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Antares low profile focuser for SCT and back-focus

Celestron Cassegrain Astrophotography SCT
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#1 rknhe


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Posted 28 March 2023 - 06:15 AM

Anyone tried Antares low profile focuser (Part: FGLPS) with SCT (e.g. with Celestron 11" EdgeHD 11 )



I am trying to figure out a few things about this focuser.

 (1) How much of a back-focus would it consume (minimum. When the draw tube at 0 position)?

 (2) Anyone tried installing ZWO EAF motor with it for remote operation?

 (3) Any field experience with this focuser, its quality, built and use with  astrophotography sessions?


SCTs have limited optimal back-focus (e.g EdgeHD 11 has 146.5mm), so looking for a low profile external focuser, that allows enough space for other astro-imaging gear (OAG, Filter/tray/wheel)



#2 GoldSpider



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Posted 28 March 2023 - 07:28 AM

I've pondered this very thing, and I don't understand how these can work at all unless the reducer is moving with the focuser.  Without that, aren't you changing the backfocus spacing every time you adjust focus?

Edited by GoldSpider, 28 March 2023 - 07:35 AM.

#3 rknhe


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Posted 28 March 2023 - 08:23 AM

Sorry, if it is repeated for many. I can speak from C11/EdgeHD perspective(also for 9.25", 11", and 14" versions). The external focuser is used mainly for a very fine adjustment and keeping back-focus spacing always around 146.5mm(+/- mm). Typically, I start with the external focuser (drawn out some)+ rest of the imaging train at nearly 146.5mm. Then use the scope's main focuser (that moves the mirror inside) to achieve focus. This conforms to the 146.5mm requirements. Then lock the main scope mirror (it is possible on edge HD versions) and don't touch the scope main focuser after that (avoids image-shift and mirror flop). Finally, achieve (very minute) focus with the external focuser(if needed at this time). So the external focuser is used only for very small changes (temperature effect etc) over a long session. This can be done with or without a reducer before the external focuser for edgeHD versions.


If the external focuser is correcting a lot, then yes, it changes the back-focus far away from optimal (146.5mm) and doesn't produce the intended result.

Edited by rknhe, 28 March 2023 - 08:23 AM.

#4 GoldSpider



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Posted 28 March 2023 - 10:03 AM

Apologies for jumping in since I incorrectly assumed you were talking about AP and using with a focal reducer.  It seems that in both cases there is a certain amount of tolerance in the backfocus requirement in both cases, and that small focus corrections wouldn't severely degrade edge performance. 


Assuming that's true, I'm still interested in seeing others' first-hand accounts of how well this focuser performs. 

#5 luxo II

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Posted 28 March 2023 - 11:46 PM

This goes on the back of the SCT and everything hangs off the back of the focuser - and moves with it.

What matters most is the spacing between the reducer and the camera - which remains constant.


SCT -> focuser -> reducer -> filter wheel -> camera, or

SCT -> focuser -> filter wheel -> reducer -> camera.


This one is poor design however in that the imaging train has to hang on a drawtube inserted in the back and secured by a compression ring and two screws. This will result in the lot flopping as the scope moves around and will play havoc with collimation. The Baader ClickLock would have been a vastly more secure way to connect the image train.


Better ones (I have one) have a male thread on the backend of the focuser so the image train is screwed on securely and cannot move, wobble or flop as the scope moves. Mine was a low-profile one (50mm) from Teleskop-Express and has a 63mm male thread on the back of the drawtube; a 63mm - 48mm adapter ring suffices to connect the imaging gear.


Only snag with mine is that it's no longer available.

Edited by luxo II, 29 March 2023 - 02:39 AM.

#6 MikeBY


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Posted 06 April 2023 - 06:08 AM

You really should find a focuser with threads if you are going to all the trouble and expense. 


What size imaging sensor?  It needs to be small to do this.


You need to change backplate if you are going to go with APS-C or Nikon DX size. That's not what I mean by small. APS-C and DX are LARGE. 


The Biggest focal reducers offered by Celestron do not support above APS-C  without vignetting.. 

Yes, you can read the specs on the big $850 FR and the big ones do have 60mm aperture, but they only support a 42mm interface and at 146.5 mm backfocus don't expect miracles.  Remember that the reducer concentrates the photons into a smaller area so the 0.7x reducer takes an images circle that starts at 42mm and it ends up as 29.4mm. That's just enough to cover the DX diagonal of 28.8mm. and you must consider about a 50% energy drop-off as the higher ncident angle light gets cut off. So for Focal Reducers other methods are needed. 


But the OP didn't say there was a FR involved. 

But Edge HD isn't a simple SCT. It's more like Petzval refractor. An additional set of optics sits inside the baffle and does 0.x flattening.  

That's why the backfocus is so restricted and really must stay at 146mm +/- 0.5mm from the rear of the scope for AP. It doesn't matter where the external focuser sits. The OP has it correct. It can only be used for microfocusing. That's in the +/- 0.5 mm realm. It might add some flexibility too in avoiding the issues encountered with small 1mm or 0.2 mm shims. 

So without a FR what matters is clear internal aperture and the ability to identify and repeat a midpoint location on the external focuser as a  zero point. That means electronic focusing. 

Why? Because A) you need to focus both directions B) The zero point is your 146mm point and you need to be able to calibrate and repeatedly find this position on the focuser.


Check 1st the actual clear aperture at the back of the scope. You may find surprising field stops where you least expect them. The SCT threaded interface on the back of the telescope is about a 39 mm clear aperture.

The conventional FR that threads on is 38mm.

The 2" visual back is 50.8mm whatever you connect to it will at best have 48mm clear (for a M48 filter thread and the thread stop brings you down to 47mm maximum. Many adapters are much less as 47 mm clear on a 50.8 mm interface is a very very thin wall of 1.9mm

Most will be closer to 44mm which is all you expect out of a M48 " 2 inch" filter. 


You can see why you need to measure. What's the point of an oversized OAG if through the focuser you're down to 44 mm? 


So you can start with sensor size or start with the scope and figure out what you end up with at the other end.  Do it on paper 1st if you can but you need clear aperture info which takes some digging.  Since you have the back of the scope and changing things gets expensive,  I would normally suggest you start there and keep everything threaded.  If you end up with too small a sensor for your purpose or have a specific camera in mind to stsrt then you need to start at the imager and work the other way. Trust me, you'll find if you have a taste for the new large sensor cameras they cost a lot more to implement than you might think. 


If you want to go full frame, IMO the best option is the Astro-Physics 2.7" system. That will leave you at 48mm to the camera if you're careful to avoid the T interface defaults. But 1st check the scope baffle or in the case of the EdgeHD the rear optical path for clear aperture to be sure it will support the final image circle. Don't be surprised if it's not supported. The SCT designs are all based on M42 which is NOT full frame.

Revisit the focuser choice after you know what clear aperture you need. You will need electronic focusing and threaded interfaces throughout. 


Good luck. 

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