Reverse Engineering the Skymax 180

[John]

I've often wondered why maksutov-cassegrains are able to deliver such sharp and high contrast views given their relatively large CO %'s. I have assumed that there must be other properties within the optical system and perhaps the lack of secondary support vanes that overcome the CO % size and allow them to excel in this role

I used to have a 150mm F/6 mak-newtonian made in Russia by Intes. 19% central obstruction on that one. Not the same sort of "mak" that is the subject of this though. Mak-newtonians are not "folded" to the extent that the maksutove-cassegrain is.

 

 

 

Edited January 6, 2021 by John

[Nik271]

On 06/01/2021 at 17:49, alex_stars said:

I agree, that is indeed disappointing, given that I got my Mak to have better contrast than a SCT. 😞 Well well, it's still a nice scope...

I have also been very pleased with the images of mine. In the summer I managed to split Zeta Herculis with it. I think the main reason the Mak does so well despite the 34%CO is because of the excellent collimation and ability to hold it well. And spherical mirrors must be much easier to produce to high standard than other more complicated surfaces. So taking the CO into account the 180mm Mak should compare well with a good 120mm APO refractor, but it will be beaten by a 150mm APO on contrast.

[alex_stars]

I agree with @Nik271. Holding the collimation and having hopefully high-quality mirrors should do the trick for the Mak design. Just for fun I plotted the MTFs for all we know including the 4 and 5 inch APOs

As we can see, the possible smaller aperture (AP=172mm instead of 180mm) is really no problem for image quality, except maybe brightness. The larger central obstruction (58.5 [34% for the 172 mm AP] ) is however indeed a problem putting the 7" Mak down to a 5 " APO for larger features. The 6" APO does already a lot better. Mind you that we hardly can actually see the very fine resolution features (maybe 0.6 - 0.7 and upward on the X axis in the image above) due to seeing.

Would we have gotten the instrument we thought we pay for, as it is advertised by Skywatcher (green line), we would have gotten an instrument which is as good or better than a 5" APO. But we did not get that.

I still like my scope, however any time I find out these rather strong deviations between specs and reality in astronomical equipment that I have paid a substantial amount for, I think that this is really the part which sometimes takes the fun out of this hobby.

Clear skies.

Alex

[Avocette]

@Captain MagentaDuring these stormy days I was thinking of having a look inside my SkyMax 150 and just wondered if you had yet found a moment to dismantle and reassemble yours? I bought mine second hand seven years ago and became convinced that ‘someone had had it apart’ probably for cleaning, when I noticed evidence of clear glue fixing the secondary baffle to the surface of the secondary mirror. I suppose cleaning the secondary mirror is more simple if the baffle is first removed. I have held back until now but have an urge to open it up to look more closely at the condition of the mirrors, even if I don’t attempt to clean them if they seem in good enough condition.

[Captain Scarlet]

Unfortunately I haven't yet attacked my Skymax 150, I haven't even managed to get back from Ireland to the UK yet!

With the 180, although I have cleaned my secondary, it was because it became necessary after discovering my own secondary baffle had been attached seriously off-centre. At that point I simply pulled the baffle off, leaving behind a ring of glued-on foam. To deal with that ring of foam, I soaked and kept soaked the area in pure acetone, and as it softened gently manipulated with a soft plastic spatula until the last shreds detached and could be lifted away. The whole area, back-plate and mirrored surface seem to be coated with a very hard coating, most likely SiO2 (i.e. quartz). So it can even tolerate rather poor treatment I think.

I replaced that baffle with one I made up myself of clear 220 micron acetate, painted matt black, and which I glued on with 3 dots of standard 2-part epoxy. I made sure that such epoxies are soluble in acetone. I have since removed that replacement baffle, cleaned up with acetone again, added flocking to the baffle both sides and then re-attached with the same epoxy.

It gives lovely views.

My points are is that it may well be that a previous owner had noticed, like me, that the original baffle was not centred and decided to remedy it. I would imagine a non-central baffle would have a disproportionate effect on transmission! Getting it back on perfectly central though is actually quite a difficult thing to judge, and my cynical self would suspect that the word "plonk" might have applied at that SW manufacturing stage.  Also that acetone is the right thing to use here, there's not anything around for it to damage, with sufficient care (and pure water and dust-free swabs!). If you do get it wrong, as I did at one stage, simply dissolving away the dabs of glue and starting again is quite easy, although time-consuming.

