Filter Size Query: For use with ASI294 & Skywatcher 0.85x Reducer

[feilimb]

I may be investing in a filter or two for use with an ASI294MC Pro, which is connected to a 0.85x Sky-watcher Focal Reducer (in turn connected to Sky-watcher ED80) and I have little idea on what size filter I should go with and how to get it into the imaging train.  Currently the ASI294 is connected to the focal reducer using the adapters which came with the camera which are:

• 11mm T2 ring (screwed directly onto the camera)
• T2 Extender 21mm
• T2-M48 16.5mm Extender

There are some other adapters in the box also however, which may assist with getting a filter in there somehow, see the listing of what is included at eg (look for 'What's in the box' section):

https://www.firstlightoptics.com/zwo-cameras/zwo-asi-294mc-pro-usb-30-cooled-colour-camera.html

I think one option I might have would be to go for this adapter (and then screw a 2" filter size into the top of it) - but I think this would put the filter quite a distance away from the imaging sensor:

https://www.firstlightoptics.com/adapters/flo-adapter-for-skywatcher-focal-reducers.html

and I would no longer be attaching the reducer to the main scope via threading, just using thumb screws which might be a bit of a concern.

Can anyone advise if they are using a similar configuration, and whether a 1.25" filter might be easier to incorporate.. and if so how!  I only want support for a single filter at a time, don't want the overhead of filter wheel etc. at this point.

[feilimb]

*bump* would anyone have any advice on this? 

[michaelmorris]

I have recently bought an ASI 294 MC and I'm using it with exactly the kit you are using.  There is a Male T2 to female 1.25" adaptor in the box which threads into the thick (10mm I think) adaptor that attaches direct to the camera.  I've read that when used this close to the sensor, a 1.25" sensor only gives marginal vignetting, which can be compensated for by using flats.   However, I've chosen to go down the route of using a single 2" IDAS light pollution filter attached to a FLO focal reducer to 2" adaptor.  I attach the 2" adaptor to the focuser draw tube using a Baader Click lock adaptor.

https://www.firstlightoptics.com/adapters/baader-click-lock-2956256-m56-celestron-skywatcher.html

This holds the focal reducer orthogonally and, for all practicable purposes, as firmly as if you had screwed it on. 

I already had both the FLO adaptor and the Clicklock adaptor and the filter I wanted was only available in 2" https://www.firstlightoptics.com/light-pollution-reduction-imaging/idas-d2-light-pollution-suppression-filter.html , so this solution seemed to make sense.

 

Edited November 23, 2018 by michaelmorris

[feilimb]

@michaelmorris thanks for the really useful information, it's great to hear you have the same gear and read how you solved the filter conundrum in that thread. I think I will go down a similar route with the 2" filters and flo adapter.

Have you had a chance to try out the idas filter yet with the asi294? 

[michaelmorris]

34 minutes ago, feilimb said:

@michaelmorris

Have you had a chance to try out the idas filter yet with the asi294? 

Yes, but I haven't yet done any processing beyond alignment and stacking in DSS. The colour cast with the D2 is a bit  blue/green with more of a colour cast than the P2, but less than with a CLS filter.

[feilimb]

Sorry to bump this again, but I am now thinking of going down the 1.25" filter route.  This is mainly as it will be cost saving - since 2" filters are more expensive, and since I may already the adapters required (using the adapters which came with the ASI294MC Pro).

Looking at the adapters which came with the camera however, I am not convinced I can get a 1.25" filter into the image train and before ordering any filters of this size it would be great to hear from anyone who has successfully used a 1.25" filter with this camera (and without filter wheel). 

Has anyone else out there succeeded in using a 1.25" filter with the adapters that come out of the box, and then attaching the camera to SkyWatcher focal reducer..?  I can attach some photos of the adapters which come with the camera in case they are useful for figuring this out..

[Graphic]

Hi, just seen this post. You could go down the route of a filter draw and 1.25 holder. Or buy the zwo efw mini filter wheel and connect this directly to the camera body, no spacers between the two, this would enable you to use 1.25" filters without suffering vignetting. The only thing I would comment on here is to buy some of the delcrin spacers from FLO, placing the thinnest between the filter wheel and the camera body, thus enabling you to unscrew them easily once finished. You would still need to take the remaining distance with you spacers to get to the 55mm distance. Hope this helps?

 

regards.

[droopy2]

Hi, a much belated addition to this discussion. I have just added an ASI294 with a ZWO EFW with 7 1.25 filters to my rig (WO GT81) and I'm getting horrendous vignette which varies from filter to filter the worse one being Ha. The camera is directly attached to the filter wheel with no shims. Wish I'd bought the 2" version now despite all the reassurance that 1.25 would be ok. It calibrates out but there is so much light being lost!

[nfotis]

It's my understanding that the distance of the filters from the sensor is a major parameter (and the scope f-ratio another).

There's a calculator for these:

https://astronomy.tools/calculators/ccd_filter_size

In general, 36mm filters work up to APS-C sensor size for most medium speed scopes. Vignetting is a problem indeed, but you can reduce its impact via flats (I think).

N.F.