Collimation Nightmare on A Maksutov Newtonian - 190MN

Frequent contributor

Oct 12, 2023

Use a cheshire EP of the proper focal length to recenter and square the secondary then adjust collimation first with slightly out of focus star (WITH AN EP (200-250x) - NO CAMERA!) by moving the star around the FOV to where the donut (fringe rings) are best concentric (usually in the direction of the flaring) and then use the collimation screws on the primary to recenter the star and repeat until the pattern AT THE CENTER OF THE FOV look symmetrical. Finally check with IN-FOCUS star at or around the Zenith for the Airy disk ring pattern being symmetrical. Use very high power here, at least 300x. Same procedure as above if the Airy disk rings don't look symmetrical.

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Topic starter

Oct 12, 2023

Forgot to say, the focal length has changed from 997mm to 993mm since so I think I need to move the primary mirror towards the back of the tube

Frequent contributor

Oct 12, 2023

I really don't think so. You are only moving the focus in.

Topic starter

Oct 12, 2023

That’s interesting. Why would the focal length have changed then do you know?

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Kevin Dixon

IOTD Staff

Oct 12, 2023

Hi Craig,

I too own a Skywatcher 190MN Mak Newt along with a few other Newtonian telescopes. When I received the 190MN, the collimation was slightly off. I used the HoTech HT-SCA2-C SCA laser collimator to achieve near perfect collimation. The resulting images are of stunning quality. I have had similar results with my other Newtonians. I highly recommend that you purchase the HoTech collimator. I am certain that you will be most pleased with the results.

Clear skies,
Kevin Dixon

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Frequent contributor

Oct 12, 2023

Craig Dixon:
That’s interesting. Why would the focal length have changed then do you know?

I'd think 0.4% variation is within margins if change in temperature is significant. Also, if the focal plane is tilted this too can give rise to errors in plate solving. Nothing I'd concerned with, to be honest, with the other issues you have now. Besides, Maksutovs are carefully engineered optical systems and the distance of the corrector from the primary is critical in defining the best aplanatic design (and thus best left alone).

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Oct 12, 2023

My experience with the 190MN:
Collimation of a MakNewt differs from collimation of a standard Newtonian. In a Newtonian, the optics can be aligned even if the optical path isn't aligned with the centre line of the telescope tube. A MakNewt has a fixed corrector, and this corrector needs to be aligned with the rest of the optics. Furthermore, the distance from the corrector to the primary mirror is fixed and shouldn't be altered.
Do NOT recenter the secondary under the focuser. It is factory set. ONLY USE the three collimation screws.
As you probably have noticed, the primary mirror's collimation screws do not have springs to push the mirror, but rather two rubber rings. This is because the distance from the corrector plate to the primary should be fixed. You should only tilt the primary slightly when collimating.

Here's my collimation procedure for this scope
1. bottom out the primary so that it is as flush to the back of the cell as possible. This gives you a baseline.
2. make sure that the focuser extension tube is fixed in place and can't move at all (assuming that you have the stock dual speed low profile focuser)
3. use a collimated laser collimator (I use a cheap laser collimator that I have collimated myself) to align the secondary. Put the laser dot inside the donut on the primary. If you turn the laser collimator in its eyepiece holder, the laser dot should stay within the donut of the primary. If it doesn't, the laser is most likely not properly collimated, or the focuser extension tube sits at an angle in the focuser tube.
4. put the laser collimator inside a barlow and put that back into the focuser. The shadow of the small ring that was factory drawn on the secondary should be visible on the primary mirror and should roughly coincide with the donut on the primary
5. adjust the primary mirror until the shadow of the centre donut of the primary mirror is visible and centred on the bullseye of the laser collimator.
6. verify collimation with a star test. The star should be centred in your eyepiece when you do this.

Hope this helps

cs,

Wim

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Cem Oğuz

Oct 12, 2023

Best advice I can give is use lazer and do not listen to people who says other tools are better. I have tested this and can say lazer is less confuzing and easy to use while giving me perfect result in 5 min.