Cheers, Magnus

[Avocette]

I have just completed a very quick (but careful) dismantling and reassembly of my SkyMax150 Pro, taken some measurements, taken some photos and blown away a very few dust motes from the optics. The primary mirror diameter   (measurement K in Magnus’s nomenclature) was 163mm overall and around 160mm if I discount the ground edge which is chamfered at about 45°. The Primary Mirror Retaining Ring (J) is 47mm. The Secondary Baffle Skirt is 52mm (H) at the wide end, and has a depth of 43mm (C&I). 

The metal tube is 2.2mm thickness and 178mm internal diameter. In my scope, this means that the eight screws, which attach the Corrector Plate Rim and the Rear Cell, distort the circular profiles when tightened appropriately.

My scope at least ten years old and possibly somewhat older. It has the ‘Black Diamond’ paint job and the 2” visual back but with a 66mm thread rather than the SCT compatible rear thread of more recent manufacture.

Here are a few photos for the curious!

 

[jetstream]

 @Captain MagentaExcellent Magnus, missed this thread unfortunately. What did you end up figuring the central obstruction was in the end? for the 180mm

Edited February 7, 2021 by jetstream

[Captain Scarlet]

5 minutes ago, jetstream said:

 @Captain MagentaExcellent Magnus, missed this thread unfortunately. What did you end up figuring the central obstruction was in the end? for the 180mm

Thanks.

It seems to be in the region of 33-35%, as predicted from dimensions and confirmed with images. A shame, as I reckon SW could have reduced it quite significantly by using a thin retaining-ring for the primary rather than the big thick thing they have used. Oh well.

[Captain Scarlet]

On 05/02/2021 at 16:28, Avocette said:

I have just completed a very quick (but careful) dismantling and reassembly of my SkyMax150 Pro, taken some measurements, taken some photos and blown away a very few dust motes from the optics. The primary mirror diameter   (measurement K in Magnus’s nomenclature) was 163mm overall and around 160mm if I discount the ground edge which is chamfered at about 45°. The Primary Mirror Retaining Ring (J) is 47mm. The Secondary Baffle Skirt is 52mm (H) at the wide end, and has a depth of 43mm (C&I). 

The metal tube is 2.2mm thickness and 178mm internal diameter. In my scope, this means that the eight screws, which attach the Corrector Plate Rim and the Rear Cell, distort the circular profiles when tightened appropriately.

My scope at least ten years old and possibly somewhat older. It has the ‘Black Diamond’ paint job and the 2” visual back but with a 66mm thread rather than the SCT compatible rear thread of more recent manufacture.

Here are a few photos for the curious!

 

Brilliant thanks for this! Allows me to fill in a few blanks in my specs for my 150.

M

[jetstream]

3 minutes ago, Captain Magenta said:

Thanks.

It seems to be in the region of 33-35%, as predicted from dimensions and confirmed with images. A shame, as I reckon SW could have reduced it quite significantly by using a thin retaining-ring for the primary rather than the big thick thing they have used. Oh well.

Yes, Im surprised too- I saw this in the pictures. A 23% obstructed Mak with good optics would be perfect IMHO. Thanks for this information. A 10% reduction in CO would be considered a huge improvement.

Edited February 7, 2021 by jetstream

[jetstream]

This might be why- from Astro Physics https://www.astro-physics.com/254f145mc

"The optical design follows the original concept of aspheric primary with secondary spot on the inside of the corrector . The simplicity of this 2-element optical system assures no collimation issues - the scope will always retain collimation and cannot be knocked out of alignment because there is no secondary mirror to fuss or adjust. The small 23% central obstruction assures the highest possible planetary contrast at high powers.

This obstruction also acts as a baffle to keep stray light from the sky from falling directly on the eyepiece. For high-power eyepieces, a 23% obstruction is enough to fully shield the field of view from this stray light. When the field of view exceeds about 1 inch diameter, light from the sky can leak past the front baffle and reduce the contrast at the edge of the field. Therefore, for low-power, wide-field viewing with 2” eyepieces we provide a larger secondary baffle that will increase the front baffle size to 32% to block this stray light in the outer portions of the field. This is needed primarily when sky light-pollution is high and you are using low-power wide-field eyepieces to observe faint deep-sky objects. Under dark skies, there will be no difference in contrast. Note: this secondary baffle threads onto the front of the corrector. The photo does not show the method of attachment on the corrector."

