Back to forums

SCT Collimation Idea

Equipment Collimation
8 replies•206 views
Michael Gorman avatar
Michael Gorman avatar
Don't worry, this is not a help me with my collimation thread. 

Obviously collimation is a pretty annoying headache that some of have to deal with. My main scope is a wide field refractor so i'm all set there unless i drop it, however come galaxy season I heft an edgeHD out to my pier. Just sneezing within 10 feet of these can knock the collimation out (I kid). But seriously it typically does need to be done every so often. More so if you're cleaning things or swapping in and out a hyperstar.  And when you finally do go to do it, you need to effectively waste a portion of a decent night fiddling with it. There are expensive laser setups, and they seem to generally work, but still they suggest checking on a bright star dead center when done on both sides of focus.

So this morning I was just shooting flats with my flat panel and looking at a few of the dust mote donuts it occurred to me, could we just use a filter with an appropriately sized dot placed in the middle during the day or with a flat panel to create a nice big out of focus donut to allow for much easier collimation?
Helpful Insightful Respectful Engaging
Lynn K avatar
Lynn K avatar
A dot placed on anything that close will not come to focus or near focus. ARE, you would already have a big one called your secondary.
 My answer is no.  Dots on elements cause donuts, not diffraction rings.

An artificial star will work. But, it will need to be dark enough to see it. I have found the Hotech laser collimator too finicky to work well.
Same with the Kendricks SCT collimater (nolonger made).

Lynn
Michael Gorman avatar
Michael Gorman avatar
Lynn K:
A dot placed on anything that close will not come to focus or near focus. ARE, you would already have a big one called your secondary.
 My answer is no.  Dots on elements cause donuts, not diffraction rings.

An artificial star will work. But, it will need to be dark enough to see it. I have found the Hotech laser collimator too finicky to work well.
Same with the Kendricks SCT collimater (nolonger made).

Lynn

Not sure the focus matters since it should be out of focus, but thinking about it more I think I see what you're saying. The mote likely wouldn't adjust because it's not on/in-front of the secondary, its more of an out of focus shadow than a representation of the light path. 

I agree related to many of the other tools the hotech helped to get my hyperstar close but it still wasn't great. It was expensive, and was still a big effort to get everything setup right. I regret buying it for sure. 

The artificial star is also difficult to make work well.  Center placement is difficult requiring large distance away, and as you mentioned still need darkness.
Ashraf AbuSara avatar
Ashraf AbuSara avatar
The dust mote is not going to be appropriate representation of the light path for a real star image. If you are trying to do daytime collimation by centring the central obstruction, your best bet is probably getting an Ocal collimator. Its not as good for SCTs as it is for Newtonians, but it will get you close. Frankly at f/10 of f/7 you should be fine getting close. But for high resolution planetary imaging nothing will substitute for real star collimation. I typically use Metaguide for that.
Helpful Concise
Lynn K avatar
Lynn K avatar
Yes Michael,  the diffraction ring pattern of an out of focus star is out of focus. But, not that much. Anything as close as you are recommending  will be so out of focus you want even be able to see it. Beside the secondary mirror will block it.

A artificial star need to ba about 50 ft. away from the scope. And a few minuets after sunset will do.

Lynn K.
Lynn K avatar
Lynn K avatar
Michael,  the diffraction rings we use to collimate scopes are cause by a point light source.  Most of the light goes into the Airy disk, but some is diffracted into rings. Any type of obstruction, adot or otherwise, in front of the optics will cause diffraction rings. 

Lynn K.
Oscar avatar
Oscar avatar
Michael Gorman:
could we just use a filter with an appropriately sized dot placed in the middle during the day or with a flat panel to create a nice big out of focus donut to allow for much easier collimation?

How is this possible?

This is the weirdest coincidence ever

2-3 weeks ago I was thinking about this same exact idea

I'm not lying

I get your logic too; it was EXACTLY what I was thinking

if a dust particle can reveal if you're off-collimated, then I think it's possible to try the filter method.

It doesn't hurt to try a prototype, with a clear-glass filter and a sharpie mark

but I still wonder how you got the idea when I never mentioned about it online

but maybe, as others said, it wont really work, idk
Michael Gorman avatar
Michael Gorman avatar
Michael Gorman:
could we just use a filter with an appropriately sized dot placed in the middle during the day or with a flat panel to create a nice big out of focus donut to allow for much easier collimation?

How is this possible?

This is the weirdest coincidence ever

2-3 weeks ago I was thinking about this same exact idea

I'm not lying

I get your logic too; it was EXACTLY what I was thinking

if a dust particle can reveal if you're off-collimated, then I think it's possible to try the filter method.

It doesn't hurt to try a prototype, with a clear-glass filter and a sharpie mark

but I still wonder how you got the idea when I never mentioned about it online

but maybe, as others said, it wont really work, idk

(Not so) Great minds think alike? It def doesn't seem as though it would work logically as Lynn has pointed out. I wasn't fully thinking it through when it kind of came to me. It would be nice if there were an easier way outside data gathering hours than expensive lasers that don't really work very accurately or artificial stars that require you to precisely aim at them from 50feet away in the dark and somehow keep them centered while making the minute adjustments. My desire just led to me to jumping to a conclusion.
John Hayes avatar
John Hayes avatar
I must be dense.  How does looking at an out of focus dot on a filter help to align the telescope?  What's the method and how does it allow aligning the secondary to within the required tolerance?

John
Well Written Engaging