200/800 Newtonian astrograph rebuilt with custom mirror cell

[Dan_Paris]

   Hello,

I would like to share with you the complete rebuilt of my 200/800 GSO Newtonian astrograph that I did last year. I bought this optical tube second hand three years ago and, while it was a good way to get into AP imaging, it was far from ideal both from the optical and from the mechanical standpoint. So I decided to change everything that was needed in order to turn it into a high quality astrograph. In the end I did not keep much of the original OTA : only the metal end rings, the end cap and the tensioning nuts for the spider. 

I first ordered a new set of optics from Romano Zen in Italy, as well as a carbon tube made by Klaus Helmerichs in Germany :

 

 

The main part of the project was the design and built of the mirror cell. The original mirror cell from GSO has many problems. The rear and side contact points are neither at the right place nor of the right material, the cell is connected to the end ring only through the collimation screws hence prone to lateral motion, weak springs, locking screws not at the right place, retaining clips add disgracious artifacts, and so on.

 

 

The cell that I designed has the following properties:

• central ball bearing made with M30 stainless steel spherical washers. It prevents any lateral motion and, since its center of curvature coincides with the mirror vertex, no change of focus during collimation. It also locks the mirror in place while the collimation screws are tightened.
• three rear contact points (enough for a 25mm-thick mirror of 200mm diameter), made of PTFE to avoid any stiction, placed at 0.5 x mirror radius
• three lateral contact points at the mirror center of gravity, that consist of stainless steel set screws with a POM ball ends. Mirror clearance can be adjusted to the absolute minimum to account for differential thermal expansion (about 0.1mm)
• a laser-cut black POM ring that acts both as a retaining ring and as a champfer mask. I chose it rather thick (6mm) so that it helps strengthening the mirror cell assembly
• collimation screws are  hexagon socket screws mounted on spherical washers, with low heads (in order to be able to store the OTA vertically without stressing the collimation screws)
• open design for quick cooldown time.
• exact same thickness as the original GSO cell to keep the position of the focus plane

Aluminium parts were made by a local machinist shop and anodized. Here it is after assembly

 

How it fits in the GSO tube end:

 

Mirror installed:

 

Mounted on the carbon tube:

 

I also replaced the spider assembly by a carbon spider sold by Tecnosky in Italy, much more rigid than the original one, and ordered a carbon secondary holder from Sud Dobson  :

The secondary holder in carbon composite has two advantages compared to a metal one: less differential thermal expansion w.r.t. the secondary mirror (hence less prone to astigmatism) and much less heat conduction, which prevents dewing of the secondary. In fact, I never had any dew on it even though I don't use any heater.

As you can see in the second picture above, the tube is entirely flocked with Protostar velvet, and there is a reinforcing plate beneath the focuser.

 

Regarding the focuser itself, I chose a large format focuser to accomodate for large coma correctors. This particular one is a 3" R&P focuser from Teleskop service with M117 connection.  It was originally made for RC scopes so I needed to shorten a bit the drawtube, and I had a custom focuser base machined by Skymeca. I removed the fine focus assembly and installed the motor flexible coupling instead:

 

 

The coma corrector sits inside the focuser with a two-sided adaptor (M78 endcap with M68 threads beneath and above) hence the imaging train puts no stress on the corrector optics. I chose a 2.5" Wynne corrector designed by Massimo Riccardi and sold by Teleskop Service, that is able to cover full frame and has a slight reducing effect (about 0.95x).

 

After about one year of use, I am very satisfied with my astrograph. Collimation is very stable, it does not shift from one part of the sky to the other, and meridian flip has absolutely no effect. I have to recollimate slightly the secondary from time to time, every two or three imaging sessions (fine tweaking with a Catseye autocollimator), but very rarely the primary mirror which is almost locked in place thanks to the cell design. The only minor drawback with this mirror cell is that one has to be careful not to overthighten the collimation screws, otherwise it could put stress on the primary. Diffraction spikes are thin and,  thanks to the champfer mask and smooth mirror figure, the star images are rather clean. Thanks to the  carbon tube, focus is rather stable during the night.  The corrector, tube flocking  and flocked dewshield make a good job avoiding unwanted reflections,  either from bright stars or from street lights. I have yet to try large format sensors, but I get tight star images across the sensor of my cameras (ASI1600 and ASI183).

 

Thanks for reading,

Dan

 

 

Edited May 13, 2022 by Dan_Paris

[CraigT82]

That’s a really nice scope, well done.  I like the primary mirror cell design in particular, very simple but very functional, do you need to lubricate the central spherical surfaces to give a smooth collimation motion? 

[Dan_Paris]

4 minutes ago, CraigT82 said:

That’s a really nice scope, well done.  I like the primary mirror cell design in particular, very simple but very functional, do you need to lubricate the central spherical surfaces to give a smooth collimation motion? 

Thanks Craig, during assembly I put a bit of lithium grease just in case.  Whether it was necessary I don't know, but I never felt any stiction. I'll check when I'll clean the mirror but I doubt that minute collimation adjustments could make grease replacement necessary.

