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Meade 7" Maksutov Collimation

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#1 mayidunk

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Posted 04 December 2012 - 02:09 PM

I was fortunate to recently purchase a used Meade 7" Maksutov ("Big Mak") OTA, it's something that I used to dream about having way back when. I still can't really afford it, but decided that it was now or never!

Anyway, this OTA has push-pull collimation screws on the meniscus cell, which many seem to feel is "the" way to collimate this scope. I have learned from reading about other people's experiences trying to collimate their "Big Maks" using these screws that, basically, those screws aren't what they seem to be, with their efforts apparently ending in failure.

My "new" baby is scheduled to arrive this week, and I've been assured that the collimation is fine. Hopefully this means that no one has ever tried messing with the front cell adjustment. However, just as a point of curiosity, given that nothing else has changed in the alignment of the optical train, if it ever came to light that someone did try adjusting the front cell, is there a surefire way for a user to test it, and bring it back into adjustment if need be without having to send the OTA out to Dr. Clay do be collimated?

:question:

#2 orion61

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Posted 04 December 2012 - 03:54 PM

Those push pull screws work fine for tweeking those. I have one and was slightly out.
Here lies the problem, one of the problems with the 7" Mak is the large image shift from the mirror, it is enough that unless you pull it apart and address it the system can be perfect pointed west but point it up and it is out again.
Also make sure the meniscus holder is not too tight!
they work best when like a refractor lens there is a tiny rattle to it.

#3 mayidunk

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Posted 04 December 2012 - 11:50 PM

Those push pull screws work fine for tweeking those. I have one and was slightly out.
Here lies the problem, one of the problems with the 7" Mak is the large image shift from the mirror, it is enough that unless you pull it apart and address it the system can be perfect pointed west but point it up and it is out again.
Also make sure the meniscus holder is not too tight!
they work best when like a refractor lens there is a tiny rattle to it.

I think this is the purpose of the mirror lock. It's for squaring the primary with the optical train after it has been focused. In fact, I've read were other people recommend setting the mirror lock so that it slightly drags, so that the primary will remain reasonably square with the optical axis while it's being focused, and then locked afterwards in order ensure that it's squared up properly. Does your's have the mirror lock?

I also read about another person who had used the push-pull screws to try to fix a slight miscollimation, and in doing so actually made it worse! They weren't able to undo what they did because the misalignment they were originally trying to fix made it impossible for them to accurately undo their adjustment. Instead of making it better by adjusting the push-pull screws, they actually made it worse by adding another alignment error to the already existing error!

This is why I posed the question, as I have a hunch that some people may be incorrectly intuiting the purpose of those screws as having the same affect as the adjustment screws on the secondary of a typical SCT or newt. From what I've been reading it seems that, while they may be adjusting the secondary's tilt, more importantly they're actually allowing you to adjust the meniscus in parallel with adjustments made to the primary as it's collimated. I don't completely understand it, but I believe that those screws may have more to do with maintaining an important relationship between the meniscus and the primary, and less to do with adjusting the tilt of the secondary, though that's also happening. The Meade has a spherical secondary, and an aspherical primary.

From what little I think I know about this, while most other Gregory/Maksutovs don't have push-pull meniscus cells, their meniscus' must still be shimmed in order to maintain that spatial relationship with the primary. In doing so, the secondary alignment pretty much takes care of itself. The crux of the matter being that the relationship of the meniscus to the primary is the more critical dimension, which cannot be correctly set by merely attempting to align the secondary spot. I think this may be why out-of-focus star images on many factory collimated Gregory/Maks appear to show them to be out of collimation, when in fact they are perfectly collimated at the focal plane.

That being said, I'm certainly no expert in any of this which is why I'm tossing it out for discussion with the hope that someone who does this a lot, like Dr. Clay, might also chime in on it.

#4 mayidunk

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Posted 05 December 2012 - 05:55 PM

Bump...

#5 orion61

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Posted 05 December 2012 - 09:03 PM

The mirror lock is only on the later tubes.
When all else fails bottom the adjusters down and start one screw at a time. It is confising if you are used to SCT alignment. but trust me it was his inability not the scope.

