19 replies to this topic

[Charlie Hein]

Indoor Artificial Star Collimation

By Derek Wallentinsen

[Sarkikos]

I think the distance is only critical if you are star testing, not just checking or adjusting the collimation.

When I first received my C6, I tested the collimation inside my house using a Hubble Artificial Star. I attached the Star to an elevated tripod head at a far corner of my house, and set up the C6 at the opposite corner. The angle was around 30 or 40 degrees. The collimation was right on, even at 600x.

Of course, since the distance was much less than that advised by Suiter for an optimum star test, the induced SA was obvious. But why should that affect the collimation per se? Star testing and collimation are not the same thing.

In any event, the collimation on my C6 was perfect out of the box when using a Hubble Artificial Star at maybe 2/3 to 1/2 the distance Suiter recommended in "Star Testing." All I used was the Hubble artificial star, not a convex mirror to enlarge the image. Later that night, when I star tested on Polaris, the test was perfect to within 5 waves.

What you have done sounds interesting, but I don't know if it's necessary for collimation, as distinct from the more sensitive procedure of star testing.

That said, if your idea makes indoor star testing easier for SCTs, I'm all for it. Now I wonder where I can get a discarded SCT secondary?


Mike

[rainycityastro]

Really interesting article. Considering the huge number of threads in the SCT forum surrounding collimation (specifically for the Astrotech RC scopes), this should allow someone to collimate using the actual equipment they will have in the field.
I intend to try this out with a CCD camera attached directly to the scope and dial in the collimation.

One question: Does it matter how accurate the figure on the convex mirror is? For collimation? For star testing?

[Sarkikos]

In the field, I would just use Polaris, and not bother with artificial stars or convex mirrors.

Mike

[shiner]

Nice, but just how many people have access to a "discarded secondary from an old SCT" ?
I have one of the artificial stars and they are very good. But the issue I find is getting sufficient distance to get the infinity focus on the scope.

[Sarkikos]

Nice, but just how many people have access to a "discarded secondary from an old SCT" ?

Yes, exactly. I don't have one. Where would I get one? Where would I get a NEW one? Would it be worth the cost?

I have one of the artificial stars and they are very good. But the issue I find is getting sufficient distance to get the infinity focus on the scope.

The full Suiter distance is difficult to attain unless you have a sizable backyard, easy access to a field or a very long hall or basement in your house.

Some will say that you can tie a Christmas ornament in a tree and sight it from your house. For many people this is not a viable solution. All the trees at the sufficient distance from me are in common areas. What would most people think if they saw a middle-aged man shimmying up a tree to hang a Christmas bulb?

But the question is, "Do we really need the full Suiter distance - or focus at infinity - just to get a decent collimation on an SCT?" I doubt it very much. I haven't seen any legitimate rebuttal to this position. Sure, there will be induced spherical aberration. So what? Does that really matter for collimation? Yes, it does matter for star testing. But for star testing you can sight on Polaris.

Mike

[rainycityastro]

Polaris might be an option. But artificial stars are often superior for star testing, no clouds to worry about (a very real concern where I live)and no seeing induced issues to interpret.

You can get convex mirrors at edmund scientific for around $40. Around 10% the price of one of those Takahashi collimation scopes.

[Sarkikos]

OK, so Edmund's is an option for the convex mirrors. But I'm still not certain that it is necessary for close collimation.

Mike

[Sarkikos]

One question: Does it matter how accurate the figure on the convex mirror is? For collimation? For star testing?

Good question. We don't want to introduce artifact errors into the mix. KISS.

IMO, for collimation the convex mirror is not necessary. For testing it might complicate the diffraction pattern with additional errors.

Mike

[orion61]

Nice, but just how many people have access to a "discarded secondary from an old SCT" ?
I have one of the artificial stars and they are very good. But the issue I find is getting sufficient distance to get the infinity focus on the scope.

I have an extra SCT SPH secondary from a meade 2080 that had good optics,(untill the corrector got broken).
Larry
PM me if interested...

[shams42]

The out-of-focus figures in the article seem to show astigmatism. But the author reports excellent optical performance. Did the stig come from the accessory mirror or its mounting?

[Snickersnee]

Not too much as long as the surface is smooth. The closer to the center of the spherical reducer the reflection of the artificial star is, the sharper it will be.

[Snickersnee]

@rainycityastro: The accuracy of the figure is not too important as long as the surface is smooth. The closer to the center of the spherical reducer the reflection of the artificial star is, the sharper it will be. The astigmatism is due to the off-center reflection.

@Sarkikos-Mike: Of course, this is for indoor use. No, as noted in the article, you don't need the full “Suiter” distance for this collimation technique.

