DSLRs for Widefield Imaging

[Gina]

I've been re-thinking my ideas for a widefiled imaging rig using DSLRs and camera lenses as a completely separate system from my main pier and mount in the observatory.  Ideas for pier and mount are discussed elsewhere - this thread is to discuss the cameras.

I have a full-spectrum modified and debayered 450D which I'm Peltier TEC cooling and including a filter wheel in the overall casing.  I was going to do the same with an 1100D but the debayering job is extremely difficult with this particular model so I'm now thinking of using an 1100D as an OSC cooled camera to complement the 450D mono with filter wheel.

Options considered so far for DSLR widefield imaging include :-

1. Single camera modified for full-spectrum and mono with integral filter wheel.
2. As above plus camera modified for full-spectrum but left as OSC.
3. Triple setup with three full-spectrum, mono cameras with integral filter wheel or filter drawer.

All these options would include fully set-point Peltier TEC cooling.

I shall probably go for Option 1. initially and see how it goes with Option 2. as the next step.  Option 1. is already covered in another thread - http://stargazerslounge.com/topic/196608-adding-cooling-and-filter-wheel-to-a-debayered-450d-mono-dslr/.

For Option 2. I was thinking of keeping the existing lens mount and plastic frame that holds the sensors orthogonal to the light path and thereby avoiding the alignment adjustment needed in the integral FW version but that leaves another problem of sealing the light path.  The latter is one of the things I'll be looking at in this thread.

[sharkmelley]

I'll watch with interest!  I've seen some of your previous threads on DSLR modding and they're quite inspiring.

Mark

[Gina]

Thank you Mark

[Gina]

I believe I have a solution to the sealing problem.  If I remove the bayonet ring and spring (plus spring and plunger) I could replace it with an aluminium plate with a hole containing an IR/UV cut filter to act as an optical window - as well as IR/UV cut filter (needed after full-spectrum mod).  An adapter with M42x1 Pentax/Praktica thread would be attached directly to the ali plate.  A 3mm ali plate would exactly match the bayonet ring and preserve the back focus.

I'll produce a drawing to show the whole cooling system but meanwhile here's a photo showing removal of the bayonet ring.

[pete_l]

If I'm reading this right option #2 is for 2 cameras: one full-spectrum mono and a second kept as OSC.

When you process the images, the definition of the image comes from the L channel. The RGB channels can be lower resolution (i.e. from an OSC, or binned) and that won't hurt the final image.

So with this option, you could shoot L and RGB simultaneously and then if you want Ha, use the mono camera with an add-on filter and shoot another session (if the cooling allows you to get a sufficiently low noise image with a DSLR sensor.

[Gina]

That's right

[Gina]

Here's a cross-section diagram.  The optical window has to be inside the camera because some of the vintage film SLR Pentax/Praktica thread lenses protrude outside the casing at infinity focus.

[Gina]

I've been thinking some more about this project brought on by considerations of my second pier as discussed in the "DIY Observatories" forum and working out how much room I would need in a mini-obsy to hold it.

There are a couple of points particularly. 

1. Camera lenses are not apochromatic with the red focus being quite a long way off from the green and blue.  This means that an OSC camera is not really suitable for astro use with camera lenses.
2. I think that for widefield imaging NB is more suitable than LRGB.

Those plus the fact that OSC cameras are less sensitive than mono leads me do dismiss Option 2. in the first post above.

So that leaves Options 1. and 3.

I have been working on Option 1. but not really got all that far as yet.  For instance I have done nothing towards the filter wheel other than buy the disc and centre bearing.

BUT as everyone here knows - I'm into triple imaging   So you know where this is going - don't you?  Yep!  Triple imaging rig with three debayered and super cooled 450Ds one with an Ha filter, the second with an OIII filter and the third with an SII filter.  This also ties in with my love of narrow band imaging   HaLRGB I use for galaxies.

However, all this depends on how well the debayered and cooled 450D actually performs.  So my plan now is to take the debayered sensor and put it in a 450D with the plastic main frame complete, to contain it in a sealed container and super cool it with Peltier TEC and dry it with silica gel.  I'll put the Baader 7nm Ha 36mm filter in it and see how it does.

[Gina]

Now I have a lathe I shall be able to turn a custom fitting for the lens and filter.  The NB filter will act as an optical window as well as filter to stop moisture getting into the sealed enclosure.  The fitting will be similar to the M42 to Canon EOS adapter shown above but without the bayonet of course.  The flange will be bigger to aid screwing the fitting to the aluminium end plate and the bayonet replaced with an extension to hold the 36mm unmounted filter.  Drawing to follow...

[Gina]

Here is a design for mono 450D with sealed dry chamber and super cooled sensor with narrow band filter providing the optical window to seal the dry chamber in the light path.  The DIY lens mount replaces the bayonet ring and carries the NB filter.  It is screwed and sealed to an aluminium front plate.  Aluminium angle is used to attach the copper dry chamber box to the front plate.  The copper box is formed from copper sheet bent up and soldered at the corners.  Drying will be provided by silica gel bags in the spaces between camera and box (not shown in diagram).

[Gina]

Slight redesign of the lens mount and front plate.

[Gina]

Yet another small design change to reduce the cost of aluminium and amount of turning for the custom lens mounting.  This can now be turned from 2.25" OD x 0.5" wall Aluminium tube

[Gina]

There is another use I have for a mono DSLR and that is with the Esprit 80ED when I get it back (in the next week probably).  That should just about cover the APS-C format sensor and would be a good test of the scope   So I shall want one with a full spectrum optical window (just IR/UV cut) which I already have.

[Gina]

450D with alternative front mounting with T2 female thread to match thread on EFW2 and with IR/UV filter fitted, for use with Esprit 80ED with field flattener.  I shall need to work out what spacing I shall require from T2 extension tubes but here is a drawing of the camera part of the imaging rig.

[Gina]

Just remembered - the back focus required for the Esprit flattener is 54.9mm and the camera is 44mm.  That leaves only 10.9mm for the filter wheel and it's 22mm so can't be done.  Anyway, the EFW2 has 1.25" filters - no good for APS-C format.  So it's back to the version with built in filter wheel then with 36mm unmounted filters   Guess I could end up with 4 mono 450Ds then    That would mean buying another used one

[Gina]

To use the mono 450D means using the cut down camera with filter wheel but I would like to have full facilities to use with the Esprit.  This means allowing for LRGB filters as well as NB meaning 7 filters.  However, there are two reasons to go for 8.  One is to allow a possible extra filter and the other is that an 8 hole version would be a lot easier to make and if I use direct drive from a stepper motor (as I would like) the number of steps between filters would be exactly 512 as the stepper gives 4096 steps per revolution.  Th drawback is that 8 holes take up more room than 7 and makes things tighter.  However, I do have a design with 8 holes in a 150mm diameter wheel (carousel).  This could be housed in a casing made of two 180mm diameter x 3mm aluminium plates.

Here is a diagram of the wheel layout.  Next step will be to design the way the filter wheel fits onto the camera.

[Gina]

Here is the filter wheel attached to the camera.  The camera has it's own front plate that is attached to the back plate of the filter wheel.  The optical window is sealed into the camera front plate so that the sealed dry chamber containing the camera and cooling system is a self-contained unit.  Unlike the carousel, this diagram is NOT to scale.

[Gina]

Drawing showing section from another angle and the stepper motor directly driving the wheel.  Again, not to scale.

[Gina]

Since this thread is about the widefield rig with SLR lenses and for that I plan to use individual cameras for the three NB imagers, I shall continue the design for the Esprit in the 450D with FW thread, which is more appropriate.