#6 Asbytec

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Posted 05 December 2012 - 11:44 PM

Not sure where to go with this, you concern is that someone might have used the meniscus adjustment screws to align the scope. Those adjustment screws do seem to be more about adjusting the tilt of the meniscus so the chief ray coincides with the optical axis. Then you adjust the primary to that same axis to achieve collimation.

I doubt the meniscus adjustment has anything to do with the primary/meniscus spacing. With a moving mirror focusing mechanism, maintaining optimal spacing is nearly impossible since no one focus point is optimal for all eyepieces. You will always adjust focus while affecting the spacing a tiny bit.

If meniscus collimation is off, maybe there is some sort of laser technique you can use to square the meniscus. The idea would be to send the laser from the secondary down the primary baffle to dead center on the focal plane. Final tilting the primary mirror would bring it's axis coincident and complete the collimation - and not only at the focal plane, but along the entire axis on both sides of focus.

That seems to suggest a way to determine if the optical axis are coincident. If you get a nicely centered shadow through large de-focus on both sides, then it would be collimated. If the shadow skews from one side to the other - and is collimated at the focal plane only - something is not square. That something could be the meniscus/secondary. So, maybe collimating at or very near focus, then adjusting the meniscus tilt until it's perfectly centered along the entire range of focus travel is one technical to try, if needed.

You can imagine, if the meniscus is tilted, the chief ray will not be parallel or coincident with the tube or optical axis - nor strike the primary dead center (imagining it had a center.) Additionally, the diverging marginal rays might even miss the primary all together. Getting that right seems to be the purpose of those adjustment screws.

You're correct, I believe, my Mak and no others I am aware of have such adjustment. They rely on the meniscus being square to the OTA through the fabrication process and allow adjustment of the primary only.

If the star is merely collimated at the focal plane, then I think Larry will agree it's because the meniscus axis and primary mirror axis are not coincident. But, this looks like something that can be "not made worse" if a good technique can be devised - certainly focusing only on the meniscus axis, first and foremost.

My two cents.

#7 orion61

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Posted 06 December 2012 - 05:08 AM

I know the OTA he's talking about and it's a good tube.
It belonged to a friend of mine.
It may need tweeked now but the optics are Grade A.

#8 Jay

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Posted 06 December 2012 - 06:52 PM

Hi Bob and Orion61,

If this is the scope you recently purchased then the collimation screws were never touched. It arrived from Meade in Irvine less than 15 miles away and was in perfect collimation. The scope was used rarely and babied when in use.

http://www.cloudynig...all_DSC0018.jpg

Thanks,

Jay

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#9 Live_Steam_Mad

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Posted 02 April 2014 - 01:28 PM

I have a deforked ex-LX50 version of this 'scope that I bought second hand in 2006. Collimation of it was something I have just been faced with for the first time :shocked:. When I bought the 'scope I was actually buying a used C11 OTA locally at Skelmersdale, about 6 miles way from where I live. The Star Test on the C11 was the most perfect Star Test I have ever seen, identical intra- and extra-focal diffraction patterns with a 15mm eyepiece (186x), the only high power e.p. that the seller had left.

After I had bought the C11 and before I left, I asked about the 7" Meade Mak the guy had and he said I could buy it if I wanted. He warned me that the coatings on the corrector were scratched by a previous owner with cleaning that was too severe. Anyway I asked what he was looking for in cost and I paid the 100 GBP he asked for.

When I Star Tested the 7" Mak at his house with the 15mm ep (so 178x) I noticed that the patterns inside and outside focus were not the same but it was fairly close. However the collimation was slightly off to one side.

After getting the C11 home I noticed that the secondary mirror (which I only checked very quickly at his house) had a patch of something on it like a fairly faint grey fungus that was reducing the contrast just a little (the optics were StarBright Coated not XLT).

BUT the 7" Mak's secondary mirror was getting pretty bad with a blue mold on it / coating decaying :bawling: ;-

https://picasaweb.go...andNewCelest...

..but hey OK what did I expect for 100 GBP when the tube if new was worth a lot of money, especially compared to a 7" Intes Micro Mak at 2K GBP. And the cosmetics were not that good, the Meade stick on decals were a bit tatty and there was an unsightly dent in the top of tube where something had been dropped on it, and there were a lot of other scratches on the tube, some deep ones.