Topic starter

Oct 12, 2023

Thanks all for the advice. I think the main problem is that the Ocal is designed for a Newtonian and I assumed it would be relevant. I bought the scope used and have since learned the following:

The secondary mirror has been adjusted in height
The corrector plate has been removed at some point
The mark on the secondary has been removed at some point, although I think i might be able to see a very faint mark where it used to be.

Therefore, my concern here is that the factory presets have been adjusted. I’m not sure if there is anything I can do about that really so I’m not sure where that leaves me.

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Oct 12, 2023

Craig Dixon:
Thanks all for the advice. I think the main problem is that the Ocal is designed for a Newtonian and I assumed it would be relevant. I bought the scope used and have since learned the following:

The secondary mirror has been adjusted in height
The corrector plate has been removed at some point
The mark on the secondary has been removed at some point, although I think i might be able to see a very faint mark where it used to be.

Therefore, my concern here is that the factory presets have been adjusted. I’m not sure if there is anything I can do about that really so I’m not sure where that leaves me.

Your image of M27 shows good stars, so the scope can't be too messed up by its previous owner. My advice therefore: don't mess with the secondary, other than tilting it using the three collimation screws. Use the collimation procedure that I described before, and see if that gives you good collimation. A star test is the ultimate proof.
The biggest problem with my 190MN has always been the extension tube that wobbled inside the focuser draw tube. You need to fasten it really tight with the thumb screw, or it will wobble. To avoid any movement while collimating, it's best to put the scope on a horizontal stand with the focuser pointing straight up.
Good luck.

cheers,

Wim

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Topic starter

Oct 17, 2023

Thanks for everyones help with this. I think I've managed to get it properly collimated. I've tested this out and think my stars are now looking better. Here's a quick screenshot from PI showing a single unstretched sub and zoomed in to the same star. The image on the left is before collimation and the one on the right is after.

Here's also a jpeg of a single auto-stretched sub showing the full frame

Sean Mc

Oct 17, 2023

That’s a great improvement!

Christopher Dietz

Oct 18, 2023

Craig, what method did you use to fix it?

Topic starter

Oct 18, 2023

It was a bit of a nightmare to be honest but in the end, I removed the secondary mirror by removing the corrector plate, reset everything and used the Ocal to get the mirrors and square as possible The offset secondary caused a few issues. I'm not sure it's perfect but it's as good as I'm likely to get it myself.

https://www.firstlightoptics.com/other-collimation-tools/ocal-electronic-collimator-pro.html

Here's the view from the local when I was done

Coenie

Oct 18, 2023

As a fellow MN190 owner, I can second Wim's advice above.

Your final image in the Ocal looks fine for the MN190. You do not see the "three concentric circles" that are the goal of Newtonian collimation. The primary mirror reflection will be towards the primary mirror side as in your image and the secondary mirror should be slightly higher up towards the corrector, not rounded under the focuser.

The 533 chip you are using is small enough to fall within the MN190's fully corrected field of view. You should not see any deformed stars in the corners. It is a different matter for APS-C sized chips though.

It would be interesting to see how your flats look. If the previous owner also changed the offset of the secondary mirror on its stalk, you should see the flat illumination circle just slightly short of perfectly centered. In all probability, it will be shifted to the right slightly. If he did not mess with the offset, the illumination would be quite a bit to the right. This does not affect the star shapes as long as you do the rest of the setup correctly.

The golden rule for this and probably all Mak-Newts is that you have to move the focuser to accomplish the correct secondary mirror placement. You do not tilt the secondary to do so as you would in a standard Newtonian.

I have a lengthy setup guide that I compiled over the last six years of using this telescope which I will share if it can help.

Looking at your images so far, you are killing it with the MN190. Keep it up!

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Topic starter

Oct 19, 2023

Coenie:
As a fellow MN190 owner, I can second Wim's advice above.

