Widefield Triple Imaging Rig

[Gina]

As many will know who have been following my numerous threads, I now have three Atik mono CCD cameras - a 460EX and two 314L+. I also have a fair number of prime telephoto SLR lenses with Pentax/Praktica M42 x 1mm thread mount. Several of these are of superb quality but all suffer from chromatic aberration at the far red end of the spectrum so any filter change between the red end and green/blue requires refocussing.

With a triple imaging system this isn't a problem since different colours will be handled by separate lenses and cameras. Remote electronic focussing is still wanted though to get the focus really accurate - the shorter focal lengths of these lenses mean that they need much more precise focussing than a longer focus scope. So I shall be providing electronic focussing for all three lenses with an Arduino driven triple stepper motor system.

With two different sensor sizes in the cameras I will need different focal lemgth lenses in order to cover a similar field of view. Otherwise a lot of sensor area is wasted. The 460 is about 1.4 times wider and about 1.5 times higher in sensor size compared with the 314L+, so a lens with a focal length of 1.4 to 1.5 times that used for the 314s would match the 460. I have lenses with 35mm, 55mm, 105mm 135mm and 200mm. 135 to 200 is a ratio of 1.48 a pretty good match. 105 to 135 is 1.29, not so good. 55 to 105 is 1.9 useless and 35 to 55 is 1.57 which would be reasonable if I had 2 35mm lenses - I haven't as yet. The latter would cover a very wide field - could be interesting. I'll try the 200/135 combination first as one of the 200mm lenses is an SMC Takumar and an excellent performer.

[Gina]

The next decision involves guiding. Up until now I have had the widefield rig (as a dual imaging system) mounted on top of the ED80s and been using an OAG with guide camera on one of those. I now intend to mount the widefield rig separately - direct onto the NEQ6 Pro mount, so I'll need to provide a new guiding system. I could use an OAG with one of the lenses or separate guidescope/lens. My feeling is that an OAG is really only of much benefit on longer FL scopes where flexure between a guidescope and the main scope can be an issue. Having the guide camera on a separate lens would mean more stars as possible guide stars. It would be easier to arrange too, I think.

With a lens for guiding, the focal ratio would be in the region of f2 to f4 so plenty of photons getting to the guide camera. I think this would mean I could use my presently redundant QHY5 guide camera as a permanent fitting thereby saving swapping the Lodestar around. Of course, I could sell the QHY5 and recover some funds but it's always nice to have a fallback position in case the Lodestar fails (however unlikely).

Other members' views on any of the above are always welcome, of course

[Gina]

I now have a design for the triple imaging WF setup. A complete rebuild of the dual imaging rig as all the holes would want to be in different places. The 460EX with 200mm f4 SMC Takumar lens will be middle bottom with the QHY5 and 55mm lens above. The 314L+s with 135mm lenses will go at the sides. I have changed to a 300mm x 150mm x 5mm alumunium panel for mounting the lenses. There will be 4 electronic focusers one for each of the 4 lenses but with a simpler design than I used for the dual imager. I am going to drive the focussing rings directly from the stepper motors with timing pulleys and belts, with the smallest motor pulleys of 10 teeth. If I find the focussing is too coarse, I'll look into micro-stepping but I'll start with 4 phase and see how it goes. I can mount the stepper motors on a bar above the lenses and provide individual belt tensioning by adjusting the motor position.

Here's a couple of screenshots of a rough SketchUp model.

[Gina]

I shall need to work out filter holders for the three cameras/lenses. I have one filter tray but I need something for the other two. It wouldn't be worth building filter wheels, even manual ones. I'll probably work out some sort of filter drawer or tray. I shall make my own as connercial filter drawers are ridiculously expensive for what they are IMO.

[Gina]

I have an idea for holding filters. This is a drawer holding three filters, one for each imaging lens/camera. A strip of plastic or aluminium about 300mm x 50mm x 5/6mm with holes in which to fit the 3 filters could slide out sideways for filter changing. The imaging cameras would be attached to a 300mm x 75mm x 3mm aluminium plate spaced away from the main mounting plate. Here's some screenshots of a ShetchUp model.

[Gina]

Now to calculate the spacing required.

Back focus required for these lenses is 45mm. Back focus of the Atik cameras is 13.5. The mounting panels are 3mm thick (not 5 as I posted earlier). Going from lens we have 3mm lens mounting plate then the spacing x then another 3mm for the camera mounting plate plus 5mm for the T mount adapter and 13.5 for camera plus 1.5mm to allow for filter. 26mm + x = 45mm. So the spacing x = 19mm. Pity it isn't 20mm.

[astrosathya]

Wow Gina!!! Now that does look out of this world. I've been following your posts since the time I was in Manchester and you do some real astronomy. Kudos.