I had no mount at the time (only just had my new Celestron CG-5 for like 2 days now) so I held the OTA on a cushion on the patio table and looked at M42, it took me aback with how much I could see of the nebula and how green it looked and the suprising amount of contrast that there was on everything and how dark the sky was in it, kinda mesmerising, reminded me of a refractor view. The Moon at 333x with my 8mm Radian was pretty good, no Chromatic Aberration and fair sharpness (fan hadn't been used yet by me).

The 7" Meade Mak OTA was very heavy at 10.5Kg so recently I took it apart and removed the cast iron disc counterweight inside it (which was 3Kg) so now it weighs only 7.5Kg and presumably cools a lot faster too now. I also tried to clean the secondary with alcohol and a microfibre cloth but it made no difference to the blue mold, but I got a long scratch in it :foreheadslap: (don't know how, it never happened when I cleaned the primary or my eyepieces with the same method).

I just last night got around to mounting this OTA for the first time on my new mount ;-

https://picasaweb.go...andNewCelest...

...and I took my first look at Jupiter. Compared to my friend's LX90 8" ACF that I was using for the last 3 years the view was fair but that might have been seeing issues, magnification I was using was 205x. It was a mild evening last night and I didn't use the fan at all as I wanted to see what it was like just like this. I used a DIY cardboard dew shield but it was a rare dry night. Damping time was 1.5 seconds with the new mount, which was pleasing. The tube is still not quite balanced in RA ('scope heavy), I need another smaller counterweight, not sure how I will solve that yet :confused:.

However I noticed immediately at 83x that Jupiter's Moons had rather obvious and pronounced coma and when I defocussed I could see that the collimation was off by a country mile! So I got the 2 Allen wrenches out that I had that fit it (two different sizes, the larger bolt locks the collimation down, the smaller one pushes the corrector out towards you at that position on the corrector, as you turn it clockwise, as you might expect.

I turned the adjustment 180 degrees at a time (the top left small bolt made the donut pattern bunch together at the bottom when turned, the right hand lower small bolt made the pattern bunch up on the left of the donut) and had to recenter the star (I used Polaris) but the adjustment was rather less sensitive than on my friend Jon's 8" ACF that I had collimated perfectly at 500x some months ago. I started to make it very much better (and noticed oddly that the size of the donut seemed to get bigger all by itself as I got the collimation closer to correct, don't quite understand why yet) and then I ran out of travel on the small bolt on the bottom and realised that I had to start all over again :bawling: :foreheadslap: (should have started from a neutral position of all small bolts partly inwards).

I did notice that the collimation was pretty straight forwards and fairly easy since you just (if you want to go clockwise for example) slacken off the large bolt 1/2 a turn, then turn the small bolt anything up to 1/2 a turn clockwise, then tighten the large bolt again). I am now not afraid of the collimation anymore.

So I screwed all 3 small bolts (well they are more like grub screws really) so that they were just a little below the surface and then tightened up the large bolts (so the meniscus mounting cell flange was flush all round against the flat inside rim of the front cell basically) and would you believe it the collimation was just about bang on dead centre at up to 333x (I didn't dare try a barlow and get it any closer to perfection!) :)

Meade glue the rear primary cell onto the OTA (I noticed that when I tried to remove it and couldn't when the screws were all out), same with the front corrector mounting cell, and they seem to have got it mechanically square quite nicely, which was a relief. Mirror shift was nearly none since I regreased the baffle tube when it was apart, with Rocol MTS 1000 (Moly DiS based), since when I looked I found virtually NO grease left (all gone to a thick wax on the rear of the mirror's baffle edge), before I regreased it there was a TON of image shift.

I then tried the 'scope on Jupiter and it was quite a sight, similar levels of detail to the 8" ACF with quite good contrast on a shadow transit of Io that was happening. So overall I am happy with my 100 GBP Meade Mak but I did notice that Jupiter had rather less tan brown color in it compared to my friend Jon's 8" ACF and the star colors were quite a lot less intense also, which is merely a function of the amount of light that was entering my eye and the degraded secondary mirror coating on my Mak. Mind you the skies last night were quite bad, poor transparency due to recent and unusual air pollution and also a lot of high thin cloud and light pollution.

