Which to trust most: collimation cap, cheshire eyepiece or laser?

[Piero]

If collimation of a, let's say, f5 Newtonian telescope without coma corrector is checked with a star test, the star must perfectly be on axis, otherwise the coma-dependent misalignment is also visible. This cannot be really distinguished by optical misalignment.

So, again, to me at least star testing is not the right way to check collimation as it is too sensitive, unless the seeing is very good, the high power eyepiece is good, coma corrector is used, and the mount tracks automatically. Even so, this test doesn't tell you anything about secondary and focuser alignments.

 

 

Coming back to the topic, my HG laser and Catseye telecat give the same reading consistently. 

Edited February 18, 2020 by Piero

[John]

I've been quite happy with my cheap plastic cheshire for the past few years. Perhaps I've been kidding myself

[Louis D]

6 hours ago, Piero said:

Even so, this test doesn't tell you anything about secondary and focuser alignments.

I've often wondered, if the secondary is tipped relative to the true optical axis, but the primary is compensatingly tipped so the center dot looks centered in a collimation cap reflection, what affect does this have on the image?  Does Suiter discuss this condition?  I ask because I'm never quite sure if the secondary is pointing exactly at the center of the primary each night even since I check it infrequently, only performing a quick primary check each night.

[johninderby]

That’s why I’m a convert to the Concentre. So simple to use but it gets the secondary spot on as when you’ve adjusted the secondary mirror to a perfect circle using the concenric rings it is now precisely centered under the focuser as well as being rotated correctly and not tilted. Even a total beginner can get the secondary perfectly set up very easily. 

One thing I have found is that everyone I know that has tried a Concentre becomes a convert to the concept.

Can of course also be used for adjusting the primary. It is a good idea to check the squareness of the focuser to the tube as well. 

[jetstream]

9 hours ago, CraigT82 said:

I dont like to be wasting precious clear-sky time with star collimation,

Great point and I don't waste time collimating like this either. The tools available to collimate a newt are fast, easy and accurate- even my 300+ lb 24" f4, after wheeling it down a ramp and across a gravel pad is easy and fast to collimate.

 

[Piero]

2 hours ago, Louis D said:

I've often wondered, if the secondary is tipped relative to the true optical axis, but the primary is compensatingly tipped so the center dot looks centered in a collimation cap reflection, what affect does this have on the image?  Does Suiter discuss this condition?  I ask because I'm never quite sure if the secondary is pointing exactly at the center of the primary each night even since I check it infrequently, only performing a quick primary check each night.

 

By 'tipped' do you mean off-axis towards the primary mirror?

 

1. A primary mirror axial misalignment will cause coma on axis (coma due to misalignment).

2. A focuser axial misalignment will cause the stars to focus at different points across the focal plane.

3. A secondary mirror (severe) misalignment will cause unequal field illumination.

 

It's rather obvious that only the first one degrades on-axis, therefore 2 and 3 reveal nothing on a star testing as this is conducted on-axis.

 

Of the 3, the last one is the less critical for visual astronomy. 

Without coma corrector, the first one is the most critical. With coma corrector, also the second becomes rather critical. All of them become more critical in faster newtonians. 

There is nothing to fear in this process. If wrong, it can be fixed and rather easily.

[Louis D]

20 minutes ago, Piero said:

 

By 'tipped' do you mean off-axis towards the primary mirror?

 

1. A primary mirror axial misalignment will cause coma on axis (coma due to misalignment).

2. A focuser axial misalignment will cause the stars to focus at different points across the focal plane.

3. A secondary mirror (severe) misalignment will cause unequal field illumination.

 

It's rather obvious that only the first one degrades on-axis, therefore 2 and 3 reveal nothing on a star testing as this is conducted on-axis.

 

Of the 3, the last one is the less critical for visual astronomy. 

Without coma corrector, the first one is the most critical. With coma corrector, also the second becomes rather critical. All of them become more critical in faster newtonians. 

There is nothing to fear in this process. If wrong, it can be fixed and rather easily.

Correct, the secondary is slightly off-axis toward the primary.  Think of a laser beam striking the center ring itself rather than the hole.  Now, the primary is tipped to send the laser beam back to the center of the laser's output.  So now, you've got some primary mirror axial misalignment induced by the secondary mirror misalignment, correct?  Would a collimation cap be immune to the effects of the tipped secondary when aligning the primary causing the primary to be aligned correctly irrespective of the secondary tip or would it result in the same situation as with the laser?

[cloudsweeper]

I reckon the whole collimation thing has been blown into something unnecessarily worrying.  I went through that before getting my 10 inch Dob, but when I got it, I just did the process by eye, then finished off the primary with a simple cap.  No Cheshire, no laser.  And the views are crisp and clear.  So for me, a cap is all you need!

Doug.

[Timebandit]

12 minutes ago, cloudsweeper said:

I reckon the whole collimation thing has been blown into something unnecessarily worrying.  I went through that before getting my 10 inch Dob, but when I got it, I just did the process by eye, then finished off the primary with a simple cap.  No Cheshire, no laser.  And the views are crisp and clear.  So for me, a cap is all you need!

Doug.

