This results from my experience with years of NB imaging with Ha, OIII and SII filters and finding that the amount of SII around is so small as to be very difficult to capture any significant amount.  NII might be a better third waveband but I can't afford another expensive filter ATM.  I have tried to develop triple imaging rigs in the past without much success.  Otherwise I have used a single rig with filter wheel.

With the lack of clear skies in the UK it does make sense to grab as much data during times without cloud as possible.  These CMOS cameras permit many short exposures as opposed to fewer long ones allowing use of short gaps in the clouds.  The use of camera lenses results in larger focal ratios and the ability to capture more data in a given time.  Achromatic optics are no problem with NB imaging as the bandwidth is so small.

This is what I have been wrestling with.

I have decided my dual rig when done will just be Ha.
It's rare to have many nights that allow much more than 3hours imaging time.
I don't mind just mono images as long as I can actually do some imaging.

I did toy with the idea of CMOS but another 383 appeared at a reasonable price and I could'nt resist.

I like mono images and often cringe for imagers you post them only to be greeted with a barrage of 'looking forward to the colour version' posts! My wife, Janie, is not so keen on them .....

Second ASI1600MM-Cool is due to arrive Monday.  There are no clear might skies in the 5 day forecast so I'm in no hurry for it.

To-do list :-

1. Sort out adapters and extension tubes plus filter mountings.
2. Mounting bracket to attach rig to EQ8 mount.
3. Lens focus ring to stepper motor gears.
4. Brackets for stepper motors.
5. Electronics box with RPi3 and stripboard with driver modules and buck converter for 5.1v RPi power supply.
6. Edit INDI software drivers for focusers and second camera.

Regarding mono images, if you want to just collect twice as much Ha data on the same object you will need two Ha filters and Astrodon 3nm Ha filters are expensive.  5nm filters are cheaper though.  I have both though I don't know how well mixing data from both would work.  Have to say, I do quite like Ha mono images - they seem to provide beautifully sharp and detailed images.  Adding in OIII for a colour image makes for a prettier result but seems to reduce the sharpness somehow.  Or at least that's how it looks to me.  A third wavelength makes for a better result and I don't mean a synthetic green.  Although some seem to get reasonable results, I haven't had much luck with that.

Another thought I had was to have a 3nm filter on one and 5nm on the other providing Ha data and NII by subtraction.  I have tried the subtraction process with two separate batches of data and it works.  Using data sets taken at the same time on the same object should produce better results.  This data would be collected after a decent run with Ha 3nm and OIII 3nm filters.  I could arrange for this if I were to use my ZWO EFWmini on one of the cameras.  This could contain 3nm OIII and 5nm Ha filters and save manually changing filters.

Another thought was to buy another ZWO EFWmini but I can't think what for on second thoughts...

I think I have a 1.25" filter holder amongst my astro stuff.  The only one on FLO seems to be Baader and only for the bigger filters (50mm etc.) - when you add up the parts needed it comes to over £150 - it would make more sense to buy a filter wheel.  Otherwise it looks like a 3D printed jobby

For some reason I decided to look at what's included with the ASI1600MM-Cool - there must have been something in the back of my mind from when I bought my first ASI1600MM-Cool camera just over a year ago.

Quote

INCLUDED IN THE BOX

• ASI1600MM Camera
• Soft Padded Case
• T2-1.25" Nosepiece Adapter
• 1.25" Cap
• 2-metre USB 3.0 Cable
• USB 2.0 Short Cable x2
• T2-1.25" Filter adapter (3mm Thickness)
• T2 Extender 18.5mm
• Desiccant Tablets
• Driver CD

I've highlighted the relevant item in green   Problem solved!!   I can use the mini filter wheel with one camera and the adapter ring for a single filter in the other.

ZWO have added several more adapters to what's in the box since this listing   This includes what would appear to be exactly right for the back focus on focal reducers & field flatterers for telescopes (with the ZWO EFW) so I'll add all the bits to my Esprit FF and carefully measure the spacing.  Plus, apart from the filter adapter ring a spacer that would appear to give the right spacing for camera lenses though this isn't shown in the diagram.  Item 1 in the diagram would be replaced by extension ring and camera lens (with or without "Russian" adapter).  Also, new since I bought my last one is a very nice soft carrying bag for the camera.  A nice addition though I doubt I'll use it. 

This is the an upgrade from the version I bought before which had a tendency for the front to come loose.