I would someday like to get the secondary coating redone on the Mak. I wonder who could do that for me?

Also what brand and model of dovetail is the long one shown in the picture in the previous post? I'd like to get one since I think they are rather more elegant than the tube rings I am using that came with the OTA for free. Or maybe I would be better off using Losmandy's D series dovetail for the 7" Mak with radius blocks and the ADM dual saddle for the CG5 that everyone says improves the rigidity and weight carrying ability of the CG-5...

Also what brand / model is that finderscope mount on there? I have no finderscope mount at present but I know that the bolt holes are definately ALL #8 by 32 TPI, UNC all over the whole of the sides of the tube.

Best Regards,

Alistair G.

#10 Live_Steam_Mad

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Posted 03 April 2014 - 04:17 PM

Well it seems that the dovetail and adapter comes from Meade themselves and I won't be able to get one now unless I wait for a second hand one to come along. It's mentioned and shown in a picture here ;-

http://www.cloudynig...1030333/page...

Farpoint make a compatible dovetail rail for this OTA with Losmandy D series size ;-

http://www.365astron...ak-losmandy-...

...as do Losmandy themselves for their D series plate ;-

http://www.optcorp.c...eade-7-mak.html

...and also Losmandy make a Vixen compatible dovetail rail for this OTA ;-

http://www.highpoint...te-for-meade...

I'd still like to know what make that finder mount is shown in the picture in that post above.

Regards,

Alistair G.

#11 freestar8n

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Posted 04 April 2014 - 02:34 AM

I would someday like to get the secondary coating redone on the Mak. I wonder who could do that for me?


Hi-

I have a mak from an lx50 that I am overhauling in a long term project. At some point I will also want to have the secondary spot re-coated. I think I sent an inquiry to one of the standard coating places and got no reply. I think many of them are not really e-mail oriented and you may need to call the guy. You could ask about coatings in the ATM section and maybe they will know. If you get it done I'd be interested to hear how it worked for you.

There are ways to remove the reflective coating - and it may be necessary to do that before having it recoated. That part I think I can do.

And thanks for the tips on collimation. When I get to that point it will come in handy.

Frank

#12 Live_Steam_Mad

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Posted 04 April 2014 - 03:51 AM

I have a mak from an lx50 that I am overhauling in a long term project. At some point I will also want to have the secondary spot re-coated.


Hello Frank,

I would be very interested to see what your secondary coating looks like in comparison to mine ;) if you could please take a picture of it (like I did above by using the manual focussing and varying it until I got it focussed on the secondary surface sharply) through the front of the tube or maybe better still with the meniscus removed and turned over to show the secondary. Here's mine shown that way ;-

https://picasaweb.go...assegrain7Ex...

I don't know how I managed to put a long scratch in the secondary when I tried and failed to remove the blue mold / chemical changed surface when cleaning the secondary later on after the above picture was taken. Mind you on my 19.5" Newt that was outside under covers for a decade or more the mirror coating got trashed so completely (looked like the surface of a pizza :foreheadslap:) that I could scratch into the coating with my finger nail. I suspect that the protective overcoat has become damaged on the Mak through constant exposure to moisture when it was mounted either on it's fork mount or when it was in the bloke's observatory in Skelmersdale (which was basically a garden shed with an EQ6 computer handset conversion version mount on a large pier). Funny how the primary looks still just about perfect though. Maybe the secondary coating was not done as well as it could have been compared to the primary coating :bawling: and this is a more common problem on these Meade Mak's than I first suspected?

I wonder how many of these older non mirror lock Meade Mak's are showing deterioration of their secondaries? Has yours gone cloudy / grey / blue?

I did see someone asking about having their Meade Mak's secondary recoated on the Meade 4M forum but they got no replies IIRC. I tried to reply but Meade don't seem to want to let me register properly on there (maybe because I am a foreigner and not USA based).

Pity that Meade no longer have these tubes in production since they would have had a setup capable of recoating them. However it would be quite expensive in postage cost to send them even just my secondary / meniscus corrector cell since it weighs quite a lot from the very thick glass. It would be worth it though for a fresh bright new UHTC coating (mine is only EMC).