 

 

Totally agree. I think a lot of wanabee or newbies to a reflector do have real concerns about collimation😱. Unless the alignment is really out , then something like cap or Cheshire(astro baby collimation guide was really helpful when I was a newbie) followed by a "star test" will usually do the job. This method has served me well for years. And luckily the OOuk mirror cells i have are really robust and hold collimation well, so my reflector rarely needs adjustment.

 

 

 

[Davey-T]

Just purchased HG laser collimator and about to embark on collimating the 150mm f/2.8 SharpStar HNT, never collimated a Newt' before so probably jumping in the deep end but gotta be easier than one of those RCs 😂

Dave

[Tim]

The Cats Eye collimation system is second to none, and is capable of giving you the precise collimation needed even for imaging with fast Newts.

Due to the way auto-collimators work, layering multiple reflections with an offset view point, rotating the auto-collimator 180° gives even more precise results.

Probably way more accurate than will be appreciated  at the average Newtonian's eyepiece, unless you are into Planetary or lunar observation.

Even with a Cats Eye though, your telescope needs to be properly set up in the first place, with focuser and mirrors all nicely positioned and securely held within the tube.

https://www.firstlightoptics.com/catseye-collimation-tools/catseye-infinity-xlkp-autocollimator.html

https://www.firstlightoptics.com/catseye-collimation-tools/catseye-blackcat-xl.html

Tim

Edited February 19, 2020 by Tim

[joe aguiar]

I think best one is high power star test

joejaguar 

[Spaced Out]

On 18/02/2020 at 15:09, johninderby said:

That’s why I’m a convert to the Concentre. So simple to use but it gets the secondary spot on as when you’ve adjusted the secondary mirror to a perfect circle using the concenric rings it is now precisely centered under the focuser as well as being rotated correctly and not tilted. Even a total beginner can get the secondary perfectly set up very easily. 

One thing I have found is that everyone I know that has tried a Concentre becomes a convert to the concept.

Can of course also be used for adjusting the primary. It is a good idea to check the squareness of the focuser to the tube as well. 

 

I've just ordered one !

I now have a cheshire, a cap, a hotech lazer and tomorrow I'll be adding this to my arsenal ! I ordered it because I have always found the secondary the hardest part to get absolutely bob on and this sounds the tool for the job.

Personally I wrestled with a cheshire when I was learning, everyone said it was the best method, but I've come to the conclusion a cap and a laser are much easier to use. Is it as accurate tho ? I don't honestly know, but my images look OK. What I do know is that it has always been the secondary that has bugged me with the collimation process so I hope this new tool will help out with that.

 

[Erling G-P]

2 hours ago, Spaced Out said:

 

I've just ordered one !

I now have a cheshire, a cap, a hotech lazer and tomorrow I'll be adding this to my arsenal ! I ordered it because I have always found the secondary the hardest part to get absolutely bob on and this sounds the tool for the job.

Personally I wrestled with a cheshire when I was learning, everyone said it was the best method, but I've come to the conclusion a cap and a laser are much easier to use. Is it as accurate tho ? I don't honestly know, but my images look OK. What I do know is that it has always been the secondary that has bugged me with the collimation process so I hope this new tool will help out with that.

 

Me too - along with a Hotech laser

We have a remarkably similar setup - I have the same two scopes & the same mount (although my Rowan Belt kit is only ordered).  Have a Chesire & cap presently. At least with the 130 PDS. you can reach everything while looking through the Chesire, but with the 200 PDS, you'd have to have arms like a gorilla to reach the primary's adjustment screws.  The description of the procedure with the Hotech laser just sounded so easy - and being able to see the results of your adjustments 'live', would be a great improvement.  The prerequisite for the laser being that the secondary is perfectly centered, which the concenter should take care off.  Can't wait to play with this during these never ending cloudy nights..

Edited February 19, 2020 by Erling G-P

[Piero]

On 18/02/2020 at 18:14, Louis D said:

Correct, the secondary is slightly off-axis toward the primary. 

[Q1] Think of a laser beam striking the center ring itself rather than the hole.  Now, the primary is tipped to send the laser beam back to the center of the laser's output.  So now, you've got some primary mirror axial misalignment induced by the secondary mirror misalignment, correct? 

 

[Q2] Would a collimation cap be immune to the effects of the tipped secondary when aligning the primary causing the primary to be aligned correctly irrespective of the secondary tip or would it result in the same situation as with the laser?

[A1] If the laser beam strikes outside the centre of the silhouette on the primary mirror, the focuser axis is misaligned. Assuming that this fact is ignored and one completes with the primary mirror alignment, the result would be that the primary mirror axis is "aligned" and the focuser axis is misaligned. To my understanding, this should show something like: 

In this figure, the focuser seems perfectly "squared" to the secondary mirror, the primary mirror is collimated, but the focuser axial alignment is still off. One could align the focuser axis first. This would make the secondary mirror look like more elliptic. Then the primary mirror can be aligned. Would this be fine? For visual, I would say so. The secondary mirror is clearly oversized and the primary mirror is still fully visible. There are cases where this cannot be sorted so easily though. If this is the case, one could move the secondary mirror slightly away from the primary mirror and collimate the focuser by levelling the small Allen bolts to the right of the focuser. The focuser Allen bolt at the bottom right needs an extra touch. This would allow to move the reflection of the primary mirror towards the centre of the secondary mirror a bit more.

 

[A2] The effect should be the same as with the laser.