@shiner and @Sarkikos-Mike:: Alternatives are suggested in the article that work the same way. Xmas tree ornaments and automotive or bicycle mirrors. Availability of secondaries: Wrecked scopes and parts are seen for sale on ebay, CN, even craigslist and at thrift stores. You certainly don't need to tie the ornament on a tree (inside?), a tripod or even a small bottle cap on a table will hold it in place for this test. Yes, some scopes near focus is too far away to be used in an indoor setting.

[Ptarmigan]

Something to consider. Rather collimate before doing any starwatching and do it inside before taking the telescope out. Collimating eats up precious starwatching time.

[bluedandelion]

The out-of-focus figures in the article seem to show astigmatism. But the author reports excellent optical performance. Did the stig come from the accessory mirror or its mounting?

The external mirror is used to "virtually" increase the distance from the artificial star to the scope. It is not part of the scope. You can get the same effect by moving the artificial star further. Any optical defects such as astigmatism must come from the scope, right?

In the results section the author says, "Observations with scopes collimated this way have been very pleasing. With my C8 at 400x, Mars 2010 opposition revealed details like the dark thread just above the south polar cap. The diffraction disks of the stars in the Double Double look circular and clean in my C5+. During the 2012 Transit of Venus, I observed the planet's faint back-lighted atmospheric halo with the C5+. The image quality of both scopes has generated fine compliments at star parties."

So he is referring to views of astronomical objects being nice through the scope. He used the artificial star to collimate the scope as best as he could during down time. The figures in the article only indicate only how the artificial star looks during the collimation steps.

Sounds like a great time saving strategy to me.

Ajay

[Gil V]

I'll give you an alternative (not to take anything away from the author). Many of us her own multiple telescopes. You can use another telescope as your artificial star - by putting a shot focal length eyepiece in it with a pinpoint light source. With this method, you can star test and collimate relatively easily. A little more difficult to set up, but it certainly works.

[Sarkikos]

Something to consider. Rather collimate before doing any starwatching and do it inside before taking the telescope out. Collimating eats up precious starwatching time.

That is exactly what I do for my Newts. The SCTs and Maks hold their collimation very well, so I hardly ever tweak them before taking them out. When I take my Dobs to a dark site, I try to arrive a little before sun down, allowing me plenty of natural light to check and tweak collimation with Cheshire/sight-tube and autocollimator. I don't trust lasers.

Mike

[zeddie]

I know this is a relatively old article, but I've just come across it. Has anyone actually tried to do this? I'm trying on a 152mm f/8 refractor. can't get the reflection of the artificial star in the FOV of the telescope. They are all at approximately the same height. I put a newtonian collimating laser in the eyepiece holder, got the laser shining directly in the middle of the mirror, and the reflection of the laser in the mirror directed onto the artificial star. Still no image in the FOV. Any suggestions would be greatly appreciated.

 

TIA!

[Baobas3360]

I have a 125mm refractor. It cannot focus within the room. Then a device with a convex spherical mirror and a light source will not work?Then you need a design with a different telescope, right?

Edited by Baobas3360, 08 December 2021 - 05:05 AM.

[Dan in NV]

I just got a GSO 6" classical cassegrain, which is proving very difficult to collimate. First I centered a cheshire's central hole in the donut on the secondary.

 

Then I used an artificial star with a 50 micron beam. Placing it at 100' (more than the minimum specified by the article) made it quite impossible to focus. In order to focus with the 2" extender, and not the other extenders, required that  I move it much farther out. I did it at ~500ft. I want to not use extenders that I'm not going to use while observing, for fear of flexure.  

 

I centered the image of the secondary in the out-of-focus image, and was able to see airy rings- very wobbly due to seeing. 

 

I then went back to the cheshire and found that now the cheshire spot was way out of center. Yikes!

 

I suspect that the donut is not in the optical center of the secondary. It may be in the physical center, but that does no good. Also, the cheshire and a laser that I have fit very loosely in the focuser. I don't think I can rely on them other than for rough, initial collimation. 

 

I need a better solution than doing the artificial star outside, or using Polaris, for that matter. These days in N. Nevada, it gets real cold real fast after the sun goes down. 20-25F. Numb fingers. So, I'm going to try this indoors method with something like a Christmas tree ornament or bike mirror, so I can get a much longer simulated distance. 

 

I found another interesting article:

 

A Procedure for Collimating Ritchey‐Chrétien

and Other Cassegrain Telescopes
Version 1.0
November 15, 2011
Rich Simons – Deep Sky Instruments

 

This one is about an iterative procedure of adjusting the secondary for stig and the primary for axial imagery, using a camera. This method uses a natural star, but maybe it will work with artificial stars.

 

I have a >50 y-old 6" f7 Cave Astrola. All I did was grossly locate the secondary and center the secondary holder with its reflection in the primary. I had both scopes side by side with similar mag, and the Cave was way better. Newts are a lot easier With the Cass I have a way to go. 

Dan