Your final image in the Ocal looks fine for the MN190. You do not see the "three concentric circles" that are the goal of Newtonian collimation. The primary mirror reflection will be towards the primary mirror side as in your image and the secondary mirror should be slightly higher up towards the corrector, not rounded under the focuser.

The 533 chip you are using is small enough to fall within the MN190's fully corrected field of view. You should not see any deformed stars in the corners. It is a different matter for APS-C sized chips though.

It would be interesting to see how your flats look. If the previous owner also changed the offset of the secondary mirror on its stalk, you should see the flat illumination circle just slightly short of perfectly centered. In all probability, it will be shifted to the right slightly. If he did not mess with the offset, the illumination would be quite a bit to the right. This does not affect the star shapes as long as you do the rest of the setup correctly.

The golden rule for this and probably all Mak-Newts is that you have to move the focuser to accomplish the correct secondary mirror placement. You do not tilt the secondary to do so as you would in a standard Newtonian.

I have a lengthy setup guide that I compiled over the last six years of using this telescope which I will share if it can help.

Looking at your images so far, you are killing it with the MN190. Keep it up!

Thanks for your input. Your guide would be most helpful, thanks. I’m planning on upgrading the focuser next so it will come in really handy

Oct 19, 2023

Craig Dixon:
I’m planning on upgrading the focuser next

I did that last spring. There are only a few focusers that I know of that are direct replacements of the SW low profile focuser. The most popular one is the MoonLite. Starlight Instruments has a FeatherTouch that, combined with an adapter, can be used with the stock focuser base plate. I ordered this focuser more than a year ago, but Starlight Instruments could never deliver. After almost a year waiting, I found an Omegon focuser that was (almost) a direct replacement, the Omegon Steeltrail 2" dual speed Crayford focuser. With my Pegasus Astro focus motor, I had to use a thin shim between the baseplate and the focuser, or the focuser knob would not clear the tube. The focuser has so far worked very well and is much better quality than the stock focuser. Most improtant, no more wobbly extension tube.

cs

Wim

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Topic starter

Oct 19, 2023

I’ve found the same after doing some research. The Baader s
diamond steel track NT is the replacement I’m intending to purchase. As I understand it though, I’ll need to drill two new holes in the tube to fit it. My main concern there is getting it exactly central

Frequent contributor

Oct 19, 2023

Craig Dixon:
I’ve found the same after doing some research. The Baader s
diamond steel track NT is the replacement I’m intending to purchase. As I understand it though, I’ll need to drill two new holes in the tube to fit it. My main concern there is getting it exactly central

Actually, there is a direct replacement, as I understand it:

Telescope Focuser 2'' Double Speed & 0 backlash For sky-watcher MN 190 maksutov | eBay

Topic starter

Oct 19, 2023

A direct replacement would be amazing. Not sure about the brand but from the images, that does look solid

Frequent contributor

Oct 19, 2023

I guess that is most likely a copy of the Moonlite.

Oct 19, 2023 · Edited: Oct 19, 2023

Craig Dixon:
I’ve found the same after doing some research. The Baader s
diamond steel track NT is the replacement I’m intending to purchase. As I understand it though, I’ll need to drill two new holes in the tube to fit it. My main concern there is getting it exactly central

I didn't consider that an option. That's why I only investigated direct replacements. Moonlite focusers have a base that fits the holes in the tube. Omegon's focuser has the same size base as the stock focuser. FeatherTouch has an adapter ring as an accessory.
https://www.starlightinstruments.com/product/a20-mn190-adapter-2-0-for-mk190-maksutov-newtonian-intes-alter-mn84-mak-newton-es8-astrograph-w-cf-ota/

this is the one that I bought:
https://www.astroshop.eu/focusers/omegon-steeltrail-2-newtonian-dual-speed-crayford-focuser/p,45073

Topic starter

Oct 20, 2023

Wim van Berlo:
this is the one that I bought:
https://www.astroshop.eu/focusers/omegon-steeltrail-2-newtonian-dual-speed-crayford-focuser/p,45073

Did this go straight on without modifying the OTA please?