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

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

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.

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.

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.

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.

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

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.

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.

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.

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?

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📷

📷 20251203_093248.jpg

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

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.

📷 Screenshot 2025-11-29 195741.jpg📷 Screenshot 2025-11-29 195803.jpg📷 Screenshot 2025-11-29 195837.jpg📷 Screenshot 2025-11-29 195901.jpg

Hey guys!

Have a similar Problem. Have a 10“ lacerta f/4 and have to screw my new Pegasus astro falcon v2 onto the octo because i can not keep the 55mm backfocus when i try it another way. So what i need is either a adapter from the m59 octo female to m68 male of the falcon. OR maybe to m54 because it is more common and there are adspters then. Did some of you guys do something like that?

Cheers Niki

Hi,

On my setup with the octoplus focuser too, I designed an adapter with freecad and found somebody to do it for me. M59 X1 to M68×1 to fit to my wanderer astro rotator lite.

For Your information with a TSGPU I can get perfect bakfocus with octoplus directly screwed to wandererastro rotator lite v2 Wandererastro ETA OAG-L EFW ASI2600MM. The TSGPU is pushing the focal plain 20mm outside it helps. Clear skies Frédéric

Frédéric Ruciak · Oct 12, 2025, 04:11 PM
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

Hi Frédéric,Welcome to the endless rabbit hole of fast Newtonian imaging! You’ve raised a fantastic question that many of us lose sleep over.To answer your first question directly: No, screwing isn't the only way, but at F4, it is highly recommended. Fast systems like your 250F4 are absolute monsters when it comes to tilt sensitivity. Even a fraction of a millimeter of sag from a heavy imaging train (Rotator + EFW + Camera) will ruin your stars in the corners.Here is my real-world experience with self-centering adapters:If you are talking about standard brass compression rings with three thumb screws, stay away. They will induce tilt under a heavy load. However, if you look into a Baader ClickLock or the TeleVue Paracorr's own tuning top/adapters, they do a decent job for lighter setups. But honestly, with a rotator and an EFW hanging off it, I still worry about flexure when the scope flips past the meridian.Here is what I would suggest to extend your setup:Have you considered getting a custom threaded adapter? Most of us in the States end up using PreciseParts. You can literally select "TeleVue Paracorr" on one side and your "Rotator/Tilt plate thread" on the other, and they will CNC-machine a single, rock-solid piece for you. It completely eliminates the guesswork and the tilt.Out of curiosity, which rotator are you planning to use, and have you already calculated your critical backfocus distance with the Paracorr? That 55mm (or 56mm depending on the version) gets eaten up real fast once you add a rotator and an EFW!Clear skies,[sophia]

[deleted] · Jul 19, 2026, 02:16 AM
You can literally select "TeleVue Paracorr" on one side and your "Rotator/Tilt plate thread" on the other, and they will CNC-machine a single, rock-solid piece for you

I’m really interested in this. I’ve never been able to get PreciseParts to create something that has 3 threaded connections.

One to screw to the paracorr threads on the inside and the telescope drawtube on the outside and have a 3rd set for a camera connection.

Can you give me a screen shot of how to do that?

I also have been curious about this set up. Been toying with it for some time. Im using a SW 250PDS. Its my visual only scope but I had to put a camera on it. The focuser is from Telescope service and has a captains wheel (RAP2DEL) which can be unscrewed and an internal M54×0.75 threads available.

I threw it at Claude and got this: I cannot visualize it at all. Like to see a screeny also to get some ideas.

📷 Focuser.jpg

I have never met a compression ring that didn’t introduce tilt.

All threaded connections are the way to go. Even if they do introduce their own tilt, it’s at least repeatable and you have the ability to compensate for them.

The only non-threaded system that works very well is the Baader Click-Lock adapters. They close around the 2” barrel similar to a lathe chuck. I have found them to be completely repeatable. Easy to test yourself with an autocollimator.

The click-locks have a ton of different variations for every thread out there. Even on imaging trains with all threaded connectors, I like to keep a click-lock handy for using some of my 2” barrel collimation tools.

