Screwing or not screwing the coma corrector to the newton focuser that's the question - Forums

Topic starter

Oct 12, 2025

Hi,

I have searched for this basic question without obvious answer. I have newtonians for astroimaging and they need a coma corrector. Up to now, I had a GPU or MPCC screwed to my focusers because I heard that it was better. BUT I am preparing a new setup and might have to choose the televue paracorr (photo) to get the right focal length after correction with a 250F4 newtonian. This coma corrector doesn’t offer an obvious way to be screwed to a focuser unless I design and ask some dedicated manufacturer to mile it for me. So here comes the question?

Is screwing the only way to achieve a centered and rigid imaging train (in my case, rotator, Electronic tilt adjuster, EFW, camera)?

What is the experience in real life of those of you who are using some self centering focuser adapter to carry their imaging train?

Thank you in advance for sharing your experience,

CS Frédéric

andrea tasselli

Frequent contributor

Oct 12, 2025 · Edited: Oct 12, 2025

I never attached my CCs to the focusers with anything other than the 3 screws (so press-fit) and I was never the worse for it, you only need to be very very careful when securing the imaging train in order to avoid introducing tilt. I have tilt corrector though.

Tony Gondola

Oct 12, 2025

Threaded connections can introduce tilt as well so to my mind, the only advantage is that it’s more secure. To counter that I have the back end secured with a simple lanyard as a fail-safe for forgetfulness.

TiffsAndAstro

Oct 12, 2025

I never realised there was a way to screw the entire image train together for newts. Main reason I've avoided them

Wil be interesting to read responses. No way would I use screws or a compression ring.

Joey Conenna

Oct 12, 2025

I have only ever used slip in coma correctors secured by a compression ring. I currently use a Feathertouch crayford with one screw pressing the compression ring tightly onto the Paracorr II coma corrector. I could use two screws for more security, but I forgo three screws as the coma corrector might not register with the drawtube consistently (it could float in the middle). I routinely rotate the coma corrector / imaging train for framing and tighten the screw to secure everything and have not had issues with tilt or things not holding. I would prefer a threaded connection, but I am living just fine without one.

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Frédéric Ruciak

Topic starter

Oct 13, 2025

Hi, thank you for your feedback, I can go for “slip and tight” without fear. For those wondering how to screw here is the link to an adapter that works with TS GPU to screw on a 54mm female thread of a focuser. It works with MPCC too. https://www.teleskop-express.de/en/adaptors-10/adapter-camera-bayonet-132/ts-optics-adapter-from-m48-2-filter-thread-to-m54x0-75-thread-4041

CS Frédéric

Frédéric Ruciak

Topic starter

8 days ago

As a complement to my first question, I designed a special adapter on freecad and ordered it on a platform called usineur.fr and it works very well so that I can screw my TV paracorr to my rotator screwed to the focuser. This is a way to combine a rotator AND keep the right distance between paracorr and sensor with EFW and OAG. I include the PDF drawing of the 2 parts of the adaptor. The paracorr is screwed in the part1 and part 2 is screwed on top. It leaves two male M54 0.75mm male on one side for the rotator and on the other side for the rest of the optical train.adaptateur M54 M72 part1 v3mm p1.pdf adaptateur M54 M72 part2 v2 p1.pdf

For me the screwed adaptor clearly made a difference compared to the slip and tight on my ONTC150F4 with a UNCN2G2 focuser. Hope this can help others having the same question.

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atlejq

4 days ago

There is actually a screw thread adapter for the 2” Paracorr available from TS.

https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/ts-optics-m68-system-connection-adapter-for-vip-2010-paracorr-to-m68-5014

I have this one attached to a Baader Steeltrack via a M68i adapter for my 8” ONTC. The Steeltrack also has the ability to fix the M68i adapter in any desired image orientation.

https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/baader-m68i-adapter-for-2-diamond-steeltrack-14688

(If this adapter is to be used on a 54 mm thread like the UNCN-G2 common on TS scopes it will need a different connection adapter from M54 to M68 and also a rotator of some sort unless you want to rotate the focuser at its base).

On my setup, the threading is used in conjunction with a 8” F/4 conical mirror with screw threads in the primary mirror cell, ensuring that the collimation is 100% fixed. Any risks of dropping the camera and its attached optics are fully eliminated.

dayglow

4 days ago

My focuser is a 2.5 inch MoonLite and presents M68 threads towards the camera. This allowed me to use

Teleskop-Express part M68-Paracorr to thread my Paracorr into the focuser.

https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/ts-optics-m68-system-connection-adapter-for-vip-2010-paracorr-to-m68-5014

I have not had issues with tilt.

— David F.

