dovetail bar the wrong way around?

feel free to throw stones

[Gina]

In putting things together I've found the previous design of the box that contains the Arduino, fuses and other power components is slightly too high and doesn't fit under the Esprit focuser, so I've redesigned it slightly and now re-printing it.  No problem, just under 2 hours to pring and using 50g of filament costing something like 50-60p

[Gina]

nice setup..is it my eyes or is your dovetail bar the wrong way around?

feel free to throw stones

Thank you   The dovetail is parallel the the optical axes of the scopes and the logo points towards the stars.

[davedownsouth]

Thanks Dave   Yes, just that - an 11" ADM dovetail screwd on with Allen headed M6 screws into threaded holes in the aluminium plate.

Quick snap...

Underside with ADM Dovetail.jpg

Nice, how thick is that steel plate?   Are you at all worried about it bending with the weight of the two scopes at the ends, or does that not really matter, as long as the 'flex' does not change during the imaging run?  NB.  You must have a better drill than I have got; no chance would I be able to get my drill bits through what looks like a pretty substantial steel plate.  [EDIT ignore questions, just re-read your first post   ]

[Gina]

You may be interested to know that flexure of the main mounting plate WAS a consideration.  At 10mm thick and 250mm wide I'm hoping the flexure will be minimal - the weight of the ST80 assemblies is a lot less than most scopes.  Clearly there will be some flexing that will change as the mount rotates in RA and that will cause image rotation .  The important thing is that this is less than the resolution of the imaging system (whether limited by pixel size, optics or seeing) during a single exposure.  Changes from sub to sub during the whole imaging run is corrected by the processing in DSS, as is change in image position.  I think it unlikely that my sub exposures will be longer than an hour, in which the RA angle changes by 15 degrees.  With the geometry and bending moments concerned compared with the rigidity of the mounting plate I think the problem will be negligible.

[Dave Lloyd]

Looking at these multi scope setups got me wondering if it's possible to construct a setup to direct the light from the scopes to one camera. Or would trying to individually focus multiple scopes be insurmountable?

[Gina]

Sorry Dave but I don't understand why you would want to do that.  The point of multiple scopes is to take more than one image at a time, in different colours.  The opposite might make sense viz. splitting the light from one scope onto several image sensors.  In fact I believe this has been done using dichrioc mirrors.

[Gina]

I have been looking at the idea of using a finder-guider.  The finderscope from the Esprit 80ED Pro is a prime candidate and I'm unlikely to want to use it on the Esprit as all finding will be done by CdC and AT.  Unlike other SW finderscopes, the Esprit one does not have a cast body and bracket combined but three parts screwed together.  This means that I could take the bracket off and screw the finderscope directly to the underside of my main aluminium plate, removing the main problem of finderscopes - that of flexing in the bracket or where it attaches to the main scope.

As can be seen in the photos below, the scope rings are screwed onto a plate which, in turn, is screwed to the bracket.  So I could either attach the scope rings directly to the ali plate with two screws each or use the plate and attach it with two screws to the ali plate.  Just using the rings would give higher rigidity and need 4 plain holes in the ali plate either recessed for Allen head screws or countersunk.  Using the intermediate plate would make the adjusting screws easier to get at but would need two threaded holes in the ali plate.  On the whole I think it best to use just the rings for best rigidity.

I will need to make an adapter to screw into the body of the finderscope in place of the diagonal and take the LodeStar X2 guide camera.  This will need to be something like 70mm - 80mm long with 50mm OD thread to screw into the finder body with a pitch of what looks like about 0.5mm.  The guide scope end could be a plain tube of appropriate ID for the LodeStar.  A thumbscrew could hold the camera in place.

I remember having an adapter for an earlier finderscope to use a webcam as a finder guider but whether this could be made use of remains to be seen.  I would have to find it first   Probably just as easy to make a new adapter on the lathe from aluminium (or even black acrylic).

One advantage of using the finder scope is that it has alignment adjustment screws which would help bring a good guide star into the centre of the guide camera field.  And this example should be easy to fit.

[DIYASTRO]

Hi Gina,

Locating your guidescope 'undercarriage ' seems a sensible move as we discussed before in previous thread.

The one you show is a nice little package but just wondered about the fact that you have mentioned that you have other lens's with larger aperture, why not use something a little larger and increase both light gathering power for those awkward shots as well as increase your 'centre of balance' via 'undercarriage' mounting. I not suggesting longer f/l but just greater aperture than what looks like 50mm.  

Boyd

[Gina]

Using a bigger lens was one of my considerations.  Yes the finder scope is 50mm aperture and 240mm FL giving a focal ratio of f4.8.  In lenses I have :

1. 300mm FL f4.5 (70mm objective),
2. 200mm FL f3.5 (60mm objective),
3. 135mm f2.5 (54mm obj),
4. 105mm f2.8 (37mm),
5. 85mm f2.8 (30m),
6. 55mm f1.8 (30mm)
7. 35mm f3.5. 
8. Canon zoom 18mm - 55mm (several)

The 200mm, 105mm and 55mm lenses are ones I plan to use in my widefield triple imaging rig as imaging lenses.  I had thought of using either the 300mm or 135mm lens for the guide scope.  The 300mm lens is big and heavy - the 135mm is much more managable and collects more photons.  But would the focal length be too short for efficient guiding?  I guess "try it and see" would be the answer