89 replies to this topic

[Procyon]

I bought a Ronchi because I'm curious and want to be sure I did not buy a lemon of a scope and would like to learn more about optics testing and abberations. I also bought some Bob's knobs for an 8" EdgeHD. Now I'm looking to buy a collimation tool if one exists or works well.

 

What are your thoughts on these or others within that price range. So that means no HoTech Advanced CT Laser Collimator for SCT Collimation which is like $400+.

 

I found a Hubble 5-star Artificial Stars Collimator and a HoTech SCA Laser Collimator for Reflector Telescope - Single Dot

 

Has anyone tried either? How useful was it? Did it help a bit?

 

Or are you better off doing the following:

 

Collimating a Schmidt-Cassegrain Telescope

Edited by NorthWolf, 09 November 2014 - 05:48 PM.

[rmollise]

I've tried both and both work very well. The Hubble things works, but the SCA is the ONLY way to collimate in the daytime in limited space.

Bob's Knobs are good, too...though as I said not long ago, I don't use them anymore. Not because they don't work, but because a properly collimated scope doesn't need to be tweaked that often, and the factory screws are fine by me for those rare occasions.

Edited by rmollise, 09 November 2014 - 06:02 PM.

[Procyon]

I've tried both and both work very well. The Hubble things works, but the SCA is the ONLY way to collimate in the daytime in limited space.

Bob's Knobs are good, too...though as I said not long ago, I don't use them anymore. Not because they don't work, but because a properly collimated scope doesn't need to be tweaked that often, and the factory screws are fine by me for those rare occasions.

Good ole unk'le Rod! I was just reading your review and article this morning on how to collimate SCT's and how the Kendrick one did not work out. Than I think I got lost with the Hotech part, it seemed like an entire adventure to get it done. I'd like to ask you some questions if you don't mind:

 

1) The hubble thing only works at night?

2) In the review you seemed to really like Bob's knobs, did they become bothersome in some way after a while? Did you get really used to collimating with the factory screws on at some point?

3) How hard or easy would you say the learning curve for the SCA is and would you recommend it? I used to use a similar laser device on my dob, but I see this one needs more tweeking around.

 

Thanks

Edited by NorthWolf, 09 November 2014 - 06:11 PM.

[korborh]

I would recommend collimating on a real star as that is needed to really tell if scope is collimated no matter what other tool you use. You don't need expensive tools, just Video camera (like qhy5L-II mono) + Metaguide free software + star. And with Metaguide you can dial in precise collimation in normal seeing.

[De Lorme]

Take a look at this site. Really made it simple with absolutely no guessing.
http://www.asterism....als/tut14-1.htm

[De Lorme]

If I didn't have Bob's nobs already on my C14" I would stay with the allen screws.
Bob's nobs become difficult to turn when getting close to zeroing in on collimation.
Using T allen wrenches would be much easier.

[jhayes_tucson]

I got a very nice demo of the Hotech advanced CT "collimator" from David Ho at ASAE a week ago and it looked very good.  It's a bit expensive, but I think that the principles are sound and that it seems to work pretty well.  Achieveing precise optical alignment isn't that hard--particularly with the right tools and the Hotech product looked pretty good.

 

I agree wiith others that the factory alignment screws on the secondary are perfectly fine.  If you tighten everything properly as you complete the alignment, the whole thing is super stable so it stays aligned even after some pretty severe jostlling.  I see little use for other knobs from Bob or anyone else on the secondary.

 

John

Edited by jhayes_tucson, 09 November 2014 - 10:14 PM.

[Procyon]

How often do you guys collimate your sct's? Too late now for Bob's knobs already ordered them, I don't think they shipped yet though, is it worth calling to maybe exchange them for something else? Hmmm like what though, and by the way where did you get the Allen key from? My 2nd hand sct did not have one, I have an allen key set in the garage though.

Edited by NorthWolf, 10 November 2014 - 12:09 AM.

[freestar8n]

Here is my brief summary of collimation tools and methods.  Obviously I prefer the use of MetaGuide with a real star - but even without that particular software, there are clear advantages to using a real star overhead, in focus, as a direct indication of true collimation rather than some kind of proxy.  And even without MetaGuide, video makes it much easier to study the star in detail as you make adjustments.

 

1)  Many web pages will describe "centering the donut" using an out of focus star.  This is fine for initial collimation - but what matters is the appearance of the in-focus star image.  A centered donut is neither necessary nor sufficient to guarantee a good in-focus star image - so it is not a final indication of optimal collimation.