I've corrected an error in the diagram.  (Spot the slightly different font )

Something I've noticed in the diagram above - comparing the bottom two rows, the OAG has to be 16.5mm unlike the one's I have which are 21mm and won't give the right back focus.  I wonder if ZWO produce ultra slim ones

A bit later...  Yess!!  They do :- ZWO Off Axis Guider (OAG)  Yippee, just what I might want for the Esprit rig if not for the widefield  

I have to say I'm very impressed with ZWO   They seem to have everything sorted

Here is a photo of the two cameras, one with EFWmini and the other with single filter in filter adapter inside the 11mm F-F gender changer extension tube with a pair of 55mm f1.8 Asahi Pentax Super Takumar prime lenses.

No clear night skies forecast so this project may take a back seat for a while depending on how I feel.

I have just won an auction on ebay for a 28mm f2.8 Pentax lens which I can put with my present 28mm f3.5 lens for dual imaging   Total just under £42 including postage.  It has a Pentax K mount so I'll have to sort out an adapter (make or buy).

Once I receive this lens and sort out an adapter I shall have doubles of all my lenses and this can become my main imaging rig.  At least until I finish the DIY fork mount and micro dome etc. but I can't see that being any time soon - far more important things to do (like this dual imaging rig and the Ha solar imaging rig).

I hadn't realised you were looking to also do a widefield pair, will be interesting to see the results.

Been sorting out extension tubes and rings for Asahi Pentax M42 lenses and the ASI1600MM-Cool cameras, one with ZWO EFWmini, the other with fixed filter held inside the 11mm extension ring with ZWO adapter (supplied with camera).

Back focus of M42 lens is 45.46mm.  Back focus of camera is 6.5mm.  We want 45.46mm plus 1mm for Astrodon filter = 46.46mm.  So distance between the faces of the lens and camera wants to be 46.46-6.5 = 39.96mm.  ZWO EFW is 20mm so about 20mm of extension wanted and the full 40mm (approx.) for the other camera.  Since the lens moves forwards for focussing extending the spacers required, the extension required with EFW is 20mm+ (and 40mm+ without) - depending on the lens focal length; longer FL lenses move more.

Here is a photo of the dual rig with 55mm f1.8 Super Takumar lenses.  All the M42 thread mount lenses have the same back focus so should be fine with these extension tubes/rings.

Been trying the cameras for focus and despite the calculations I can't get focus on either of them - one is past the infinity setting and the other is past the nearest setting.  There is something very odd going on here as both imagers are correct by calculation and camera to lens distances match within a fraction of a millimetre.

I've managed to get both of these lenses to focus on the distant horizon within their focussing range by swapping extension tubes around.  Since these are 55mm the longer focus lenses should be alright.  I have yet to check the 28mm f3.5 lens and the 28mm f2.8 lens has a Pentax K mount which will need an adapter or two so I don't yet know about that.

The Pentax-M 28mm f2.8 lens arrived today - very good condition   Fits the adapter I bought for it too   Now I just need to find the EOS-T2 adapter that I bought a while back for another lens

I checked focus with the M42 screw mount 28mm f3.5 lens and it was on the limit - may need a bit more spacing.  I've ordered a 12-16mm adjustable T2 extension.  I checked the spacing and I reckon 14mm would be about right.

Since I have the back focus sorted out for the 55mm lenses I think I shall go with those for my first dual imaging rig.  I need to have the minimum to do so as to be ready ASAP in case of a clear night (oh the optimism!! ).  I'm hoping I shall feel well enough to get on with things soon.  ATM I'm only doing what's necessary.  I keep looking at my projects to see if I feel up to making any progress.

With medical problems out of the way and now having my "GinaRep Giant Mk2 3D Printer" virtually finished and working pretty well I am now looking at getting back to sorting out a dual widefield imaging system.  The bigger 3D printer means I can design and build a bigger micro dome capable of accommodating a larger imaging rig (well that's what I'm hoping).  This dual rig will go on my DIY fork mount on my second pier - Skywatcher Pillar Mount.  The EQ8 in my main observatory will be used for telescopes only.

I think I shall be combining the micro dome (or it might be a clamshell) and DIY fork mount with this thread as I think the entire dual widefield imaging rig would be best combined into one blog.

Here is an initial design for the camera clamp and Dec rotation wheel for use with the fork mount.  A hole through the clamp between the cameras will take a bolt and nut to clamp the cameras.  The camera holes are just a clearance fit on the body of the cameras.  The end parts will go into ball bearings housed in the ends of the fork of the fork mount.  I've shown a complete disc for the Dec wheel but the Dec axis will only move a maximum of 129° - from Polaris to the southern horizon.  Dec drive will be by either a belt or friction drive.  With good polar alignment the Dec axis would only be used for pointing the rig at the start of an imaging run.  All movement would be in the RA axis as this is an equatorial mount.

Clamp bracket added.  Plus ball bearings.  Drive disc will go on square cornered end.