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John Stone

4 days ago

Frédéric Ruciak:
As a complement to my first question, I designed a special adapter on freecad and ordered it on a platform called usineur.fr and it works very well so that I can screw my TV paracorr to my rotator screwed to the focuser. This is a way to combine a rotator AND keep the right distance between paracorr and sensor with EFW and OAG. I include the PDF drawing of the 2 parts of the adaptor. The paracorr is screwed in the part1 and part 2 is screwed on top. It leaves two male M54 0.75mm male on one side for the rotator and on the other side for the rest of the optical train.adaptateur M54 M72 part1 v3mm p1.pdf adaptateur M54 M72 part2 v2 p1.pdf

For me the screwed adaptor clearly made a difference compared to the slip and tight on my ONTC150F4 with a UNCN2G2 focuser. Hope this can help others having the same question.

I'm trying to wrap my head around how this can work.

It looks like you're taking up 3.5mm of back focus from the camera side lens of the Paracorr

Which would leave only 56mm - 3.5mm = 52.5mm of back focus for your OAG/FW/Camera. Considering these are typically 55mm.. you're 2.5mm too long (1.5mm too long if you use 3mm thick filters).

I suppose you could buy the PegausAstro FW and OAG which would consume 33mm (+4mm for the M54F telescope connection) = 37mm and replace the tilt plate on your camera with ASG Photon Cage (3mm) bolting it directly to the FW and now attach your camera w/o tilt plate 12.5mm and we get: 37mm + 3mm + 12.5mm = 52.5mm which leaves 1 mm for tilt adjustments if you use 3mm thick filters (less for other filters).

Is this what you ended up doing on your system?

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Frédéric Ruciak

Topic starter

4 days ago

Hi John,

Your question is 100% relevant. In its standard configuration the TV paracorr comes with a dedicated adaptor from the TV IS standard to M54, this adaptor is compatible with the backfocus recommendation of TV of course, it’s coming from TV for the paracorr ;-). The part1 on my adaptor puts the paracorr flush to its flange and the part 2 is mimiking the TV standard adaptor from IS to M54. Nevertheless you are right that I have to take a special care to the distance between the bottom of the TV paracorr and the edge/flange of the part 2 where the rest of the imaging train will start. In fact I have both the TV adaptor and my dedicated one I will compare and share. In any case if there is a 1mm to be gained I have plenty of margin by adjusting the height of part 1 along the paracorr and reduce the height of part 2 along the light path.

Thank you for raising this question.

CS Frédéric📷

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Frédéric Ruciak

Topic starter

4 days ago

atlejq · Dec 2, 2025, 11:25 PM
There is actually a screw thread adapter for the 2” Paracorr available from TS.

https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/ts-optics-m68-system-connection-adapter-for-vip-2010-paracorr-to-m68-5014

I have this one attached to a Baader Steeltrack via a M68i adapter for my 8” ONTC. The Steeltrack also has the ability to fix the M68i adapter in any desired image orientation.
https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/baader-m68i-adapter-for-2-diamond-steeltrack-14688
(If this adapter is to be used on a 54 mm thread like the UNCN-G2 common on TS scopes it will need a different connection adapter from M54 to M68 and also a rotator of some sort unless you want to rotate the focuser at its base).

On my setup, the threading is used in conjunction with a 8” F/4 conical mirror with screw threads in the primary mirror cell, ensuring that the collimation is 100% fixed. Any risks of dropping the camera and its attached optics are fully eliminated.

Hi Atlejq and Dayglow, I missed this adaptor in my search, thank you for pointing it. The only limitation is the M48 thread towards the camera. Regarding the M68 on the other side I can design another adaptor M59 to M68 as the purpose is to screw everything on an octoplus focuser for my new lacerta photonewton 250.

CS Frédéric

John Tucker

4 days ago

Not sure if this is relevant to the current convo, but I’ll put it out there in case someone finds it useful.

I have a 2018 vintage TS Optics “PowerNewt”, which is basically an ONTC Newtonian with a “V-Power” focuser having M65 threads. I purchased a non-threaded Starizona Nexus reducer/corrector (0.75x) for it, and found that I could not get reproducible placement using the compression ring adapter that came with the focuser.

I came up with the following method to thread the corrector to the focuser:

I screwed one half of one of those cheap variable-length extension tubes onto the corrector threads, thereby effectively increasing the length of the thread.
I then screwed this M48 female to M65 female adapter onto the thread, with the M65 threads facing toward the telescope. https://www.astroshop.eu/reduction-rings-adaptors/ts-optics-65mm-to-m48-female-thread-adapter/p,53295#:~:text=Product%20description,the%20TS%2DOptics%20UNC%20focusers.
In my particular case, instead of threading the adapter directly to the focuser threads, it made sense to use an M65 extension tube in between.

The whole thing screws nicely onto the focuser. If desired, you can reduce the amount of backfocus burned using this technique by removing the excess thread length from the adapter used in step 1 with a belt sander.

The technique may be limited to cases where the focuser thread is relatively large or in which there is no rim on the corrector, as the adapter for threading to the focuser has to reach over any rim on the corrector.

Backfocus does not seem to be a problem if you are creative. The M48 Gerd Neumann tilt corrector is only 11mm path length, and has female threads on both sides, potentially opening up arrangements in which the 11mm female T2-T2 ring found on ZWO cameras becomes superfluous. TS Optics and Blue Fireball sell relatively short path camera rotators.

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