With a dedicated astrograph like the Celestron RASA, the camera is mounted to the front of the telescope. Imaging at such fast speeds (f/2.2 with the RASA 11″) requires the camera be positioned at the optimal back-focus distance, otherwise the stars won’t be consistently focused across the entire image. In this post I’ll show how I’ve connected my ASI294MC and ASI1600MM-C cameras to a Celestron RASA 11″.

First off, it’s important to note the back-focus distance varies based on the Celestron RASA model you have. This post applies to the RASA 11″ only. With the RASA 11″, Celestron states the back-focus distance – from the top of the mounting plate on the scope to the camera sensor – is 72.8mm.

I use two different astrophotography cameras – a ZWO ASI294MC Pro (a cooled, one-shot-color camera) and a ZWO ASI1600MM-C (a cooled, mono camera). Both of these cameras have a back-focus distance of 6.5mm, or 17.5mm with the included T2 extender ring.

I decided to use the Baader UFC (Universal Filter Changer) system – a series of different sized telescope and camera adapters, plus extensions, that can be combined together and attached to a Baader filter slider base. The filter slider itself features a small magnet, so when you insert it in to the slider base, it stays in place.

Diagram showing the various Baader UFC camera and telescope adapters, extensions, along with the filter slider base. Image from the Baader website.

Even if you image with a one-shot-color camera, it’s important to use some kind of filter as most dedicated astrophotography cameras don’t block UV/IR rays, which can bloat stars and adversely affect your images. If you live in a heavily light polluted area, you may choose to use a filter that reduces the effects of light pollution – when imaging with the ASI294MC Pro I normally use an Optolong L-Pro filter, then narrowband filters when using the ASI1600MM-C. These filters block UV/IR wavelengths.

In order to achieve the correct back-focus distance of 72.8mm, I ordered the following adapters for my RASA 11″. Each extension features a small dovetail bar along the edge, and comes with screws to attach them together. I found this is a solid design, that clamps each extension together well without introducing tilt. I’ve listed the total depth of each part of the chain, along with the part numbers, and they’re listed in order going from the telescope to the camera:

  • 3mmBaader UFC S70 11″ RASA Telescope-Adapter (2459126) – this is the adapter that gets attached to the front of the RASA 11″ using the large threaded ring included with the RASA telescope;
  • 15mmBaader UFC S70/15 Extension (2459142) – a 15mm extension ring;
  • 16.64mmBaader UFC S70 VariLock (2459145) – a variable depth extension that adjusts from 15-20mm, and can be locked in position once set. I have mine set to about 16.6mm – I measured mine using a digital caliper;
  • 5mmBaader UFC S70 Extension 5mm (2459141) – a 5mm extension ring;
  • 13mmBaader UFC Base Filter Chamber (2459110) – the filter chamber that the filter slider is inserted in to. I use 2″ filters, so ordered the 2″ filter slider (2459112); 
  • 2mmBaader UFC T-2 (m) Camera-Adapter (2459115) – the adapter plate that attaches to the filter slider base. The camera’s T2 ring interfaces with this adapter.

Then, on the camera side:

  • 11mm – T2 ring – the ZWO cameras come with an 11mm T2 ring. This attaches to the T2 thread on the Baader slider base above;
  • 6.5mm – camera back-focus – the camera sensor itself is positioned 6.5mm from the very front of the casing.

When you are calculating the total depth, remember that the filters themselves add a small amount to the total back-focus distance. The general rule of thumb is to add a third of the filter depth (the glass thickness) to the back-focus. I use 2mm thick filters:

  • 0.66mm – filter thickness is 2mm, which equates to about 0.66mm added to the back-focus.

Taking all the above measurements in to account, the total comes to the required 72.8mm.

The final configuration showing the various parts of the Baader UFC system being used with the RASA 11″ to get to the required 72.8mm of back-focus.

You could do this a cheaper way by just positioning the filter slider base right on top of the RASA telescope adapter, then using various T2 rings/extensions to attach from that to the camera, however this would introduce more vignetting, as the filter is located much further away from the camera sensor.

Also, rather than buying the three separate 5mm, 15mm and 15-20mm extensions listed above, you could go with the one-piece Baader 40mm extension and find a T2 ring that is ~7.64mm in depth (assumes your filter depth is 2mm, which adds 0.66mm back-focus), to replace the 11mm T2 ring that comes with the ZWO camera.

Ultimately, I went with the adapters listed above because I wanted something that was precise – the Baader adapters are high quality, and the precision appears good, and I can set the exact depth I want by using the adjustable 15-20mm extension piece, and alter it further if the focus isn’t consistent across the field of view. If I ever use filters of a different thickness, I can also easily tweak this depth if needed.

This setup allowed me to position the filter slider close to the camera, and if I decide to ever upgrade my camera to a larger sensor, or use different thickness filters, I shouldn’t have to change much with my setup.

I hope the above helps. Let me know in the comments below if you have any feedback or questions!