BTW here are some more pictures of my OTA ;-

https://picasaweb.go...CassegrainEx...

https://picasaweb.go...ightRemovalC...

...and here is the stupid big heavy cast iron lump that I pulled out of the OTA to make it cool much faster ;-

https://picasaweb.go...ovedFromMead...

Was yours the OTA only version without the weight or does yours have the weight in it? With the weight the OTA is not really thermally viable for observing the planets IMHO. BTW I reversed the fan (flipped it over!) to force air into the scope instead of pulling air out.

Cheers,

Alistair G.

#13 freestar8n

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Posted 04 April 2014 - 04:05 AM

Hi-

My overhaul is a long process because the 'scope is at a different location and I don't have access to it right now. It is completely disassembled, partly to remove the weight - which I have documented in CN previously. The reflective secondary coating deteriorated but it was kept in very bad conditions - so I'm not too surprised. I completely removed the meniscus and the secondary baffle - so next - when I get to it - I will remove the secondary coating and have it redone - somewhere.

My hope is that with a new coating and the weight removed it will be a nice scope. Certainly there are many people who like them and want one.

Frank

#14 Live_Steam_Mad

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Posted 04 April 2014 - 04:58 AM

Hi Frank,

Here is the other person asking about recoating their Meade 7" Mak on the Meade 4M forum. Maybe you could reply to them, I can't, the Meade forum system won't let me since the moderator didn't approve me (since I am in UK not USA I assume) ;-

http://www.meade4m.c...secondary-spot/

I wonder if there is anyone who has ever had their Mak secondary recoated (any brand of spot type secondary) ? :confused:

I hope your primary is OK?

How bad is your secondary? Mine is maybe down to 60 per cent reflective (maybe about a 5.5 or 6" aperture Mak as a result). I was thinking of maybe sending it to Galvoptics UK but they did my 19.5" and that went very bad after maybe only 5 or 6 years outdoors. I'll ask about coating in the ATM forum sometime. Not now since I need the 'scope for Mars coming up. And I would need to buy a replacement (SCT maybe) with a good Star Test in the mean time so I have something else to use whilst it got a recoat.

Best Regards,

Alistair G.

#15 freestar8n

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Posted 04 April 2014 - 05:21 AM

Hi-

The primary is fine, but the secondary is fairly corroded - not scratched. I had blamed it on the way it was stored, but I didn't realize there were two other people in need of a recoating. So maybe there is an issue with the overcoat - if there is one.

I won't be seeking a recoating or anything until I return to the project at the end of the year - at the earliest. But if anyone does find someone to recoat it I would be interested to hear how it goes.

Maybe they just aluminized the center of the meniscus - then coated the whole thing with anti-reflection layers - without first putting Al2O3 or SiO2 on the aluminum. Or the Al is completely exposed. I don't know. If it isn't overcoated - that will make it easier to remove.

Frank

#16 Live_Steam_Mad

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Posted 05 April 2014 - 05:41 AM

Can some kind soul please tell me where I can buy that finder bracket that is pictured above on that 7" Meade Mak ;-

http://www.cloudynig...all_DSC0018.jpg

...or maybe someone has one that they want to sell?

If not, where can I buy a finder bracket for a Meade 8x50 finder (I have just the 8x50mm finder but no bracket after a friend upgraded their finder) that I can fit onto my Meade 7" Maksutov (same OTA tube diameter and #8 by 32 TPI UNC hole sizes and threads as Meade's 8" LX50 and LX200 SCT OTA's) ?

I Google searched for hours but couldn't find anything that said that it would definitely fit. My Meade Mak was rather tricky to align last night on my CG-5 mount without it's original 6x30 finder and bracket! Only because at 83x in my lowest magnification (32mm TV Plossl) eyepiece I could hardly find the 2 alignment stars without a finder, it took me like 10 mins to find each alignment star, whuch sucked :foreheadslap:

I already last night after much frustration trying to find a bracket for the 8x50, ordered a 6x30 Meade brand crosshair finder in Meade blue, model 635 (also seems to have item code 07414 associated with it, not sure why) from Ebay which I was told by the seller definitely fits the older Meade 8 inch SCT's. I'll report back on if it's any good and if it fits and what the eye relief is like when it arrives.

