Dual Imaging Rigs - my thoughts and ideas

[Gina]

Without the facility of a "Dual Imaging" forum I have had to put a fair amount of thought about which forum to post about this. Apart from imaging and telescope aspects this will also involve some DIY Astronomy in that I propose to talk about designs for mounting a dual imaging rig for widefield.

I now have 2 Atik mono 314L+ CCD cameras (nicely matched of course) and a rather motley selection of glassware/optics. In order of focal length these are :-

1. 35mm f3.5 Asahi Super Takamar lens x1
   
2. 55mm f1.8 Asahi Super Takamar lens x2
   
3. 105mm f2.8 Asahi Super Takamar lens x1
   
4. 135mm f2.8 various x2
   
5. 200mm f2.8 Asahi SMC Takamar lens x1 plus 200mm f4 Vivitar lens x2
   
6. 400mm f5 ST80 achromatic telescope x1 and 510mm f6.3 ED80 semi-APO telescope x1
   
7. 1000mm f5 MN190 astrograph x1
   

No. 7 is highly unlikely to form any sort of dual imaging arrangement.

To be continued...

[ollypenrice]

You do have Registar?

Olly

[Gina]

Yes, I do.

[Gina]

So yes, I can use images from differing focal length optics and combine them. This is fundamental to using say ED80 and ST80 at the same time as a dual imaging rig initially. I have yet to discover how well the ST80 will perform as an Imaging scope - I've only used it for guiding so far. Anyone reading this used the ST80 for imaging? I realise that being achromatic it will be unsuitable for luminance, it might even struggle with RGB filters but I think it might cope quite well when used for narrow band. I may later look out for a second hand ED80 to match the one I've got or possibly the Esprit 80 when it becomes available.

Possible scenarios.

ED80 and ST80 - For LRGB the ED80 could be used for the Luminance and the ST80 for RGB. This would indicate using a simple IR/UV or LPS filter with the ED80 and the filter wheel running RGB filters in sequence for the ST80. So in this case the sequence of ED80, FR/FF, OAG, EFW2 and 314L+ won't be used (even though this combo has been calculated to give virtually the exact distance between FR/FF and camera sensor) as we want the EFW2 for the ST80. Thus a spacer of about 22.3mm is required to replace the EFW2. Baader 15mm plus 7.5mm T2 extension tubes give 22.5mm - near enough. OTOH I shall need to incorporate an IR/UV filter somewhere. For NB I would be inclined to use the EFW2 with the ED80 and a new, home made FW with the ST80.

Lenses - Being achromatic I wouldn't attampt LRGB but RGB or NB only. With the required lens to sensor spacing I will probably use the ST80 for guiding and then the EFW2 plus a 0.6mm delrin spacer and T2 to M42 Pentax/Praktica thread, provides the correct spacing for a lens. I shall make my home made FW the same thickness as the EFW2 so this too will provide suitable spacing.

More to follow...

[Jessun]

Sounds interesting Gina!

ED80 and ST80 - For LRGB the ED80 could be used for the Luminance and the ST80 for RGB.

Knowing nothing about mono, just reading this, would it make more sense to do the RGB through the ED, and the Lum with the ST. My thinking being that it could be beneficial to keep the colours as tight as possible, all stars the same size through the filters...

Then as far as I understand Luminance is king in the final stack, and colour can even be binned...

All a bit confusing for a OSC (One Shot Clouds) like myself! Good luck anyways! The only wish I would have for such a setup would be to synchronized dithering for the two cameras. Craig Stark has hinted that this may be included in a future Nebulosity.

/Jesper

[Gina]

Sounds interesting Gina!

Thank you

Knowing nothing about mono, just reading this, would it make more sense to do the RGB through the ED, and the Lum with the ST. My thinking being that it could be beneficial to keep the colours as tight as possible, all stars the same size through the filters...

Then as far as I understand Luminance is king in the final stack, and colour can even be binned...

