Filter drawer for RASA 8 and cameras with 17.5mm back focus

[fwm891]

Recently I bought a RASA-8 from a fellow club member (my second RASA 8 having sold the first - fool that I am). Whilst with my original RASA I was using an ASi533 it was possible to fit a filter drawer between the RASA's camera mounting plate and the 533 because the 533 has only a 6.5mm back focus. However now I have an ASi2600MC Duo which has a 17.5mm back focus. Searching the web I can't find a filter drawer/slider for the combination of a RASA+17.5mm BF camera.

Anyone not knowing why there's a problem. The RASA-8 has a total working back focal distance of 28.73mm with the ASi533 and it's 6.5mm back focus you're left with a hefty 22.23mm to fit a filter drawer into. For cameras with a 17.5mm back focal distance you're left with just 11.23mm. Then there's the thickness of the camera mounting plate, the filter depth....

So. My aim was/is to be able to place unmodified mounted filters between the camera mounting plate and my camera without having to remove the camera each time a filter needed changing.

This prototype (MK 1) will take filters with a total depth (side wall + 48mm thread) no greater than 7.8mm For me this means that my IDAS NBZ and Askar 6nm Ha/OIII filters are okay. My Baader UV/IR cut filter is too tall and if I'm going to use this filter then I will have to reduce the total depth of the side wall to 7.8mm or below. I can't see how I can get any more free space for deeper filters.

What I've done:

I've made a new camera mounting plate to replace the 42mm camera adapter plate supplied the the RASA originally (I still need to slot this plate to access the RASA's tilt adjusters) and cut a channel across the plate to the level of the clamp (Photo 02). The adapter could have been made to the same thickness as the clamp's retaining edge but I felt that by making it thicker and adding a rebated edge down to the thickness of the clamp's retainer it would give me a better positional register for the adapter/camera assembly. I can still rotate  the adapter/camera assembly for framing if needed.

The unit is in two parts: the new adapter plate and camera mounting plate. On the MK 1 prototype only two 2.5mm machine screws hold these parts together. On the final four possibly five 2.5mm machine screws will be used to register and secure the two parts together. The upper camera mounting plate has the deeper channel and the 54mm camera mounting thread. The channel depth has been taken to almost it's maximum as there is only 0.42mm of metal left between the channel and the camera front face which I flet should be left to protect the camera from the filter drawer entry/exit motions.

Currently waiting on some metal to make the filter drawers themselves and some shorter 2.5mm machine screws so the don't go right through on MK-2 !

TBC...

[fwm891]

The metal for the filter drawer/slider arrived and I've had a play.

It proves that a filter will go between the rasa 8's mounting plate and a **camera with a 17.5mm back focus. I'm restricted to certain filters due to the filter's physical depth and that will probably alway be the case with mounted filters.

**(with my ASi2600MC Duo anyway).

I now need to go back and make a couple of minor alterations to the main body and to the slider. Add some finger grips to enable the sider to be placed/removed safely and a small magnet to act as a retainer so the filter remains in place.

A light blocker around the closure area to ensure there are no false gradients or patches generated across frames.

I also need to make a better holder to retain the slider whilst machining the 48mm filter thread. One which will stand up to making more than the single prototype.

TBC.

Thanks for looking.

[globular]

Looking very good 

If you're up for making more than just for yourself then I'd be interested in buying one or two. (Not for a RASA but for minimising back focus usage with binoviewers on an EdgeHD.)

[gorann]

Impressed! Looks very promising. Maybe you need to blacken it all too. Keep us posted.

[fwm891]

2 hours ago, globular said:

Looking very good 

If you're up for making more than just for yourself then I'd be interested in buying one or two. (Not for a RASA but for minimising back focus usage with binoviewers on an EdgeHD.)

@globular I’m able to build this because my camera has a 54mm threaded connection and I’ve been able to modify the rasa’s camera connecting plate to suit. It won’t work for a 48mm or smaller connection because there’s no support for the male thread into the camera.

To connect to your binoviewers I don’t know what connections are involved.

[fwm891]

26 minutes ago, gorann said:

Impressed! Looks very promising. Maybe you need to blacken it all too. Keep us posted.

Thanks Gorann, it will be fully blackened inside and out.

[globular]

5 minutes ago, fwm891 said:

@globular I’m able to build this because my camera has a 54mm threaded connection and I’ve been able to modify the rasa’s camera connecting plate to suit. It won’t work for a 48mm or smaller connection because there’s no support for the male thread into the camera.