The two finderscope bracket mounting holes on the Meade 7" Mak OTA are spaced 1.23" between the centres of the holes, which are both threaded #8 by 32 TPI UNC. I know both of these points for certain after I was forced to buy new 3/8" (9.5mm) long bolts for the holes since my OTA came second hand without finder attachment bolts or finder bracket or finder :bawling:

Regards,

Alistair G.

#17 Live_Steam_Mad

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Posted 05 April 2014 - 08:35 AM

Well it seems I have found a finder bracket for my Meade 8x50mm finderscope that looks very much as if it fits from the description given ;-

https://www.astronom...r-scts_p4386...

The Meade brand 8x50 finder that I was given (from my friend's LX90 ACF UHTC GPS non-LNT) has an external diameter of 55.2mm so it should fit the finder, and the bracket should fit the Meade 7" Mak OTA. But I will confirm that before ordering one.

Regards,

Alistair G.

#18 Live_Steam_Mad

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Posted 23 April 2014 - 08:49 PM

OK so the new Meade 6x30mm viewfinder arrived so I can complete my Meade 7" Mak as far as making it into a practical instrument, but I certainly had my fair share of problems with the Meade quality control on the viewfinder ;-

http://www.cloudynig...4895456/page...

...the eye relief is fair (I can see 70 per cent of the field with glasses on), the view is sharp and contrasty (of land based objects at least) and the cross hairs are sharp. It worked wonders in allowing me to get onto the guide stars on my CG-5. The supplied Meade viewfinder bracket fit my 7" Meade Mak quite well.

Best Regards,

Alistair G.

#19 Live_Steam_Mad

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Posted 23 April 2014 - 09:10 PM

I got concerned a few nights ago because I was seeing some astigmatism at high power (333x) on the brighter stars when defocussed small amounts, I thought maybe my corrector wasn't centred.

So I loosened off the pull collimation bolts and then centred the corrector / secondary by using the interior jaws of a digital calliper set to 0.677" and made all 3 sides the same distance away. This centred my corrector mechanically in the front corrector cell. Of course this meant I had to recollimate the thing now...

Here is a picture of what I did ;-

https://picasaweb.go...CassegrainF1...

...and here is a video showing the amount by which I could shift the corrector backwards and forwards in it's front cell just before I centred it and tightened the pull bolts ;-

https://www.youtube....h?v=By--tzEqeDU

OK so 2 nights ago I managed to collimate it at 103x (need to do that at higher magnification for better planetary detail, but ran out of time) and I noticed that I still see some astigmatism when defocussed very small amounts at high power :confused: :(.

Maybe the astigmastism is "ground into the glass" and best primary to secondary rotation will only minimise it not remove it completely? Maybe a previous owner (it was bought second hand, VERY used) has already rotated the corrector glass in it's holder assembly? At a later date I will try rotating the corrector and seeing if the astigmatism goes better or worse... I have to be aware though that the Spherical Aberration (which is currently not perfect at high power) can be altered by primary to secondary mirror rotational orientation (or at least is the case in e.g. the older Celestron SCT's since they null the system by refiguring the secondary in select places).

A few nights ago before all this with very good collimation (well almost, not quite showing round Airy disc at 666x and a little flaring to one side, but quite good Airy Disc at x333), the contrast on Mars was really quite high and resolving power quite good on at 206x when I was getting a really great view of Libya, Sinus Sabeus, Sinus Meridiani, Mare Acidalium, Chryse, Hellas, and the North Polar cap, at 0:58 BST (UT+1) on 20th April 2014.

Certainly seems to easily beat my friend's 8" ACF on Mars (from what I remember of his 'scope in 2012). And that is at lower altitude (31 versus 45 degrees approx) in the sky, and with me lower down (400 feet versus 133 feet), with the secondary coating on my 7" Meade Mak badly corroding with a blue cloud on it, and with mucho horrific scratches on the corrector, a bad long scratch on the secondary, and with a 39.6 per cent central obstruction :shocked: on this 7" Mak that I measured (secondary baffle maximum outer diameter of it's conical shape compared to front corrector aperture) ! Compared to the ACF's 37 per cent Central Obstruction. The ACF was totally cooled (mild dry night, his LX90 is left outside permanently under a cover).