[Peter Drew]

This is exactly what I used to do when I made Maksutovs commercially.  The screw -on  larger disc was used for terrestrial purposes where contrast reduction due to sky flooding would be noticeable with a small obstruction.  The only practical way to eliminate this problem for both astro and terrestrial use was to employ the Dall modification that included an erector transfer system into the primary mirror support baffle tube.  This enabled a much smaller CO without any sky-flooding.      🙂       

[Captain Scarlet]

@Peter Drew I found this post of yours from a few years ago:

”Dall used an erector transfer system which picked up the primary light cone further from the primary than usual and then transferred it to the eyepiece. This typically reduced the secondary obstruction to about 15%, gave an erect image and was completely free from sky flooding even with a single spine tube. The inside of his aluminium tubed Maksutovs were unpainted!. I made a number of Maksutovs to this design and can vouch for their high contrast.”

What was the system? A roof prism inside the baffle tube?

Edited February 7, 2021 by Captain Magenta

[balticsensor]

Amazing post! Thank You very much for the effort!

I was planning to buy the Skymax 180 as my 2nd scope and compliment to my Evostar 120 achromat - lots of accessories that I could carry over from the refractor to the Maksutov - but now I just don't know. The primary thing that bothers me is the diameter of the secondary spot, 37mm. I was planning to use my 2" William Optics 40mm 72° eyepiece for the widest field, however, it's field stop is 46mm; I'm afraid it's going to vignette severely. That leaves me with my 32mm Televue Plossl; but I'm afraid that just won't cut it - 36' TFOV is nowhere near 64'. I was hoping to get those widefield sharp to the edge star images in my 40mm, but now that's compromised.

But the aperture (and image quality) fever remains. What now?..

I want to keep my 120mm achromat for the wide fields. But also, I want to crack those small DSOs and planets with bigger high-quality aperture. I can almost see myself getting a C8, but having tried the views of it, I was really underwhelmed with the edge performance (and the contrast, somewhat). Speaking of which, would the C8 also vignette my 40mm 72° eyepiece?..

A large Dobsonian, for me, means two things: (1) rare usage due to bulk, 5th floor dwelling and local atmosphere; (2) granted, it's bigger, and yet it's another optically compromised design much like the achromat. For my second scope, I want the best image possible for the given aperture.. Doesn't have to be massive, like I said, due to climate conditions and lifestyle. Like observing on the 1st night with the achromat, and on the 2nd night to cherry-pick the most attractive targets with my bigger scope with the best possible image quality and contrast.

Anyway, sorry for polluting this specialized topic with beginner-grade questions. The bottom line for me is this: would the Skymax 180 vignette my William Optics 40mm SWAN? If so, what would you recommend I buy to satisfy the "crack those small DSOs and planets with bigger high-quality aperture" and "rare usage due to bulk, 5th floor dwelling and local atmosphere" criterions? The C8 on EQ-5?..

Edited February 12, 2021 by balticsensor
Three edits, three additional paragraphs.

[chiltonstar]

The best view I've found with my 180 Mak for looking at DSOs is with the 38mm Panaview 70 degree FoV eyepiece. If there is vignetting, it's certainly not obvious to me and the view is glorious. The eye relief is not the best, but certainly ok. 

Chris

[Nik271]

I often use my Mak with the 32mm Panaview, again giving 70 degree FoV. Haven't noticed vignetting. I know there must be some because of the field stop is larger than the visual back opening, but the vignetting is not noticeable to me in dark background.

[balticsensor]

Thanks! I guess I wasted a lot of breath. You know how the excitement goes...

[Robindonne]

Very useful thread!  Thanks for that. 
While reading about the “not so small as described” central obstruction, one question came to my mind.  
When you overlap these two pictures, isnt the baffle way to wide manufactured?  And probably give you more aperture when that thing is removed or replaced for a less conical one?  It looks like they made the angle a 90 degree on the meniscus while the actual angle the light follows inside the tube is way less than 90 degrees. 
 

Edited June 19, 2021 by Robindonne

[Captain Scarlet]

Part of my initial (misguided) motivation for removing the baffle was to reduce the Central Obstruction to an absolute minimum. It was only later that I realised that the CO% was actually defined by the (unnecessarily wide) retaining ring for the primary mirror. So the baffle can be anything up to that value without spoiling things (meniscus-refraction adjusted).

