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What is the truth about collimation?

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#1 John Hoare

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Posted 07 June 2004 - 10:02 AM

Hi,

I'm thinking of purchasing a new scope (discussed in the thread Your opinion on 5" Mak selection? ). I've already got a NexStar 4, which has a sealed tube with no way of collimating it, but I've had no problems with it. My short list consists of the Orion Apex 127 (or SkyWatcher equivalent), which appears to allow/need collimation from time to time and the Celestron C130 Mak, which appears to be another sealed tube. There is a marginal possibility that if the deal were right I might consider a Celestron C5 or an Intes or Intes-Micro 5" Mak.

My questions are, how likely is it that these scopes might have collimation problems that need attention from time to time? How fequently does it come up and what tools are needed to do it in daylight?

#2 rszabo

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Posted 07 June 2004 - 10:39 AM

This doesn't answer your question, but I wanted to let you know there is a good deal on an Intes Micro M503 on Astromart right now. See this ad.

I had one, and the views through it were terrific. I highly recommend it. I miss that little scope!!

Onto your question: Maks generally don't need much collimation. I've never had to collimate mine.I don't know how difficult it would be to collimate the Celestron or the Orion, but to collimate the Intes all you need is a screwdriver. The directions in the manual make it pretty simple. Best of luck with whatever your choice may be!!

#3 Victor Kennedy

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Posted 07 June 2004 - 02:02 PM

Collimation is a minor pain in the **** It's easy enough to do; all you need is a small hex wrench (if you have a Meade/Celestron SCT; I have no experience with any others) and a few minutes of tweaking.

My C8 didn't need it for over a year, even spending a fair bit of time bouncing around in the back of the car, but last time I used it (about 6 weeks ago) it needed collimating. So (lazy me) I've been using my refractor instead :o

#4 Guest_**DONOTDELETE**_*

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Posted 07 June 2004 - 02:12 PM

Well, i own a Celestron C11 and put Bob's Knobs on the secondary to aid in collimation. I highly recommend the Knobs for the Celestrons as it prevents you from pointing a screwdriver at your secondary/corrector plate in the dark while you are tired! Besides that, they make it a whole lot easier. Now for the downside...I still haven't been able to do better than a rough collimation due to my seeing. That means that my high power views look a bit fuzzy and never "snap" into focus. But in all honesty, a smaller aperature is going to be less fussy about collimation than a mid-sized scope (probably about 10" and up). I would absolutely recommend any Celestron SCT. I have been able to look through several and they offer terrific views. I don't know about the Mak's (I just don't have any experience with them).

Clear Skies!

#5 gazerjim

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Posted 07 June 2004 - 03:35 PM

No experience with Maks, but I could sometimes swear that Gnomes sneak in at night and change the collimation of my 9.25. And yes, it is a pain in the ******, even when seeing is decent, and impossible when seeing is bad. It's also absolutely critical for good ccd imaging of planets. I put collimation of SCT's in the "Under Addressed Issues" box.
Ditto the value of after market knobs (read instructions first!). I got mine from Scopestuff near where I live. There are some pricey gadgets on the market for collimating SCT's--anyone know how well they work?

Jim

#6 John Hoare

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Posted 07 June 2004 - 04:18 PM

Is it the secondary or the correcting lens (meniscus) that gets disturbed? The reason I ask is that in smaller Maks the secondary tends to be figured into the meniscus, could that make a difference?

I will likely be carrying this scope around a lot, so the less that can go wrong with it the better.

#7 wilash

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Posted 07 June 2004 - 06:27 PM

I have the Intes Macro M503. The secondary and primary can be collimated. I have had the telescope for two years and I have yet to collimate it myself.

John, the only way an the Orion Mak's secondary can go out of collimation is if the meniscus goes out of collimation. Very unlikely unless you are going to use the scope as a rugby ball. Of course, if the meniscus is out of collimation, I'm not sure how easy it would be to put back. Usually the meniscus is in a fixed cell - or it is in photographic catadioptric lenses and my M503.

If you are going to use this for photography have you thought about this:

http://www.cloudynig...ro-rubinar.html

It focuses like a camera lens (it is a camera lens). I have tried using my M503 to photograph waterfowl. The knob focuser is really not designed for photography. The hand used for focusing and releasing the shutter is the same. And the focus is slow compared to a lens.