Edited May 13, 2022 by Dan_Paris

[markse68]

That’s really nice Dan. So the only parts left from the GSO are the tube end rings? 🤣

Mark

[Dan_Paris]

Thanks Mark!

 

10 minutes ago, markse68 said:

So the only parts left from the GSO are the tube end rings? 🤣

 

and 4 nuts 🤣

Edited May 13, 2022 by Dan_Paris

[Astroscot2]

You can't beat a well made Newtonian.  Very nice

[CraigT82]

Just seen the image your included in the OP... Wow! Beautiful diffraction pattern, and a real plethora of galaxies in that image

[Dan_Paris]

Thanks @Astroscot2 and Craig!

 

4 hours ago, CraigT82 said:

Beautiful diffraction pattern

 

I chose this framing (with mag. 2.4 Phecda included)  in part as a "stress test" for reflections, diffraction patterns, halos and other disgracious artifacts...

 

 

 

[benzomobile]

On 13/05/2022 at 09:17, Dan_Paris said:

   Hello,

I would like to share with you the complete rebuilt of my 200/800 GSO Newtonian astrograph that I did last year. I bought this optical tube second hand three years ago and, while it was a good way to get into AP imaging, it was far from ideal both from the optical and from the mechanical standpoint. So I decided to change everything that was needed in order to turn it into a high quality astrograph. In the end I did not keep much of the original OTA : only the metal end rings, the end cap and the tensioning nuts for the spider. 

I first ordered a new set of optics from Romano Zen in Italy, as well as a carbon tube made by Klaus Helmerichs in Germany :

 

 

The main part of the project was the design and built of the mirror cell. The original mirror cell from GSO has many problems. The rear and side contact points are neither at the right place nor of the right material, the cell is connected to the end ring only through the collimation screws hence prone to lateral motion, weak springs, locking screws not at the right place, retaining clips add disgracious artifacts, and so on.

 

 

The cell that I designed has the following properties:

• central ball bearing made with M30 stainless steel spherical washers. It prevents any lateral motion and, since its center of curvature coincides with the mirror vertex, no change of focus during collimation. It also locks the mirror in place while the collimation screws are tightened.
• three rear contact points (enough for a 25mm-thick mirror of 200mm diameter), made of PTFE to avoid any stiction, placed at 0.5 x mirror radius
• three lateral contact points at the mirror center of gravity, that consist of stainless steel set screws with a POM ball ends. Mirror clearance can be adjusted to the absolute minimum to account for differential thermal expansion (about 0.1mm)
• a laser-cut black POM ring that acts both as a retaining ring and as a champfer mask. I chose it rather thick (6mm) so that it helps strengthening the mirror cell assembly
• collimation screws are  hexagon socket screws mounted on spherical washers, with low heads (in order to be able to store the OTA vertically without stressing the collimation screws)
• open design for quick cooldown time.
• exact same thickness as the original GSO cell to keep the position of the focus plane

Aluminium parts were made by a local machinist shop and anodized. Here it is after assembly

 

How it fits in the GSO tube end:

 

Mirror installed:

 

Mounted on the carbon tube:

 

I also replaced the spider assembly by a carbon spider sold by Tecnosky in Italy, much more rigid than the original one, and ordered a carbon secondary holder from Sud Dobson  :

The secondary holder in carbon composite has two advantages compared to a metal one: less differential thermal expansion w.r.t. the secondary mirror (hence less prone to astigmatism) and much less heat conduction, which prevents dewing of the secondary. In fact, I never had any dew on it even though I don't use any heater.

As you can see in the second picture above, the tube is entirely flocked with Protostar velvet, and there is a reinforcing plate beneath the focuser.

 

Regarding the focuser itself, I chose a large format focuser to accomodate for large coma correctors. This particular one is a 3" R&P focuser from Teleskop service with M117 connection.  It was originally made for RC scopes so I needed to shorten a bit the drawtube, and I had a custom focuser base machined by Skymeca. I removed the fine focus assembly and installed the motor flexible coupling instead:

 

 

The coma corrector sits inside the focuser with a two-sided adaptor (M78 endcap with M68 threads beneath and above) hence the imaging train puts no stress on the corrector optics. I chose a 2.5" Wynne corrector designed by Massimo Riccardi and sold by Teleskop Service, that is able to cover full frame and has a slight reducing effect (about 0.95x).

 

After about one year of use, I am very satisfied with my astrograph. Collimation is very stable, it does not shift from one part of the sky to the other, and meridian flip has absolutely no effect. I have to recollimate slightly the secondary from time to time, every two or three imaging sessions (fine tweaking with a Catseye autocollimator), but very rarely the primary mirror which is almost locked in place thanks to the cell design. The only minor drawback with this mirror cell is that one has to be careful not to overthighten the collimation screws, otherwise it could put stress on the primary. Diffraction spikes are thin and,  thanks to the champfer mask and smooth mirror figure, the star images are rather clean. Thanks to the  carbon tube, focus is rather stable during the night.  The corrector, tube flocking  and flocked dewshield make a good job avoiding unwanted reflections,  either from bright stars or from street lights. I have yet to try large format sensors, but I get tight star images across the sensor of my cameras (ASI1600 and ASI183).