My Newtonian telescope’s 2-inch rack and pinon focuser has internal (female) M54×0.75mm threads. I use a Starizona Nexus 0.75x Newtonian Focal Reducer/Coma Corrector which has M48×0.75mm male threads on the camera end. The Nexus is 80mm long. I use a Pegasus adapter (PEG-ADAPT-M54M-M48M) to connect between the focuser and the filter drawer I use (and ultimately to the camera) and simultaneously accommodate the Nexus, which allows the Nexus to live (mostly) inside the focuser. This works because the Pegasus adapter is male M54×0.75 to male M48×0.75 AND the M54 side has internal (female) M48×0.75 threads, which matches the Nexus threads. (A similar M54 to M42 adapter is also available from Pegaus.) But my focuser is relatively shallow. To achieve focus and to avoid having the Nexus stick all the way through the focuser and into the light path inside the telescope tube, I use a 30mm Baader M54×0.75 Extension Tube with real 51 mm inner clear aperture, just enough to fit over the Nexus. The end result is that the Nexus is solidly connected to the focuser using threaded connections with about 50mm of the Nexus inside the focuser and 30mm outside (camera side) the focuser draw tube. I’ve checked images for tilt with ASTP (and other tools) and so far so good. Obviously similar connections can be made without threaded connections, but I’ve found this arrangement once established to be quite consistent and relatively easy to maintain.

I’ll share this here since it’s relevant: I just finished designing the optical train for my second imaging newt. (10” f/4 ONTC) It uses screwed connections throughout and only needs 1 adapter from Precise Parts. My current newt uses the 3” Paracorr. While the Paracorr is great it’s not easy to get a threaded connection to the focuser (FTF 3215B-A). I also wish the Paracorr didn’t have the 1.15x barlow effect. The new imaging train uses the TS Optics 3” corrector and maintains the native 1000mm FL.

Focuser:

FTF3215B-A

https://www.starlightinstruments.com/product/ftf3215b-a-feather-touch-true-3-0-diameter-dual-speed-focuser-1-5-draw-tube-travel-and-brake/

Can use this with a precise parts adapter to connect to the coma corrector adapter

Precise parts "Starlight" "FTF 3215/3235 Series focuser without end cap" to "APM" "Riccard reducer/flattner 0.75x M82"

Focuser to CC adapter adapter:

Precise parts: "Starlight Instruments" "FTF 3215/3235 Series focuser without end cap" to "APM" "Riccard reducer/flattner 0.75x M82"

https://www.preciseparts.com/ppmain//index.html

Choose "Starlight Instruments" on left and "APM" on right. Click 'next'.

Choose "FTF 3215/3235 Series focuser without end cap" on left and "Riccard reducer/flattner 0.75x M82" on right. Set "Adapter Effective Length" to "0".

Coma Corrector:

TS-Optics NEWTON Coma Corrector TSKORW3

https://www.teleskop-express.de/en/astrophotography-and-photography-15/imaging-correctors-for-telescopes-138/ts-optics-newton-coma-corrector-1-0x-riccardi-design-3-connection-8311

64mm backfocus for 250mm f/4 scope

Coma corrector adapter:

TS-Optics M82-M68-M68

https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/ts-optics-adapter-telescope-side-m82x1-male-and-m68x1-female-camera-side-m68x1-male-15741

-9mm = 55mm

OAG w/ Tilt Plate:

ZWO M68 Tilt Plate

https://agenaastro.com/zwo-m68-sensor-tilt-adapter-for-asi-6200mc-mm-pro-cameras-m68-plate.html

-5mm = 50mm

ZWO OAG-L

https://agenaastro.com/zwo-large-off-axis-guider-for-full-frame-cameras-with-1-25-holder-m68-oag-l.html

-17.5mm = 32.5

EFW:

ZWO EFW 7x2 for 2" filters

https://agenaastro.com/zwo-efw-7-2-filter-wheel-for-2-50-4mm-filters-2025.html

-20mm = 12.5mm

Camera:

ZWO 6200MM

-12.5mm = 0mm

RIP: My Formatting (2026-2026)

John Stone · Jul 19, 2026, 04:19 AM
[deleted] · Jul 19, 2026, 02:16 AM
You can literally select "TeleVue Paracorr" on one side and your "Rotator/Tilt plate thread" on the other, and they will CNC-machine a single, rock-solid piece for you
I’m really interested in this. I’ve never been able to get PreciseParts to create something that has 3 threaded connections.

One to screw to the paracorr threads on the inside and the telescope drawtube on the outside and have a 3rd set for a camera connection.

Can you give me a screen shot of how to do that?