BTW I wanted to correct / update the collimation info... The top left push screw makes the inner dark disc go left when screwed in, the bottom push screw makes the inner dark disc go down when screwed in.

Best Regards,

Alistair G.

#20 Live_Steam_Mad

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Posted 26 April 2014 - 11:34 PM

OK so tonight I did some more on the collimation on my 7" Meade Mak and worked my way up through 166x then 206x on Polaris and got the collimation so good so that I could barely tell if there was any misalignment at 206x, maybe the dark centre disc had to go just a tiny bit upwards, quite difficult to tell. Mars was crisp and high contrast.

The top push screw definitely makes the dark disc go left when screwed in, and bottom left push screw definitely makes the dark disc go upwards when screwed out. That is with the OTA on the left of the mount as I look at it from the front, when the OTA is on Polaris. I tightened the pull screw (locks the corrector ring) firmly after collimating at 206x.

At 333x with the 8mm Radian it was quite difficult to see any misalignment at all. In focus was a good crisp Airy disc with one outer ring of mid brightness. Maybe a slight tendancy for the ring to flare to one side, difficult to say. In focus at 666x on Polaris I can see the companion and a pretty good Airy disc, nice and round, with a couple or three of rings, with a slight flare to one side of the rings.

The collimation screws are suprisingly insensitive, you can turn them like 1/2 turn at 206x and it's tricky to tell how much the inner dark disc has shifted!

Well I've got it this good. Next will be infocus collimation I suppose, using Metaguide. It runs on my laptop and my camera works (Philips PCVC740K, 640x480) with it all, so we'll see how I get on next time if I am brave enough to attempt even finer collimation. Certainly I seem to be able to master the collimation this far just fine. If anyone needs a Mak collimating, just come by my home and I will do it for nothing! As long as I get to view through the ep at a planet or two ;)

BTW if I can see Polaris' companion with a very bad secondary mirror coating then I can only imagine how bright and colorful the view is through an 8" Mak. Also if I am getting this much detail and contrast on Mars then I can only drool at how much an Alter 815 will show me, I've just got to have one :D

Best Regards,

Alistair G.

#21 Live_Steam_Mad

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Posted 27 April 2014 - 01:46 PM

I checked and it looks like I am only up to beginning stage 2 collimation (Thierry Legault - The collimation) ;-

http://legault.perso....fr/collim.html

I'll try a brighter star (Arcturus, high in the South in the late evening at this time of the year from here in NW England) and higher magnification and try stage 2.

Cheers,

Alistair G.

#22 orion61

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Posted 27 April 2014 - 02:39 PM

With mine I found it easiest to just loosen the jack screws and flatten it totally out. I then just had to move 1 screw about 1/2 a turn till it was perfect!
Before the cell had a gap of about 2mm afterword virtually none!
It is very nice to have those adjustments.
I also found the mirror lock threw off my alignment at high powers so I had to collimate it to either use the Mirror lock 100% of the time visually or only use it at prime focus
low mag where it didn't show up. I am NOT a fan of that system! All it takes is a slight CCW twist to focus to lock mine in just fine..
I have owned 3 of those 7" and as good as they were optically they ALL had annoying image shift! But they are so darn good I quickly got over the little I couldn't remove bu thinning Super Lube a bit and re-greasing the baffle tube.. I was surprised how well my Advance GT (CG5)
held it, even tho it still had the LX200 CW inside..8lbs

#23 Live_Steam_Mad

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Posted 28 April 2014 - 06:20 PM

Last night I focussed on Capella and went up to 666x, and I could not see any definite misalignment of the central dark disc in the rings or any overlapping of the rings. Then it clouded over. I will try a higher up bright star (Arcturus) and try again but it looks like I have done stage 2 collimation as far as I am humanly able to tell. At 666x in focus I got a pretty good Airy disc with only slight flaring of the rings to one side.

I did try connecting my Philips PCVC740K camera (ICX098BQ 4.6 x 4mm CCD) to my OTA with MetaGuide software and I checked and the DSLR calculator online says I am at 440x magnification with the camera! Mars had some detail on the laptop screen even in bad seeing, then I tried the camera on a star and didn't see what I expected to see (I was expecting the pictures in stage 2 collimation on Thierry's page) then it clouded over again. The camera way of doing it seemed like a lot of effort and difficult to achieve.

