How can I introduce a filter wheel to imaging train?

[RobertI]

Hi all, I am looking to introduce a filter wheel (preferably motorised) into my imaging train for RGB and some narrowband imaging and want to know what my options are. 

Currently my imaging train consists of the following:

Lodestar camera -> 2” to 1.25 “ converter -> Baader SCT to 2” adaptor -> Meade F3.3 SCT fit Reducer -> Moonlite SCT to 2” adaptor which fits into draw tube.

Something like this:

The camera body to reducer distance must remain constant at 35mm. Apart from that I have plenty of focus travel inward and outward.

Can anyone advise on what my options are for a motorised filter wheel which provides the correct connectivity and retains the correct camera to reducer spacing?

Many thanks.

Rob

[Davey-T]

There are a few  filter wheels around that will fit with the 3.3 F/R but it doesn't lend itself to imaging with "modern" equipment, you might be better of getting a 6.3 f/r.

Dave

[RobertI]

Hi Dave, thanks for that. Just to clarify, what I currently do with this setup is Electrocially Assisted Observing which involves taking brief exposures of 10 to 30 seconds and stacking them in real time to view a live image on screen. The 0.33 reducer and sensitive Lodestar works well to get the most out of the brief exposures although I sometimes use a 0.63 reducer for brighter objects and sometimes no reducer at all. So for the moment I don't anticipate changing this configuration. Do you think I will have problems with a filter wheel?

[ollypenrice]

Your avrerage filterwheel is about 20mm thick so why couldn't that just come out of the Baader adapter distance in the graph above? You'd then build up the missing distance with extenders available from FLO, Baader, TS, etc. I've a feeling Ime missing something. (I often do!)

Olly

[Astrojedi]

Is that diagram for a SCT or RC scope? If using a SCT I would recommend attaching the 3.3 FR to the SCT and then using a t thread optical train. Most filter wheels have t threads. You can then get a t thread to a C mount adapter at the end. You will get better focus and alignment and to the optical axis and less flex with t threads.

Most filterwheels specifications will tell you what their optical length is.

If using the setup above as per your diagram I would recommend the following train (note I have switched which side is the scope).

scope -> 2" focuser -> 2" to SCT adapter -> Meade 3.3 -> SCT to t thread adapter -> Filterwheel w/ threads -> (additional t thread spacers if required) -> t thread to c mount adapter -> Lodestar

[RobertI]

Thanks for the replies.

Hiten - it's an RC6, sorry meant to say that. OK, so if I understand correctly I remove the Baader adaptor and replace with the filterwheel plus T thread adaptors either side. It's all screwed together so I get the correct sensor-reducer distance with spacers? Makes perfect sense.

Olly - you weren't missing anything, the solution seems to be exactly as you describe! I should also have added I have never used a filter wheel so wasn't sure what potential traps were lying in wait for me, but fortunately seems straightforward.

Thanks to you both.

Rob

[DoctorD]

Hi Rob

I agree with Olly - T adapters is the way forward.

SCT to T adapter, Filter wheel, T to 1.25", Lodestar

This will allow you to fine tune the distance between the reducer and sensor to get the correct spacing (use a parafoal ring on the Lodestar.

Alternatively:

SCT to T adapter, Filter wheel, T spacer, T to CS, Lodestar

This will be more ridged but you will need to get the right T spacer to get the correct distance between the CCD and reducer. A low profile T to CS adapter will give you more flexibility on choice of T spacers.

I use this second arrangement with my Lodestar for EEA (albeit without the filter wheel)  as it keeps the CCD secure (I found that my 1.25" adapter allowed the Lodestar to move slightly from side to side)

HTH

Paul

[RobertI]

Thanks Paul, I hadn't thought about the different options at the camera end (ie: T to 1.25" as opposed to T to CS).

I like the idea of the T to CS option, I have had a look on the web and have identified a filter wheel, low profile adapter and spacer which will get me to within 1mm of what I need so I think this route is workable, however everything screws together and I will not be able to rotate the camera relative to the reducer - this may not be a problem but I have read a post from someone on SGL or CN stating that their Meade 3.3 reducer is not even around the central axis, so may get coma in one orientation but not in another. I doubt whether this will be a problem for me and I guess I can test it on mine to see if it exhibits the same problem. 

But the options are very clear now and many thanks for the input.

Rob

[DoctorD]

Hi Rob

My SCT to T adapter allows me to rotate the camera with respect to the reducer - however it's not low profile, so may cause issues with spacing when used with the filter wheel.

Might be worth looking around.

Here's my set up - note that the reducer sensor spacing is incorrect in this photo - I've removed the colimatable 10mm spacer and am still experimenting with the spacing.

HTH

Paul

[RobertI]

Thanks Paul, it's really helpful to see your actual setup, and it's also interesting to know that collimatable extensions rings and rotatable adapters exist! That gives further options. Obviously my challenge is the small camera to reducer distance (forced upon me by the limitations of the F3.3 reducer which only seems to produce decent results for me at F4.5) but it looks doable.

Thanks agian for your help.

Rob

[RafaelP]

First post here!

Paul,

I have never seen a collimatable adapter like that before.  What is the brand?  I tried to find it online with no luck....

~Rafael