LX200 7" Maksutov collimation off ???

chris shaw · Post by **chris shaw** » Mon Jul 19, 2004 11:13 am

Dear all

Why doesn't the LX200 Maksutov or any Maksutov for that matter, need frequent collimation? Is it possible to bang it out of collimation since the secondary is a aluminized spot on the miniscus lens in front and there are no parts that can be adjusted except perhaps the primary mirror itself.

The question came up after I accidentally banged the scope with my head last night on the OTA after standing up from a squatting position under the scope. The OTA suffered no visible damage, slewing was ok which proved the gears were fine. But a quick, perhaps inaccurate star test, with my 26mm plossl on antares showed good diffraction rings but an almost imperceptible skewing of the rings to the right side. I tried to center the star as best as I can to the center of the plossl to do the star test.

I wonder whether my head knocked the collimation off or did I do the test wrongly. I have never bothered to do a star test on my Mak before since it cannot be collimated except at the factory, so no point.

Can anyone give comments.

Chris

ps. Outside of the star test, Antares looked pinpoint and so did the stars of M6 and M7, so I wonder whether my star test was accurate or the bad collimation is so slight it doesn't really make an observable difference??

Post by **acc** » Mon Jul 19, 2004 12:39 pm

Hi Chris
Don't get unduly worried about the slight skewing, since a slow scope like your mak is relatively insensitive to slight mis-collimation. After all, the in-focus stars you observe are sharp and that's all that matters. Relax and enjoy your scope!

cheers
cc

kayheem · Post by **kayheem** » Mon Jul 19, 2004 12:52 pm

acc wrote:Hi Chris
Don't get unduly worried about the slight skewing, since a slow scope like your mak is relatively insensitive to slight mis-collimation. After all, the in-focus stars you observe are sharp and that's all that matters. Relax and enjoy your scope!

cheers
cc

On the other hand, I beg to differ. Sure, the final F-ratio is high (i.e. slow) but the primary mirror is a fast mirror. It is the secondary that increases the F-ratio. The primary mirror on a typical F/10 SCT is about F/3.5. Hence, it is critical for collimation to be spot on in an SCT.

Chris, I assume there was a difference in the star test before and after the bump. Could it be due to the diagonal you were using? How about repeating it straight through?

I think there aren't many 7" MCT users here in Singastro. You could ask our local Meade dealer for advice or post the question on the appropriate yahoogroup.

Rgds,

Kay Heem

chris shaw · Post by **chris shaw** » Mon Jul 19, 2004 4:08 pm

Thanks for the advice Kay Heem and cc. I did not do a star test before the bump, so I don't know if it is inherent or is it the diagonal. But my question is - why do they say that Maks are generally more resistant to losing collimation? Since the secondary is but a silvered spot on the meniscus front lens, how does a Mak lose collimation?

I know there aren't many Meade Mak users here, but surely there are simply Mak users.

Chris

chrisyeo · Post by **chrisyeo** » Mon Jul 19, 2004 4:13 pm

I think since the primary is movable, it is the primary mirror that can be knocked off-center form its axis. I suppose also that that's hard to do, that's why maks are said to hold collimation well.

My guess,
Chrisyeo

kayheem · Post by **kayheem** » Mon Jul 19, 2004 4:47 pm

chrisyeo wrote:I think since the primary is movable, it is the primary mirror that can be knocked off-center form its axis. I suppose also that that's hard to do, that's why maks are said to hold collimation well.

My guess,
Chrisyeo

Hmmm...thanks for reminding me about the moving mirror bit. Try to repeat the star-test straight thru and with a different eyepiece. If the problem is solved or direction direction has changed , it may just have been due to mirror shift/flop, inherent in scopes using moving primary mirrors. At what magnification did you do your star test?

Kay Heem

chris shaw · Post by **chris shaw** » Mon Jul 19, 2004 5:18 pm

I did my star test at 34X per sq inch or magnification of 242X. I know star tests generally recommend much higher magnifications but I don't see the logic. I can already see diffraction rings clearly at 34X per sq inch. At 34X per square inch I can clearly see that the right side of the rings seem to be a very tiny bit bunched up when I do off focus.

Best
Chris

Post by **acc** » Mon Jul 19, 2004 6:14 pm

Hi Chris

Since you wrote that the skewing is "almost imperceptible", then please try not to worry about it..

Read the following article:

http://www.weasner.com/etx/warnings/col ... rning.html

Cheers
cc

chris shaw · Post by **chris shaw** » Tue Jul 20, 2004 10:30 am

THanks cc for the article. Did not know that slight donut offset is inherent in most Maks when star test is performed even though the scope is perfectly collimated. Hope that is what it is and my accidental jarring of the scope with my head did not knock the primary off its axis!!!
Chris

kayheem · Post by **kayheem** » Tue Jul 20, 2004 11:17 am

chris shaw wrote:THanks cc for the article. Did not know that slight donut offset is inherent in most Maks when star test is performed even though the scope is perfectly collimated. Hope that is what it is and my accidental jarring of the scope with my head did not knock the primary off its axis!!!
Chris

Yeah, read the article too. Thanks, cc.

A couple of points then:
If a slight offset is inherent in most maks,

1. Would that mean that concentric rings signify that it is out of collimation?

2. How do we know that it is currently offset/skewed in the right direction by the right amount?

Kay Heem