I got a look at Jupiter visually and it seemed very sharp with details within the belts.

I think the next step is to get a bright star high up at 333x in focus in the 8mm Radian and see if I have a continuous ring around it with no flaring. If I do or I can't tell because of seeing then I will try using a camera for the in focus image (which is 440x). Then if I get it to a continuous ring around the Airy Disc I will try maybe collimation at 666x and then I am done, it's just about perfect then.

BTW does anyone know of a focal reducer that has a serious amount of reduction (0.33x) and NO field correction (so not the Celestron / Meade F3.3 corrector / reducers for SCT's) ? I need to get the focal ratio down with my camera.

Best Regards,

Alistair G.

#24 Live_Steam_Mad

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Posted 04 May 2014 - 09:52 PM

Well I went for it tonight, after seeing a super sharp Mars and Saturn low down with a ton of cloud with more cloud coming over I decided to collimate at 666x in focus on Arcturus!

It worked quite well, I was fiddling only with the push screw which is right near the word "Meade" on my corrector retaining ring. I got it so that the flare to one side was reduced. I found that screwing that one out made the flare go worse on the top left of the Airy disc's 1st ring, and screwing it in reduced the flare. I first increased and then later reduced the flare to lower than when I started, then ran out of time but I am getting closer. It's quite easy to do.

1/4 of turn at 666x makes a small-ish difference to the amount of flaring to one side and even with 1/2 a turn at this magnification the flaring doesn't change a huge amount, and the star doesn't move out of the field of view with 1/2 a turn of movement! It's the least sensitive telescope I ever used for collimation!

Also I would swear that the minor Astigmatism that I saw before at high power is rather lower now, but I think it's because it was a very mild night and I ran the fan a lot and the OTA was totally at ambient and any astig. from cool down is gone and there's just a little bit remaining. I am seeing a bit of trefoil astig. at 666x though.

Cheers,

Alistair G.

#25 Live_Steam_Mad

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Posted 05 May 2014 - 07:45 PM

Hey I just noticed something about my OTA versus the one in the large picture on page 1 of this topic, mine is like the one in the LX50 advert shown below and has "MAKSUTOV-CASSEGRAIN CATADIOPTRIC" in one whole continguous block of text, whereas the newer ones with the mirror lock have the "MAKSUTOV-CASSEGRAIN" and "CATADIOPTRIC" in separate places. Also my 2 holes in my front retaining ring are in different places to the one with the mirror lock on it.

I think (from looking at the pictures that I took when I was removing the huge internal weight) that I put the corrector back on in the right place (there were 3 possibilities)? I notice that the white alignment mark on the inside of the meniscus retaining assembly (where you have to pull the corrector assembly out to see it) is almost 10 degrees clockwise past the top pull (lock, i.e. the larger of the two) collimation bolt, I think, going from the picture I took - see below.

My top pull (lock) collimation bolt is to the LEFT (counter clockwise) of the very top of the OTA tube bolt by about 11 degrees. That in total puts my corrector meniscus just about dead on with the primary as regards the rotation of the primary relative to the secondary, so my primary and secondary are aligned rotationally to what they where when they came out the factory, I assume. That should mean the least astigmatism (in theory). My primary did not rotate on the outer baffle tube that it was attached to, I made sure to check for that when I had the OTA disassembled, so the primary is in the original factory rotational position since the focusser shaft / mirror back focusser plate keeps it in position, rotationally. I notice that the OTA on page 1 has the top of the OTA tube bolt in line with the top (pull / lock) collimation bolt, unlike my top collimation bolt. Here is a composite picture of the 7" Mak from the LX50 advert (left), then the one on page 1 (middle) then my OTA (on right) ;-

https://picasaweb.go...CassegrainIn...

https://picasaweb.go...assegrain7Ex...

...BTW I did NOT put that white mark on there. Either someone else or the factory did. I can't exclude the possibility that a previous owner rotated the meniscus independent of the corrector retaining / clamping assembly, also.

Regards,

Alistair G.






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