With the baffle completely removed you can see directly from the front of the scope and see light past the visual back, bypassing the secondary-mirror path. That should not be "allowed". In that configuration, looking at a bright object like the Moon will cause all sorts of serious extraneous light problems. Part of the purpose of the wider baffle therefore must be to block off that direct line of view through the VB.

M

[nfotis]

So, it seems that Synta engineers made one compromise by raising the central obstruction in order to offer a more usable telescope with 2" eyepieces and looking at the moon.

Well, I can live with that (I hope to go tomorrow at the rooftop with my Skymax 180). Obsessing over theoretical values doesn't add to the enjoyment of using it.

N.F.

 

 

[retroformat]

On 27/05/2020 at 10:24, Captain Scarlet said:

...
8.    Unscrew the 3 screws on the flange around the focus-knob, and set them and the flange-plate aside.
9.    Unscrew, by hand, and set aside the exposed brass focusing assembly (ACW) all the way off (perhaps 20+ turns!). Also notice and remove a black rubber washer underneath the assembly. Be careful of the evil black grease!
10.    The threaded rod, covered in black grease, will now be poking up through the hole in the back plate.
 

 

 

Dear Magnus, thanks for your excellent report.  I have the exact same telescope as yours, and will soon undertake a disassembly to look for what I hypothesize is an off-center primary mirror assembly (due to bent collimation screws/threads in the rear cell/plate... and all of that due to a side impact on the OTA).  But I have a question/concern.  Your excellent pictures give a good idea of how everything is held together.  For example, it is evident that the entire primary mirror assembly, along with the rear plate, is held onto the OTA with only the collimation screws.  And you give warnings about that as well.  Your pictures also suggest that the only thing holding the primary mirror/outer baffle onto the rear plate is... the focus threaded rod.  This threaded rod moves the primary back and forth upon the inner baffle, the mirror being free to slide upon that central baffle, but held in check only by the focussing rod.  Yet in your disassembly instructions, you non-challantly have us remove the "brass focussing assembly (ACW) all the off (perhaps 20+ turns!)"   From your pictures, it looks like this would completely free up the primary mirror, so that it would then slide down the inner baffle, and fall onto the corrector plate.  I am obviously missing something... what am I missing?  Is a step missing prior to step 8.?

 

 

[Captain Scarlet]

2 hours ago, retroformat said:

Dear Magnus, thanks for your excellent report.  I have the exact same telescope as yours, and will soon undertake a disassembly to look for what I hypothesize is an off-center primary mirror assembly (due to bent collimation screws/threads in the rear cell/plate... and all of that due to a side impact on the OTA).  But I have a question/concern.  Your excellent pictures give a good idea of how everything is held together.  For example, it is evident that the entire primary mirror assembly, along with the rear plate, is held onto the OTA with only the collimation screws.  And you give warnings about that as well.  Your pictures also suggest that the only thing holding the primary mirror/outer baffle onto the rear plate is... the focus threaded rod.  This threaded rod moves the primary back and forth upon the inner baffle, the mirror being free to slide upon that central baffle, but held in check only by the focussing rod.  Yet in your disassembly instructions, you non-challantly have us remove the "brass focussing assembly (ACW) all the off (perhaps 20+ turns!)"   From your pictures, it looks like this would completely free up the primary mirror, so that it would then slide down the inner baffle, and fall onto the corrector plate.  I am obviously missing something... what am I missing?  Is a step missing prior to step 8.?

What's not clear from the pictures is that there is an O-ring sitting in a groove on the inner baffle tube precisely to stop the outer baffle tube (and primary) going past a certain point and coming off. I do refer to it in step 15, but you're right it should be mentioned it earlier by suggesting that one checks if it's actually there. Unfortunately that post is too old now for me to edit, though I shall edit it on the version on my website.

Good luck with your repair.

Magnus

Edited May 16, 2022 by Captain Scarlet

[Stu]

1 hour ago, Captain Scarlet said:

What's not clear from the pictures is that there is an O-ring sitting in a groove on the inner baffle tube precisely to stop the outer baffle tube (and primary) going past a certain point and coming off. I do refer to it in step 15, but you're right it should be mentioned it earlier by suggesting that one checks if it's actually there. Unfortunately that post is too old now for me to edit, though I shall edit it on the version on my website.

Good luck with your repair.

Magnus

If you let me know what needs adding Magnus, I can do it for you.

[Captain Scarlet]

Thanks Stu I’ll draft up a paragraph to fit in before that section

[retroformat]

Thank you both!  Excellent information.