17 thoughts on “Attaching ZWO cameras to a Celestron RASA 11″ using the Baader UFC system”

  1. Hi Dan,

    This article hit a sweet spot for me. Just got the RASA 11 and the ASi071MC Pro. To work with the 2″ L-pro filter, I am using the Starizona slider. The slider mounts directly to the camera (M42). The slider’s camera side is M48, so plenty of clear aperture to the sensor. The problem I am getting with vignetting appears to come from the rather narrow 16.5 mm M42 ZWO extension tube (which has M48 female to RASA plate and requires the use of the M48 adaptor ring to the slider). I am thinking of ditching the narrow optical path RASA plate/ZWO extender and using the Baader system from telescope to existing slider housing. I am confused as to what to order and through what outlet in the US. Can you point me to a vendor? I will need about 40mm from scope to slider ending in a male M48 male thread. So I expect to use the Baader RASA adaptor, the 15mm extender, the varilock, BUT would need an adaptor to drop down to the M48 slider. The only M48 adaptor I see on the Baader site is for their slider housing. Any thoughts? Or who to talk too? My other option is to just do the whole thing as UFC. If I did that, I think I would prefer to attach the filter slider directly to the camera to minimize vignetting on my slightly larger sensor. Was there a reason you do not do that on your setup?


    1. Congrats on your new RASA 11! I originally started with the ZWO extension tubes and was experiencing vignetting, I believe because of the narrow extension tubes.

      I don’t believe there’s a Baader adapter that offers either a M48 or M42 male threaded ring at the end, without use of their filter slider. You may want to go with an entire Baader setup – might be easier, as I’m not seeing an immediate option using the Baader configuration and Starizona filter slider.

      I purchased my Baader UFC bits from Agena Astro here in the US – just search for the Baader part numbers I listed above on their website, and you should find them.

      I could have mounted the camera a bit closer to the filter slider, but I wasn’t too concerned as I use 2″ filters and the sensor size isn’t too big on the cameras I use (ASI1600MM-C and ASI294MC-Pro). Also, the Baader UFC only provides the option of male threads on the filter slider side, so I’d need some kind of ring to attach the camera to the filter slider anyway. If I upgrade to a full frame camera at some point in the future, then I may need to mount the camera closer to the filter slider.

  2. Correction, if not obvious, the Starizona filter slider connection on the telescope side is M48. The camera side and camera are M42.

  3. Hi Dan,

    Thanks for the info. I will probably get the whole setup sometime soon, with the expectation that I will eventually get a full sized camera as well.

    My question about the close mounting for the slider on the camera was partly to do with a concern of mine regarding the appearance of the brightest stars in my images. In addition to a broadening that might be expected for intense star images, I am also getting a halo as well. Well, maybe you could call it a disk of lower intensity surrounding the star image. My concern is the potential from internal reflections between the filter and the camera window. I know that they are coated with antireflective, but nothing is perfect and the close proximity of the two surfaces might promote internal reflections that otherwise would get spread out and diffused if the filter was spaced well away from the camera. So I may be causing one problem (reflection) by trying to solve another (vignetting). I think the Baader system should solve all of this.

  4. Hi Dan,
    Exactly what Alan is describing above is what I am experiencing. I have a bad reflection around any slightly bright star. I am not sure where it is coming from but I do believe it is the filter and the window reflecting off each other. Are you experiencing any large halo around your stars with the Baader system.. As we spoke earlier I am using the Starizona system which is causing me a good amount of vignetting. I am seriously considering the Baader system if it corrects both issues.


    1. Hi Bob,

      I don’t normally experience large halos in my images when using the Baader system and highspeed narrowband filters, for example I don’t see any in this image: https://www.astrobin.com/411685/B/.

      However, I did once notice a large halo when imaging the Elephant Trunk: https://www.astrobin.com/411823/ – the bright star in the upper left had a fairly large halo around it, which I managed to mostly hide. If you look closely you can see a small white ring around the star, then a fainter, larger ring around that where I tried to fix/hide it. Maybe that one was particularly bad because it was very bright? This crop of the “before” version shows the halo, before I fixed it: http://www.nightskydan.com/wp-content/uploads/2019/09/ElephantTrunk-Halo.jpg.

      In the example above, the halo only showed up in the OIII data. The Ha data was fine.

      Apart from that one, I haven’t noticed bright halos in my narrowband images.

      1. I get them – but to be expected on such bright stars. I didn’t get any on the crescent because none are really bright stars. I’m using Baader and stuff to use my full frame OSC camera. I get only a little vignetting in the corners. It would be easily removed with flats, but I don’t bother doing flats or darks.

        I can photoshop out the discs – but I don’t find them objectionable – as they are a result of SUPER bright stars. I’m a retired pro photographer/studio owner that have seen such flares for over 30 years.


        Also – to all let me share that we have a RASA facebook group that is quite active. join us.

  5. Wish I had seen this last year. I have nearly same configuration but had issues as I have full frame sensor camera. Please with mine now over all – but took a year to get things solved (even sent it back for repair).

    1. Glad to hear you’ve got your issues solved now! Took me some time to figure out the best adapters to use, before I finally settled on the ones above. I’m pleased with how it performs so far.

  6. I still have some issues with an uneven focus across the field. I do not lock the mirror as I’m remote. I think if I did it might flatten the field. It’s not always a problem and can come and go. I suspect it’s mirror flop because of this. Still with a 21mp file of 70mb I can “hide a lot of sins”.

    1. SEnt it in under warranty and C fixed it up nicely – I’ve heard so many of the C11’s had to go in for fixing due to the rotating lens array in the corrector plate. They covered shipping to and from and It’s great now.

  7. For the 294mc pro what filters do you use? I currently have an L Pro and Lenhance. Would you recommend either with the RASA 11?

    1. With the RASA and ASI294MC-Pro, I use the L-Pro for broadband targets like galaxies, and now mostly use an Altair QuadBand filter for narrowband targets.
      I think the Ha bandpass of the L-eNhance filter is a bit too narrow for the fast RASA, as I get a stronger Ha signal when I use the wider bandpass Altair QuadBand filter.

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