 

2)  Some devices will bounce a laser inside the 'scope somehow and use that as a true indication of collimation - but beware of any system that does not sample the entire wavefront, or that relies on some reference in the ota, such as the focus tube.  One exception is the various laser devices that are based on first collimating the system with a star - and then making sure the laser spot bounces off the secondary the same way.  This relies on a real star for collimation - and just uses the laser to make sure the secondary is still set that way.  That seems like a good system to me because it doesn't try to indicate "collimation" - it just indicates the system is aligned as it was when it was collimated.

 

3)  Many people claim the Airy pattern requires exceptional seeing to be useful - but that is one area where MetaGuide has a specific advantage - since it uses realtime image processing to reduce the impact of seeing.  Once you have the Airy pattern in view, you are looking at what really matters for collimation - and with a real star and the full wavefront.

 

Frank

[REC]

I've tried both and both work very well. The Hubble things works, but the SCA is the ONLY way to collimate in the daytime in limited space.

Bob's Knobs are good, too...though as I said not long ago, I don't use them anymore. Not because they don't work, but because a properly collimated scope doesn't need to be tweaked that often, and the factory screws are fine by me for those rare occasions.

 Ron, I have a 2" SCA from Hotech to collimate my DOB...can I use the same devise on my SCT as well?  Wasn't sure if it was the same model for both Newts and SCT's?

 

Thanks,

Bob

[jhayes_tucson]

If you really want to use the full wavefront for alignment, the folks at Innovation Foresight (IFI) are working to introduce a Shack Hartman sensor to actually measure wavefront errror through the whole system using a star as a source.  It works pretty well, although I think that it's a bit overkill for aligning a SCT.  It might make more sense for a RC scope.  In any case, it lets you see exactly how good your system really is and how much a small tweek in alignment might affect the wavefront.  Thor Labs also sells a SH sensor (camera  + lenslet array) and software to do much the same thing but you have to  do a lot of design to make it as turnkey as the ISI system.  In either case, adding the capability to measure wavefront data will set you back by the cost of a C11 Edge OTA so it's not inexpensive.  Of course compared to a cheap interferometer that might run $50k, it's dirt cheap.  

 

PixInsight also has a nice tool to evaluate alignment.  The tool produces a contour plot of star size over the whole field.  When things are well aligned, the minimum star size should be on axis and the contours should be roughly circlular and centered.  That tool is less useful for alignment (particularly for more complex systems like RCs) and it's most useful as an "after alignment" assesment.

 

John

[freestar8n]

I don't mean to measure the full wavefront - but to use the full wavefront in assessing collimation.  A star overhead and full aperture will do this.  At the same time - a relatively low cost Shack Hartman sensor sounds great and I imagine would have some value for quantifying performance in-situ with a real star - so I look forward to results from that.

 

For the same reason, I am not a fan of Bahtinov masks for focusing - and instead I rely on direct measurements of star sizes through focus with the full aperture.  That is how most autofocus routines work.  Any mask is only using part of the wavefront for focus.

 

As for assessing stars in images - I have my own tool for that - but for me it is similar to studying a real star in focus.  Even without a tool you can scan in image and study/measure the stars around the field to get a true indication of imaging performance, and to reveal any tilt, coma, or astigmatism present.  I like to go by the real stars in the real image plane for collimation and image assessment - rather than proxies.

 

Frank

[Eddgie]

My own advice is to just learn to collimate using stars.  There are many good reasons for this.

 

First, it is not at all difficult.

 

Second, if you collimate with the telescope pointed to close to the horizon, critical collimation will change when you point into the sky.

 

Third, if you collimate correctly, and if you properly tighten the collimation screws, an SCT should hold collimation for years.  I went mayby 3 years with my C14 and never had to touch collimation.  I collimated my EdgeHD once after I had to remove the corrector to clean it.

 

I don't transport the scopes on rough country roads, but otherwise, they would go in and out a lot with noting special in the way of handling.

 

If you are having to collimate an SCT a lot, then something is either wrong with the collimation proceedure being used, or something is wrong with the telescope.  An SCT could go a lifetime without collimation.  When I read that people have to collimate their SCTs a lot, I always suspect that they are not doing collimation correctly to start with, and most SCTs will not be in perfect collimation at all possible positions in the sky.  Most that are collimated at angles closer to the horizon will not be in collimation at zenith.   My guess is that the most common reason for people collimating a lot is becuase they are collimating it in different positions than they are using it, and are not turning the focuser properly when testing (always appoach focus using CCW turn of the focuser to lift the mirror into a consistent angle).

Edited by Eddgie, 10 November 2014 - 12:03 PM.

[Geo.]

After seeing one corrector broken because of accessory collimation knobs' lack of adequate clearance to the dust cover, I've never used them. However, if you find you have Phillips head screws replace them with socket heads and an Allen headed screwdriver.

 

I've never been able to get my head around all the flips and flops the light does as it gets from the light to my eye, so I came up with this method that only requires remembering one fact. 