Hi John, in fact it is a two parts adapter, you can have a look at the pdf and you will understand that the televue is screwed between the two parts. There was a question about this design reducing or not the backfocus for the televue paracorr, I was prepared to ask for a variant with 1mm less but it proved to provide very good results in hocus focus as it is with my ONTC 150F4.

Hope this gives you some ideas, Frédéric

adaptateur M54 M72 part1 v3mm p1.pdf

adaptateur M54 M72 part1 v3mm p2.pdf

adaptateur M54 M72 part2 p1.pdf

adaptateur M54 M72 part2 p2.pdf

Feedback from my experience having the setup remotely installed now.

With the TSGPU coma corrector and the imaging train I described previously (octoplus rotator TSGPU ETA OAG L FW camera) everything screwed with dedicated adapter from the octoplus to the rotator and TSGPU to rotator and ETA of course (with standard ones) I consistenly have round enough stars up to the 4 corners on my ASI 2600MM once I ran the ETA once. And it is stable up to now. For me screwing is no more a question :-)

I attach a light subframes to let you see. This is not absolutely perfect but this is among the best configuration since I use newton for astrophotography.

Capture d’écran 2026-07-19 à 23.31.53.png

Jon Main · Jul 19, 2026, 07:06 PM
I’ll share this here since it’s relevant: I just finished designing the optical train for my second imaging newt. (10” f/4 ONTC) It uses screwed connections throughout and only needs 1 adapter from Precise Parts. My current newt uses the 3” Paracorr. While the Paracorr is great it’s not easy to get a threaded connection to the focuser (FTF 3215B-A). I also wish the Paracorr didn’t have the 1.15x barlow effect. The new imaging train uses the TS Optics 3” corrector and maintains the native 1000mm FL.
Focuser:
FTF3215B-A
https://www.starlightinstruments.com/product/ftf3215b-a-feather-touch-true-3-0-diameter-dual-speed-focuser-1-5-draw-tube-travel-and-brake/
Can use this with a precise parts adapter to connect to the coma corrector adapter
Precise parts "Starlight" "FTF 3215/3235 Series focuser without end cap" to "APM" "Riccard reducer/flattner 0.75x M82"

Focuser to CC adapter adapter:
Precise parts: "Starlight Instruments" "FTF 3215/3235 Series focuser without end cap" to "APM" "Riccard reducer/flattner 0.75x M82"
https://www.preciseparts.com/ppmain//index.html
Choose "Starlight Instruments" on left and "APM" on right. Click 'next'.
Choose "FTF 3215/3235 Series focuser without end cap" on left and "Riccard reducer/flattner 0.75x M82" on right. Set "Adapter Effective Length" to "0".

Coma Corrector:
TS-Optics NEWTON Coma Corrector TSKORW3
https://www.teleskop-express.de/en/astrophotography-and-photography-15/imaging-correctors-for-telescopes-138/ts-optics-newton-coma-corrector-1-0x-riccardi-design-3-connection-8311
64mm backfocus for 250mm f/4 scope
Coma corrector adapter:
TS-Optics M82-M68-M68
https://www.teleskop-express.de/en/adaptors-10/adapter-m68-205/ts-optics-adapter-telescope-side-m82x1-male-and-m68x1-female-camera-side-m68x1-male-15741
-9mm = 55mm
OAG w/ Tilt Plate:
ZWO M68 Tilt Plate
https://agenaastro.com/zwo-m68-sensor-tilt-adapter-for-asi-6200mc-mm-pro-cameras-m68-plate.html
-5mm = 50mm
ZWO OAG-L
https://agenaastro.com/zwo-large-off-axis-guider-for-full-frame-cameras-with-1-25-holder-m68-oag-l.html
-17.5mm = 32.5
EFW:
ZWO EFW 7x2 for 2" filters
https://agenaastro.com/zwo-efw-7-2-filter-wheel-for-2-50-4mm-filters-2025.html
-20mm = 12.5mm
Camera:
ZWO 6200MM
-12.5mm = 0mm

Hi Jon, I hesitated but finally choosed the lacerta 250F4. The octoplus is a good focuser and it comes with two caps and I was able to adapt a fan on the rear one so there is no parasite light entering from the back. I also like the precise optical design balancing the full illumination circle limited to APSC with a reasonable obstruction. And it holds collimation very good :-) which is mandatory for a remote installation.