#8 Starman1

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Posted 07 June 2004 - 08:55 PM

The thing about commercial MCT's is that they are, in general, the reverse of SCT's. When SCT's get out of collimation, the only thing you can collimate is the secondary. With MCT's, the only thing you can collimate is the primary. Yes, I know bigger MCT's usually have a separate collimatable secondary, but the 5" and down variety rarely do.
You can collimate REAL close in the daytime. Here's how:
http://www.mira.org/...ures/collim.htm
At night, a simple touch up on a bright star above 45 degrees MAY be called for. Frankly, I get it so close using this method (especially with the card), that I can't improve on it at night, even with the EZ Telescopes Collimation Reticle eyepiece (reviewed elsewhere on this site). So, fancy lasers notwithstanding, the old eyeball is still pretty good.
As for WHETHER you'l need collimation, my Orion 127SVP MCT came perfectly collimated, has been transported in a car to a dark site about 50 times, and is still perfect. Chances are likely yours will be similar.
I only collimate my SCT once a year when I "pop-the-top" to clean the corrector plate on both sides, and I check it every time I go out--but it never needs it, either. 'course I carry it in a padded bag in the back seat, not in the back of a truck on a gravel road. If the screws are snug, it shouldn't go out that often or by that much.
But the daylight method is sensitive and you can take all the time in the world to make it perfect.
A note: when I use an artificial star, my collimation shows perfect when the daylight method shows it a little off. After correcting using the daylight method, it STILL shows perfect with an artificial star. Perhaps the daylight method is more sensitive! Cool.
Good luck with the Orion 127MCT on SVP. It's a killer scope for the price.
Don

#9 John Hoare

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Posted 08 June 2004 - 01:44 AM

If you are going to use this for photography have you thought about this:

http://www.cloudynig...ro-rubinar.html

It focuses like a camera lens (it is a camera lens). I have tried using my M503 to photograph waterfowl. The knob focuser is really not designed for photography. The hand used for focusing and releasing the shutter is the same. And the focus is slow compared to a lens.


That's a very interesting scope. Here's the original manufacturer's link: http://www.lzos.ru/en/astro_lub.htm#1. Of course with a Coolpix 4300 direct attachment is out, so eyepiece projection or a new camera would be my next step. Three problems: budget, budget and budget :( Must investigate angles.

#10 John Hoare

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Posted 08 June 2004 - 12:09 PM

You can collimate REAL close in the daytime. Here's how:
http://www.mira.org/...ures/collim.htm


Thanks for the explanation Don, and a great link.

#11 Mike Hosea

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Posted 08 June 2004 - 07:19 PM

Yes, I know bigger MCT's usually have a separate collimatable secondary, but the 5" and down variety rarely do.


Although true as a matter of correlation with aperture, I think it would sound a little less curious to note that there are two main classes of MCTs on the market, those with aluminized spot secondaries and those with separate secondaries. The ones without a means to collimate the secondary are the ones with aluminized spots (e.g. ETX, Questar, and the ones Orion sells nowadays), and in fact the end user is not usually intended to collimate anything in that case, although the factory-intended means of collimating the primary can typically be accessed and understood easily enough. If, on the other hand, the secondary is a separate optic, then it would be bizarre if it were not collimatable.

Aluminized spot secondaries still need collimating in the factory, of course, but I think this is usually accomplished on an optical bench using shims rather than with some kind of screws.

So it turns out that most of the larger MCTs have separate secondaries (Intes, Intes-Micro, and TEC), while most of the smaller ones have aluminized spots. However, exceptions that come to mind are the Intes Micro Alter M503 (a 5" MCT) and the 10" AP.

#12 Matt Looby

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Posted 10 June 2004 - 10:39 AM

Hello,
I resolved 2 craterlets on Plato this week thru my M503.
Not bad for a 33% obstructed 5" mak. This scope is as good as it gets in the 5" class. there are no short-comings. I highly recommend it.

In regards to collimation, it is very critical step for
high resoultion and it is a process conducted under different conditions of seeing. Really requires minor tweaks
and I always check collimation before lunar observing. I have found that all the MCTs I've owned needed some collimation, and It's very easy to do.
Thanks,
Matt Looby

#13 Scott

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Posted 10 June 2004 - 07:26 PM

I have a Mewlon 180( which is a DK) and have used the daylight method to collimate, however, with any scope that capable of 500X +, precise alignment is a must and I find that method good but not precise. I use an art. star! I'm curious about that Digitec at the top of this page...anybody own one?

#14 Starman1

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Posted 10 June 2004 - 09:48 PM

Scott,
I own the Digitec artificial star, and as I mentioned above, the daylight method actually improved on the artificial star (used with the EZ Telescopes Collimation Reticle Eyepiece), as the artificial star indicated alignment, but the daylight method showed it slightly off. After collimating using the daylight method, the artificial star still showed it collimated, so the daylight method was, I believe, more sensitive.
I used a card (with a small hole in it)mounted on a tripod stand to do the daylight collimation, and I fussed with is a while (20 minutes).
Star testing showed perfect star images, but seeing wasn't good enough to really see the difference between the 2 methods.
One caveat: diagonals are rarely a perfect 45 degrees, and a perfectly collimated SCT or MCT can show out-of-collimation star images when the diagonal is attached. You can either shim the diagonal mirror or, as I did, try 5 units until I got one that was close to perfect.
Don