To connect to your binoviewers I don’t know what connections are involved.

Connecting an M54 into my train won't be an issue - I've accumulated lots of adapters over the years - but if you're modifying an existing rasa connecting plate to make this then you won't have a surplus of them to make more. No worries.

Keep posting updates - can't wait to see the finished working version.
 

[tomato]

Great job, I take my hat off to you to make a slide in filter using the amount of back focus you have to work with. 

[fwm891]

3 hours ago, globular said:- but if you're modifying an existing rasa connecting plate to make this then you won't have a surplus of them to make more. No worries.

Keep posting updates - can't wait to see the finished working version.
 

I’m making new rasa camera connecting plates because they need a channel cut across them to accept the filter slider. Plus I’ve modified how the clamping nut registers against the camera mounting plate.

[globular]

Very impressed.

It looks like the filter screws into the slider (rather than just drop in)?
So I'm guessing you'll make a slider for each of your filters for quicker, easier changes?
And a nice storage box to slide them all in for safe keeping will be on the cards too no doubt?

[fwm891]

I ended up making a support jig for the different stages of producing the filter trays/sliders.

The first photo shows the three stages: left most is the sizing of the blanks. Here I position rough sawn blanks each held in place with three 5mm cap head screws. Blanks are over size by about 0.5 - 0.75mm all round. Mid position is where the main maching takes place the central hole is cut, the 180° outside curve and the rebate the filter screws down onto. The righthand area is where the 48mm filter thread is cut.

Photo 2 shows a blank being sized to 50.75 x 65 mm with a 2mm radius on each corner. (The tape over the other areas is to keep the swarf out until they're needed.

Photo 3 shows 5 filter trays/sliders ready for the threads to be cut. That should give me 6 trays/holders.

Thread cutting tomorrow and a small mod to the mid section of the camera adapter. Big clean up and blackening all the parts.

TBC...

Photo 1

Photo 2

Photo 3

Edited October 23, 2023 by fwm891
Headers added

[fwm891]

I've cleaned all the engineers 'blue' off the pieces now and taken off all the burrs so it's now waiting for the final clean-up before being anodised (black). Three more photos of it fitted back on the camera. The corners of each of the filter holders sits just inside the clamp nut's outside diameter so shouldn't cause any extra diffraction effects. There will be a small tuck-away lever to insert/remove the filter holders which will also act as a lock to stop the filters moving. I managed to get my Baader UV/IR cut filter in but only by machining 0.8mm off the leading edge of the filter's mounting (I did take the filter glass out first).

Photos are fairly self evident: My filters in their holders, Interior view with the UV/IR cut filter and a view into the filter slot with a filter in place.

(the inserted filter in the bottom photo is the wrong way up!!!)

Thanks for looking

Edited October 28, 2023 by fwm891
Text added

[fwm891]

First imaging test with the filter drawer 15x 240s filter: Baader UV/IR cut.

Parts still to be black anodised, star images are good right to the corners. This has been cropped a little for composition but pleased with things so far.

[w4m0330]

Great work!

I am curious how the guiding works? Could rasa 8 sufficiently illuminate the small chip in the 2600mc DUO? More interesting to hear how dual band narrowband, e.g., IDAS NBZ, works for guiding?

[fwm891]

On 20/01/2024 at 19:05, w4m0330 said:

Great work!

I am curious how the guiding works? Could rasa 8 sufficiently illuminate the small chip in the 2600mc DUO? More interesting to hear how dual band narrowband, e.g., IDAS NBZ, works for guiding?

Thanks.

Yes the Duo guides with NB filters like the NBZ (which I have) also with a 6nm Askar HaOIII (D1). I have had to increase guide exposures from 2 sec to 2.5/3 sec depending on star fields. The RASA 8. will illuminate the second chip and at f2 guide stars are brighter.

[Crispy5573]

Hi I've just received an Astrodevices filter drawer and a tilt corrector for my RASA 8. These are 3D printed with carbon fibre re-enforcing. I believe they are no longer in production but there are a few left if you search. The drawer was designed (like yours) specifically for the RASA 8. Although they are out of production if you go to their website they have the drawings of the individual parts. I thought it may help with your project or give you an idea or two to develop yours further. Having said that you're making a great job of it so far. Well done. 

[ollypenrice]

Lovely job, Francis.

Olly