The idea was to use the best scope for Luminance. Achromatic means the scope isn't corrected fully for chromatic aberration and the focus varies over the spectrum and you can't get it to focus well enough for the full spectrum Luminance bandwidth. Therefore, I plan to do RGB with a change to the focussing when changing filters. I realise, of course, that it can't simply be left unattended for all 3 filters. With lenses, there is CA correction for G and B and the same might apply to the ST80 - I don't know. If so I could do the red, change to green, refocus and do a sequence of G&B while I went to bed. I'll have to see how it goes - it may be that using binning the colours means the focus is less critical. If I could get away with an unattended run I'd be very happy.

All a bit confusing for a OSC (One Shot Clouds) like myself! Good luck anyways!

Thanks

The only wish I would have for such a setup would be to synchronized dithering for the two cameras. Craig Stark has hinted that this may be included in a future Nebulosity.

/Jesper

I haven't tried dithering as yet.

[ollypenrice]

The only thing I'd even think about for more than a second with the ST80 would be narrowband. RGB would be awful and luminance is just simultaneous RGB. It has evil CA but might just cope with the near monochromùatic light of NB filters with Registar applying the flatter field of the ED80 afterwards. Great guide scope but that's about it...

Olly

[Merlin66]

IMHO the ST80 is a great little scope for a quick look see or for guiding - not for imaging......

One of the telelens would be better. The ED80 is OK.

I made up my own dual bar 10mm aluminium plate and a couple of the Vixen saddle from Skies t' Limit.

(I think/ hope Sara is still getting use out of it) That way you can get the two dovetail aligned properly.....

[Gina]

Thanks chaps I rather suspected that - I wouldn't expect much at that very low price. For dualling with the ED80, I have a 300mm f5.5 telephoto lens I could use though I think the SMC Takumar 200mm f2.8 used at f4 would be a better bet. I could use that for RGB or even binned 2x2 for NB giving an effective 400mm FL. I'd be losing image resolution though so may not be worthwhile for NB. I'll have to experiment - the ST80 might just be better for NB.

Overall, I think lens for RGB and probably ST80 for NB for use with the ED80 - until I get something better.

[Gina]

Just bought a second Super Takumar 105mm f2.8 lens to match the one I've got. I've been looking out for another for quite a long time - they seem to be rather scarce - plenty of 135mm lenses - but the 105 fits just nicely between the 55mm and 200mm Takumars. One has now just come up on ebay but at twice the price I paid for the other one. So I had the dilemma of - do I pay an extra £70 and buy it now or wait in hope of a cheaper one. I decided to strike while the iron's hot! It could be months before another became available.

[Gina]

I've been designing my new widefield rig to add the second camera, filter wheel and lens plus associated bits and pieces. The plate attached to the scope rings will be extended as will the plate at right angles that the filter wheels will be attached to. Both plates will be 300mm x 200mm x 3mm. I have been using SketchUp to help with the design. This is just a rough stress to show the two filter wheels (EFW2 and my make) attached to the upright plate with attachments for the cameras. Lenses are simply represented by cylinders. The EFW2 will be held against the plate with simple clamps and mine will be bolted on.

Here is a screenshot of the SketchUp model - I'll try to upload the ShetchUp file itself in a zip file or somehow.

[Gina]

I'm sure I've seen SketchUp files uploaded here (and since the forum software upgrade) but I'm dashed if I can work out how. Nor can I find an example. Oh well, not that important I guess...

Here's a view from a different angle.

[Gina]

I've decided that fitting the ST80 with electric focuser for imaging would be a waste of time so I'll use it for guiding only until it gets replaced with an ED80. I'm going to concentrate on the widefield rig for now with ST80 & Lodestar guiding and also try out the OAG with ED80 + FR/FF.

[Gina]

I'm currently looking at trying widefield and scope (ED80) at the same time. The EFW2 is on the WF camera so I don't have a FW for the 2nd camera ATM. I do have T2 entension tubes to put the camera at the right distance from the FR/FF but seem to have nowhere to put a filter. I would at least need a UV/IR blocker. Oh well... back to the drawing board Of course, if I had a lathe I could make an adapter...

[Gina]

OK SketchUp via dropbox link :- https://dl.dropboxusercontent.com/u/39971951/Telescopes_%26_Cameras_07.skp

[Merlin66]

Gina,

I think 3mm aluminium plate is very light for this job....6mm or even 10mm would be FAR more robust....