#15 Scott

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Posted 11 June 2004 - 08:48 PM

Don, Do you bother bringing the center of the hole in the card right up to the center of the secondary, before moving it back across the room...or just approximate? I have the set screw style 2" adaptor for my Williams diagonal...which is most likely not as precise as the screw in. How do I know its truly a 45?? Regards, Scott

#16 Starman1

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Posted 12 June 2004 - 10:22 AM

Scott,
The card is placed at a distance away from the front of the scope that allows the secondary to be just a trace smaller than the visible shadow of the secondary. That way, you can take your time to position the card
perfectly to make the two perfectly concentric. This is accomplished by moving the card and the scope until they line up perfectly. As for distance, I set that up before I position the card.
As to how to gauge a true 45 degree, collimate on any form of artificial star during daylight (artificial star, ornament reflection, etc) with the eyepiece straight through, then compare the "star" image with the diagonal in place. If the collimation appears to be off, you can shim the diagonal because you have the adapter style. There is always play in the slip fit between the adapter and the diagonal tube. Aluminum foil makes an excellent shim material--a small patch of it can correct for angle changes if you put a piece between the body of the diagonal and the end of the adapter tube, or between the adapter and the outside of the diagonal's insertion tube, etc. If you have the scope collimated in the straight-through mode, AND with the diagonal in place, it is 45 degrees and you needn't worry about the angle.
I have noticed that heavy eyepieces flex the diagonal or adapter enough to spoil the perfect collimation. Perfectly collimated with a 20mm Plossl is NOT perfectly collimated with a 20mm Nagler2. The extra weight actually flexes the diagonal and its attachments enough to change the angle of the diagonal by an amount that makes the collimation change. I don't think the thread-on diagonals are any better in that regard--but maybe the WO thread-on one is, because the extension of the thread-on tube is VERY short, so it may not flex as much.
I may experiment in the future. If so, I'll write comments about what I find.
Don

#17 Scott

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Posted 18 June 2004 - 09:13 AM

Don, I'm able to achieve perfect, concentric rings in all but the outer most ring, using the daylight method. This is the same problem that PJ talked about in his article www.cloudynights.com/howto/cassegrain.htm . Using my art. star, alignment is good but slightly out. After correction...the "daylight" is off! I've contacted Tak Texas and they say that "daylight" is for rough alignment only...star testing is the answer. Have you had the outer ring (between the reflection of the scope itself and the inner edge of the OTA opening) problem? Regards, Scott

#18 Starman1

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Posted 18 June 2004 - 10:27 AM

Scott,
I can get all the rings perfectly concentric. The star test (with diagonal) shows the collimation is off. The star test w/o diagonal (very awkward and difficult) also shows collimation is off, but only by a minor tweak of one screw. When I remount the diagonal, I'm off. Mind you, this is the closest to perfect diagonal I've had (out of 5), where the 45 degree angle is concerned. I can only presume that the collimation test (both daylight and star) is sensitive enough to expose even a small error in tilt in the diagonal. By loosening the diagonal and manually moving it, I could improve the image, but only slightly.
So I had a choice: spend many hours shimming the diagonal mirror (no easy feat in a Televue diagonal) or re-collimate to the diagonal, even if this shows the scope is off. Presuming this would introduce out-of-collimation images near the edge of the eyepiece field, I went ahead and collimated with the diagonal in place, using the EZ Telescopes Collimation Reticle eyepiece.
Here's what I found:
The edge of field stars (in a Nagler-width field) are not identical on both sides of the field. One side displays more coma than the other. But the difference is slight. And, the difference is up/down. The side to side images appear clean. This implies sag is the problem.
But the centered star images are not oval--they are round with round diffraction rings. And the image of Jupiter is very sharp. During moments of good seeing, the detail is tremendous.
This only works because I do NOT rotate by diagonal. I use the scope in ALT-AZ mode only. If I rotated the diagonal, the edge-of-field aberrations would rotate with the diagonal and it's likely even the centered images would change.
Which means the mechanical line-up done with the daylight collimation may not be able to compensate for an off-center diagonal, or a tilted baffle tube, or any other mechanical misalignments the scope may have.
I'll have to think about it.
How can a perfectly concentric set of images result in mis-collimation?
How does improving the star image in nighttime collimation throw the alignment off as seen in the daylight test, and what type of sag takes place in the mechanical attachments that causes the mounting of a near-perfect diagonal and eyepiece to throw off the collimation?
One thing that would help is for manufacturers to make a collimatable diagonal mirror for SCT's to allow us to tweak the star images.
I'm still experimenting with mechanical alignment (moving the secondary, etc). I'll let you know what I find.
Don


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