 

Then all you need a artificial star or distant point of light and a finger or an artificial finger like pointer, if you can't reach the far end of your optical tube while looking through the scope.

 

OK, your out of focus point of light is off center. Place your finger in front of the corrector pointing in the same direction as the secondary shadow is offset.

 

The secondary is tilted toward your finger. You want to push the edge of the secondary closest to your finger into the scope and pull the edge farthest out. This will require loosening one or two screws and tightening one or two others. Righty-tighty pulls the secondary out and lefty-loosey lets it be pulled in. 

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[Procyon]

Phew, Eddgie you saved the day,

 

My own advice is to just learn to collimate using stars.  There are many good reasons for this.

 

First, it is not at all difficult.

 

Second, if you collimate with the telescope pointed to close to the horizon, critical collimation will change when you point into the sky.

 

Third, if you collimate correctly, and if you properly tighten the collimation screws, an SCT should hold collimation for years.  I went mayby 3 years with my C14 and never had to touch collimation.  I collimated my EdgeHD once after I had to remove the corrector to clean it.

 

I don't transport the scopes on rough country roads, but otherwise, they would go in and out a lot with noting special in the way of handling.

 

If you are having to collimate an SCT a lot, then something is either wrong with the collimation proceedure being used, or something is wrong with the telescope.  An SCT could go a lifetime without collimation.  When I read that people have to collimate their SCTs a lot, I always suspect that they are not doing collimation correctly to start with, and most SCTs will not be in perfect collimation at all possible positions in the sky.  Most that are collimated at angles closer to the horizon will not be in collimation at zenith.   My guess is that the most common reason for people collimating a lot is becuase they are collimating it in different positions than they are using it, and are not turning the focuser properly when testing (always appoach focus using CCW turn of the focuser to lift the mirror into a consistent angle).

Phew, thanks for this, after reading a few posts I thought I was going to have to learn rocket science, some of it isn't easy for an amateur.

 

Second, the artificial star is probably useless to me as it looks like you need at least 50 feet room distance (From the OTA to the Artficial star) to perform a collimation with an artificial star.

 

Third, would you try a Ronchi after you've made sure your scope is collimated or before? From my understanding the Ronch's sole purpose is to detect aberrations and nothing to really do with collimation, correct?

 

That leaves me with either collimating the SCT with a real star but in the dark, which is ok because you may only have to do it once in a blue moon, or spend 100$ and get the hotech sca laser collimator. I think I'll take Eddgie's advice and try a normal star. As for Bob's knobs, I will have to find out what you guys mean about being hard to tighten the final steps.

[REC]

So George, you then tighten the screw nearest to your thumb shadow then?  That will push the bulge back and the star will have concentric rings and your done , correct?  How much magnification of a star are we talking about, 150x or so with a C8?  If we can't use Polaris what typical magnitude star would you recommend, 2-3 range or brighter?  Is Vega too bright as it is overhead or something more around the 50* mark over the horizon?

 

Thanks for your ideas:)

 

Bob

[Procyon]

What about this collimation tool? It says it's helpful in collimation of SCT's/

 

Celestron Specialty Collimation Telescope Eyepiece Tool

 

The Celestron Cheshire Collimation Eyepiece is ideal for precise collimation of Newtonians and helpful for aligning Schmidt-Cassegrains. This special eyepiece fits into 1¼" focusers or diagonals.

Alignment is easy using the small opening on one end and thin cross hairs at the other end. The collimation eyepiece is more accurate than the simple Collimation Tool.

[Eddgie]

For collimation, the artificial star does not have to be any further away than necessary to be able to focus into an Airy Disk.   The only time the distance is critical is for testing for spherical aberration.  

 

But I would never collimate using an artificial star close to the ground.   The first time you check collimation in the sky, it will be off, and you will want to re-collimate.

 

This is what I said earlier. A lot of people chase collimation.

 

When you have an SCT level, the mirror carrier (the part that slides on the baffle) is resting evenly on the baffle.  

 

When you collimate in this position, you collimate for the angle that the f/2 primary mirror is resting at.

 

Now, when you point the scope up into the sky, here is what happens.   The primary carrier settles at a slight angle.  One side of the primary mirror is being supported by the focuser rod.   The weight of the mirror causes it to sag against the baffle so that it is now sitting at a slight angle.  Bang.  your collimation has changed.

 

If your SCT has zero mirror slop, you might get away with collimating level, but in most cases, you won't.

 

It is also important to use CCW motions of the focuser when checking collimation because this ensures that the mirror is always "Lifted" on the same side and settles at the same angle.

 

It is an imperfect design, but when used on the sky, it works well enough.   If you collimate on a ground target and I read later that your collimation is always changing, I will try hard to avoid saying "I told you so."