 

Thanks for reading,

Dan

 

 

Bravo!  There are only just one defect in your excellent astrographic elaboration :  Wynne-Riccardi  coma corrector costs, perhaps, more than the rest of the telescope 🙂

Beppe 

Edited May 19, 2022 by benzomobile

[Dan_Paris]

On 17/05/2022 at 17:05, benzomobile said:

Winne-Riccardi  coma corrector costs, perhaps, more than the rest of the telescope

More than the mirror set yes but not than the whole telescope 😉

[Swoop1]

Triggers Astrograph?

😉

Very nice rebuild and cracking image!

[Captain Scarlet]

Very nice indeed, I'm impressed. I've gone a similar route with 2 Newtonians now. One started life as a blue-tube Skywatcher 300p, the other as an Orion Optics VX8.

The SW now has an Orion primary, a Hubble Optics secondary, a Helmerichs tube and a Steeltrack focuser. Only the SW primary cell and spider remain. I plan to design and build my own primary cell as its inferiority to the rest of the set-up now really bothers me, for example the 6 radially-spaced and far-too-short M4 cell-tube attachment bolts are inadequate, and one has stripped its thread, so really there are only 5.

The VX8 now has a Helmerichs tube and Steeltrack focuser. The original Orion 8" primary cell is I think quite good and the spider is fine too. The secondary is 63mm which is too big for my visual use so I plan to get a Zen 50mm (as you suggested from another post ).

They are both a utter pleasure to use though, I had the 300 out last night. I can feel your joy in making the very best of what has clearly become a really fine instrument.

Magnus

[benzomobile]

18 hours ago, Dan_Paris said:

More than the mirror set yes but not than the whole telescope 😉

ça va sans dire 😉

Beppe

[ONIKKINEN]

That straight and even diffraction pattern around Phecda is the stuff of dreams for a fast newtonian. Beautiful scope and image!

[Dan_Paris]

19 hours ago, Captain Scarlet said:

. I've gone a similar route with 2 Newtonians now. One started life as a blue-tube Skywatcher 300p, the other as an Orion Optics VX8.

Thans Magnus, I like newts since they're easy to upgrade and tweak ad infinitum (and doing so is part of the fun!)

 

10 hours ago, ONIKKINEN said:

That straight and even diffraction pattern around Phecda is the stuff of dreams for a fast newtonian.

 

Thanks!  I think that mirror smoothness is an important factor, I believe that Romano Zen did a good job 😉

 

Edited May 20, 2022 by Dan_Paris

[alpal]

This is a great thread.

Thanks Dan and lovely telescope.

[Dan_Paris]

Thanks @alpal ! I'm now thinking about making a bigger one 😉

[alpal]

On 08/02/2023 at 01:09, Dan_Paris said:

Thanks @alpal ! I'm now thinking about making a bigger one 😉

You've done well.

I even mentioned you on 2 threads on other sites:

I think you are already there as Dan_I  ?

https://www.cloudynights.com/topic/861155-is-there-demand-for-premium-imaging-newts/page-8

and here:

my post 03-02-2023, 12:07 AM

https://www.iceinspace.com.au/forum/showthread.php?t=203108&page=2

I think will copy your mirror cell  design one day - it's excellent.

cheers

Allan

 

 

[CraigT82]

23 minutes ago, alpal said:

I think will copy your mirror cell  design one day - it's excellent.

It certainly is, simple but very effective as the images show. Should be the ‘standard’ design for Newtonian mirror cells 

[alpal]

3 minutes ago, CraigT82 said:

It certainly is, simple but very effective as the images show. Should be the ‘standard’ design for Newtonian mirror cells 

Yes - I agree.

It seems to be the answer to all the problems.

 

[Alan White]

Impressive build Dan, shows great dedication and also the skills to deliver such a build.
Based on so little left of the original though, why not just build the rest though?

Newtonians are indeed very flexible and can be adapted by those who can do such things.
It almost tempts the Refractor owner to reflect 😉 on ones choices.....

The image quality looks super that your are obtaining, nice end result.

 

[Dan_Paris]

3 hours ago, Alan White said:

Impressive build Dan, shows great dedication and also the skills to deliver such a build.
Based on so little left of the original though, why not just build the rest though?

Thanks Alan, it was my first built so it was reassuring to start from something that existed and not from a blank page. I could also test the parts before the project was finished by mixing them with the old ones.

In any case, it was a scope that I bought second hand for cheap and I resold the unused parts (mirrors, cell, steel tube etc.) for about the same price.

4 hours ago, alpal said:

I think you are already there as Dan_I  ?

Yes indeed!

 

Edited February 27, 2023 by Dan_Paris

[newbie alert]

Looks amazing Dan, well done on the build and the images that you're producing.. top draw..