[Gina]

Gina,

I think 3mm aluminium plate is very light for this job....6mm or even 10mm would be FAR more robust....

I've been using 2mm aluminium sheet so this is an upgrade

[Gina]

I've found and ordered one of these :- http://www.astro-engineering.com/prod/AE/filter/lens-adaptor/ac631.html on ABS - for my widefield dual imaging rig. It's an adapter that takes a 1.25" filter in a tray with Pentax/Praktica M42 female thread one end and T2 male thread the other. This will enable me to use any 1.25" filter with an Asahi Takumar prime lens with a 314L+ mono CCD camera. It will need an extra T2 extension tube. Currently I have an M42 to T2 adapter about 8mm thick then the EFW2 (22.3mm) then 314L+. So that's about 30mm total. The new adapter is said to be 17mm so it seems I need about 13mm extension tube. Since lenses are achromatic I will need to refocus when changing filters so a filter wheel is of less benefit. When running unattended after midnight I plan to use the same filter for the rest of the night.

As for scopes, I'm about to buy a second Evostar 80 ED DS pro complete with matching FR/FF, second hand from a member here. This will replace the ST80 on the mounting system. One scope will have OAG and EFW2 attached to the FR/FF and the other my home made filter wheel. I shall not now be using using it on my WF rig as it's job will be taken over by the new adapter mentioned above.

[Gina]

Second Evostar ED 80 DS Pro plus matching 0.85x reducer has arrived. Now to mount it It's all gradually coming together

[Gina]

Gina,

I think 3mm aluminium plate is very light for this job....6mm or even 10mm would be FAR more robust....

I've decided to take your advice and have ordered a 169mm x 287mm x 10mm Aluminium Plate for the main supprt plate that bolts onto the dovetail bar and will be carrying the pair of ED80 scopes. Mounted on top of the scope rings will be the widefield rig with 3mm plates - unless I decide to change to a 6mm plate on top of the rings Having a plate to connect the top of the rings will stiffen up the connection between the two ED80s. Guiding can only be applied to one, of course. This is FLO OAG and Lodestar - it works well.

[Merlin66]

Good call!

I think you'll find plenty of other areas to worry about as the built progresses ;-)

[Gina]

The second ED80 is now mounted and I'm working on the dual imaging widefield rig.

Here are a couple of photos showing the ED80s mounted. The second shows the currennt single imaging widefield mounted on top of the ED80 tube rings so that I can continue to do widefield imaging, albeit with just one camera. I haven't finished my home made filter wheel yet so can't yet dual image with the EDD80s. The widefield rig will be using a filter tray initially which I've got so WF dual imaging will come first. I may make another (smaller) FW for the WF rig.

[Gina]

Here's a couple of screenshots of the SketchUp model of the widefield dual imaging rig, with one camera using the EFW2 and the other the filter tray which is shown diagramitically as just a cylinder.

[Gina]

Making progress on the widefield dual imaging rig. Here's a photo. The gold camera is attached to the EFW2 with the lens attached to that with an adapter. I have yet to arrange clamps to hold the EFW2 to the plate. It's just propped up for the photo. The red camera connects through a T2 extension tube to the filter tray then the other lens goes through the plate and into the M42 female thread on the filter tray. There is enough thread to take the thickness of the plate and thereby provide a mounting.

[Gina]

Been doing more work on my dual imaging setup. The widefield part is pretty much done and I've been drilling the 10mm thick replacement plate that will sit on the mount. I now have the ED80s attached to the plate and the widefield rig attached to the ED80 rings. Now looking at the narrow Vixen dovetail and thinking "I don't like the look of that - a weak link in the system". So I'm thinking of getting an ADM Losmandy style dovetail bar. Wider so less load on the angle clamp when sitting sideways. Also, close inspection of the angle shows the Losmandy to have more of an angle so a better clamping system. Not cheap but considering the value of gear hung on it, I think it worth the extra insurance. I haven't weighed the complete assembly but it feels as heavy as the MN190 which is 10KG. I think the MN190 could do with a Losmandy type dovetail bar as well.

Some photos...