 

[Eddgie]

Please excuse this very crude drawing, but I just wanted to try to give you a visualization of why collimation will usually change if you collimate the scope while the OTA is pointing near the ground...

 

Of curse this drawing greatly exaggerates the play between the mirror carrier and baffle, but remember, we are talking arc-minutes of angle from a collimation standpoint.   The translates into a few thousandths of an inch of play between the carrier and the baffle.  The carrier has to have some play or there would be no room for a grease film which is part of the problem... The carrier settles into the great after you stop focusing if you don't jack up the mirror using counter clockwise focuser motion. 

 

I have actually seen my collimation change before I started doing CCW motion to test.  If I approached using clockwise motion (dropping the mirror into postion rather than lifting it into position), I could actually see the collimation change as I watched the mirror settle into the grease on the bottom side away from the focuser rod.  On a cold night, this can take 15 or 20 seconds. 

 

Again, I keep repeating this, but it is only because I believe it to be a big reason why people are always collimating.   I read this all the time, that SCTs need a lot of collimation.    And yet, my scopes, all of them, have held collimation for years and years.

 

But I used to fuss around with collimation all the time!!!!!  It was not until I figured all of thes things out that I learned to collimate using the sky and using CCW motion of the focuser.

 

But this means when doing high resolution (planetary) observing, final focus should always be approached using CCW focuser motion.  This helps ensure that the mirror always comes to settle at the same angle against the primary.

 

Even here, collimation can get off when viewing close to the horizon, but I just choose to ignore that, and not look at planets when they were to low in the sky anyway, because seeing is many times worse close to the horizon vs near zenith.  I find little pleasure in having  50 miles of atmosphere between me and the target when it is down low, when straight above me, I am only looking though about three miles of atmosphere.  It matters little to me that collimation is not perfect when my scope is pointed at lower angles because the seeing always dominates.. There is never perfect seeing for me when the angles are below about 40 degrees from the horizon. 

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Edited by Eddgie, 10 November 2014 - 05:19 PM.

[REC]

What about this collimation tool? It says it's helpful in collimation of SCT's/

 

Celestron Specialty Collimation Telescope Eyepiece Tool

 

The Celestron Cheshire Collimation Eyepiece is ideal for precise collimation of Newtonians and helpful for aligning Schmidt-Cassegrains. This special eyepiece fits into 1¼" focusers or diagonals.

Alignment is easy using the small opening on one end and thin cross hairs at the other end. The collimation eyepiece is more accurate than the simple Collimation Tool.

 

I have this one and use it in my Newt, but never in my SCT.  Always just use a star as others have pointed out.  What Ed is saying about have the scope pointed up rather then level make sense.

[Procyon]

So from what I understand from you knowledgeable guys, the best way is either a real star, the $400 advanced hotech collimator, or the $100-150 SCA dot collimator. Fair enough! Thanks I got my answer, now I need to read on how to collimate, must not forget that part lol.

Edited by NorthWolf, 10 November 2014 - 09:51 PM.

[De Lorme]

Thanks Ed for pointing that out. I didn't even consider turning the focuser CCW when focusing.

De Lorme

[rmollise]

Good ole unk'le Rod! I was just reading your review and article this morning on how to collimate SCT's and how the Kendrick one did not work out. Than I think I got lost with the Hotech part, it seemed like an entire adventure to get it done. I'd like to ask you some questions if you don't mind:

 

 
1) The hubble thing only works at night?
2) In the review you seemed to really like Bob's knobs, did they become bothersome in some way after a while? Did you get really used to collimating with the factory screws on at some point?
3) How hard or easy would you say the learning curve for the SCA is and would you recommend it? I used to use a similar laser device on my dob, but I see this one needs more tweeking around.
 
Thanks

 
The Hubble artificial star works in the daytime, sure, but you'll have to have enough space to get the scope to almost come to focus. Many folks don't have homes/yards quite large enough. Also, as Ed says, it's good to have the target elevated. 
 
David Ho has simplified the operation of the SCA quite a bit since I first tried it.
 
They never became bothersome....it just became a matter of not having to collimate often enough for them to make that much difference. I bought a set for my NexStar 11 GPS back in 02, but never did get around to installing them.
 
Before you decide on the SCA, have a look at Mr. David's excellent instructional videos on Youtube.

Edited by rmollise, 10 November 2014 - 05:50 PM.

[Procyon]

Informative video https://www.youtube....h?v=EwTrvznz5mA

Edited by NorthWolf, 10 November 2014 - 10:14 PM.

[Procyon]

https://www.youtube....h?v=hqRVIDj4aZA There's no way an out of focus star image on my scope looks as bad as this one...