[Gina]

Thank you very much Sathya

[RAC]

Myself I would keep things as simple as possible, the less stuff = the less stuff to go wrong. I would have them all manually focused. This only takes a minute or two and what's even better is that you don't need to change focus as you will be using three filters separately. That would simplify things a lot. And you could be up and imaging as soon as you build the lens holder.

[Gina]

Myself I would keep things as simple as possible, the less stuff = the less stuff to go wrong. I would have them all manually focused. This only takes a minute or two and what's even better is that you don't need to change focus as you will be using three filters separately. That would simplify things a lot. And you could be up and imaging as soon as you build the lens holder.

The focussing needed is so precise and the focus control by rotating sleeve so coarse that a special method of focussing is essential. I have tried all sorts of manual methods in the past without success. Focussing needs to be much more accurate for shorter focal lengths. The only thing that has worked - and worked well is motorised focussing with a reduction drive - either 12v DC motor or stepper motor. Arduino controlled stepper focussing works best as results are more repeatable. I haven't gone as far as automatic computer controlled focussing as I find a control box in the warm room next to the display works fine with continuous fast forward and reverse with single step for fine control. I go just past focus single stepping forward then an extra button for 10 steps backwards goes back a known amount and then single stepping forward to the best focus. This means I always use the same direction in the final stage to get around backlash in the gearing.

Must add though that I do prefer to keep things simple.

[RAC]

I use lenses and just do it manually with a small bahtinov mask. I just have Artemis on repeat and select just a small box around the star. Helps if you can see the screen at the same time though.

[Gina]

I've been doing some clearing up and sorting out my huge hoard and found a 2mm thick aluminium sheet, 250mm x 100mm that would mean that 20mm spacers could be used. I just need to check that it will be long enough the hold the outermost rotatable T2 adapters. I have found a source of 18mm spacers though.

[Gina]

OK... Been working things out. The 250mm plate will be long enough - just. As for width, the 100mm wide will be better because the hole required for the adapters is 60mm which doesn't leave much from a 75mm wide plate (just 7.5mm eeach side). I've also got some plastic strip 65mm x 6mm which I could use to hold the filters - that would fit in nicely.

[Gina]

I use lenses and just do it manually with a small bahtinov mask. I just have Artemis on repeat and select just a small box around the star. Helps if you can see the screen at the same time though.

I use the FWHM facility in Artemis for focussing - I find it more accurate.

[Gina]

I'll use manual focussing for the guiding lens, I've decided. So that's just 3 steppers rather than 4. Direct belt drive to lens with timing belt and pulleys.

[Gina]

I think I just about have everything to complete this now and as soon as it gets cool enough

[Gina]

Camera mounting schematic.

[Chris]

Gina, you never stop amazing me!

[Gina]

Managed to do a bit towards the construction. Marked and drilled pilot holes in the lens and camera mounting plates and the filter carrier. Drilled and tapped holes in the lens plate for the pillars and clearance countersunk holes in the camera plate for 4mm screws for the pillars. Then I screwed in the pillars and screwed the camera plate onto the pillars. This photo shows the assembly at this early stage, with the plastic filter lolder in position.

[DIYASTRO]

Gina,

Its been a long time since I have posted much on the boards but do occasionally scan SGL topics to see what other people are up to. You are busy as usual! Just thought I would reply to your interesting build of multi camera lens for widefield imaging.

I have been messing around with multi scope configurations for a while and just to jog your memory have a peep at page 10 of Chris's (yesyes) spc900 LX mod thread. (Should be some pics of scopes with multi lenses, More pics of my Lx cameras on his thread on pages12,13,18,24 which may refresh memory since its well over 18 months ago!)

I have given up on crayfords and other focus units for small camera lenses, as you say you need very fine focus adjustment for these and a more accurate system is required.

I have been trying out a system using threaded rods driven by pulleys and powered by servo motors via high ratio worm drives. I will try and find time this afternoon to take some pics of what I mean and post in seperate thread. Will send you PM when able, to say that I have posted them.

It may well be that it would suit one or two of your lenses/cameras.

Boyd

[Gina]

Haven't been able to do much to this project with the heatwave - I'm very susceptible to high temperatures and have to keep cool and rest a lot. This means that jobs requiring energy are out in hot weather. I can do lightweight jobs and planning/design but anything too physical is just not on. I'll post again when I have some progress to report

[Davey-T]

Haven't been able to do much to this project with the heatwave - I'm very susceptible to high temperatures and have to keep cool and rest a lot. This means that jobs requiring energy are out in hot weather. I can do lightweight jobs and planning/design but anything too physical is just not on. I'll post again when I have some progress to report

Your next project then Gina air con for the obs'y

Dave

[Gina]

Your next project then Gina air con for the obs'y

Dave

Yes, you're probably right

[Gina]

We got up about 6am this morning and after breakfast I was able to start working again on the widefield rig construction before it got hot. Now have the big holes cut in the camera and lens mounting plates.

[Gina]

Bit more progress today. Cut the wooden brackets and drilled and tapped holes to fix the dovetail bar to the base plate.