RASA 8 and ASI2600 MC DUO filter Mod

[fwm891]

I'm trying to help a friend out with a problem in getting filters between his new ZWO ASI-2600MC DUO camera with it's 17.5mm back focal distance and his RASA 8 corrector lens assembly. The total working distance here is 28.73mm (according to Celestron) so with the 17.5mm back focus taken off gives 11.23mm. Fortunately a 2mm filter thickness is required when using the RASA 8 so no additional spacings are needed (just need to remove the factory fitted one).

Originally I was going to machine an adapter from a solid aluminium disc but the RASA 8 comes with a 'C-mount' which just happens to be the correct depth once the 'c-mount' is removed, meaning I don't have to machine the radial slots for the RASA collimation adjusters.

The ASI2600MC DUO has an M54 female front thread to accept any attachments. So I needed to put a 55mm minimum central hole in the RASA's C-mount plate to give a clearance to the new insert's 54mm thread.

A piece of ali rod was then turned to match the internal dimension of the C-mount and it's centre bored out to 45mm in prep for cutting the 48mm filter thread later. The outside was the turned down to 55mm over an 8mm distance to be a snug fit for the hole in the C-mount plate.

The outward 5mm was then threaded to 54mm x 0.75mm pitch to match the threaded face of the camera. This would take the threaded section below the approx 4mm thickness of the C-mount RASA adapter plate to prevent any thread fouling.

The ali rod was then parted off with a small excess. The 45mm threaded section was inserted through the C-mount adapter plate and a 54mm extension tube used to lock the insert in place inside the C-mount plate.

The extension tube could then be placed in the lathe and the inner face of the insert cleaned up to size and the 48mm x0.75mm internal thread for the filters cut. The 5th image has a schematic inserted giving a cross section (not to scale) of the insert.

Internal view - Filter in place

Internal view - Filter removed

External view showing the 54mm male thread to fit the camera and the filter thread

Internal parts view

External parts view - Cross section (not to scale) to give an idea of the insert shape.

The adapter mounted on an ASI 2600MC DUO to show clearance to guide sensor

Edited August 14, 2023 by fwm891
Text to photos added

[tomato]

Nice job.👍

What do the stars look like on the peripheral guide sensor at f2? I had lovely sausage shaped stars using an OAG with an Esprit 150 focal reducer, but PHD guided on them OK.

[fwm891]

39 minutes ago, tomato said:

Nice job.👍

What do the stars look like on the peripheral guide sensor at f2? I had lovely sausage shaped stars using an OAG with an Esprit 150 focal reducer, but PHD guided on them OK.

Not had any weather breaks to try it yet…

[Starman0103]

On 14/08/2023 at 03:16, fwm891 said:

Not had any weather breaks to try it yet…

Ran across this thread as I was searching to do this very thing.  How did it work out?

[fwm891]

Hi, This has worked really well, my only problem came with trying to use an older Baader filter that was too deep in the body but all the modern 2 inch filters I’ve tried fit okay.

Guiding I thought might present problems with NB filters (6nm) is my narrowest at present. Even then with the duo I have plenty of stars to guide with. At least I haven’t imaged anywhere that’s not given me guide stars. NB filters may need longer exposures for guiding that may be a problem with harmonic drive mounts. I have an iOptron HEM27 which is okay with 2 second guide subs but prefers under 1 second.

 I machined my filter holders in ali but they could be printed easily enough. 
 

Good luck with the build.

Clear skies 

Francis