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Good way to collimate without using a star?


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Just wondering as I try to get my head around collimating my SCT (or at least checking it) - seems like taking the scope outside and dealing with all of the extra variables such as the atmosphere, tracking, dew, etc, it would be preferable to keep it indoors or at least do it outdoors during the day.

 

Would a stationary light source a fair distance away be a suitable surrogate for a star?  Say an LED from a battery charger or such, sitting on a fence and defocused?

 

Plenty to read here about the laser collimators - I guess that would be one thing to try, though that seems to have enough of its own variables such as is the collimator collimated?  Slop in the eyepiece holder, etc.

 

Interested in everyone's thoughts.  I am learning to use the tracking, but I don't trust it enough at this point to make adjustments to the optics.

 

thanks!

 

 

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  • 2 weeks later...

I tend to agree with Carbon Brush on this matter. I'd check out the scope history first, particularly if it was bought used. SCT's can sometimes need collimating but it isn't an easy job. Not unlike Maksutov Cassegrains, collimation isn't the same issue as with Newtonians. In my experience SCT's don't need regular collimation. It may not be a collimation issue at all.

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@Carbon Brush - your approach is a good one:  "what problem are you trying to solve?", and in this case I am not convinced that I have a problem.  In terms of scope history, I am Ground Zero - so I can choose to assume that the factory setup and shipment were all perfect, or I can check it.

 

I've been reading up on astrophotography, and most articles seem to start with a strong recommendation to check the collimation.  I don't know that I have a problem, but I will say that I have had difficulty getting stars to a nice pinpoint focus like I'd expect.  I've left the scope out for several hours beforehand to come up to temperature, the seeing has been 4/5 on the nights i've used it, so most likely its a case of "it can't hurt to check".  I don't want to fight other issues down the line if I'm unsure of a firm foundation.

 

The more I've read, the less I'm inclined to use the artificial star and the more I think I'll give it a go on Polaris.    Cheers guys!

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FWIW some Maksutov Cassegrains can show a bright first diffraction ring. This isn't a collimation issue necessarily. SCT's can sometimes show this. 

 

CxzChGc.jpg

 

All of my Synta-made Mak's show a bright first diffraction ring. It tends to be the nature of 'Gregory Spot' types. Some Mak's have alternative secondaries. Although they tend to be a tad expensive.

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How does the scope perform visually? It might give a clue to do somethingl ike I tried.

 

I have an Intes MN78 (Mak Cass). Known for excellent optics.
One good viewing night night I just kept pushing magnification.
For 'ordinary' viewing it was a good night. Planetary, splitting doubles, etc.
I was trying to establish the scope limit.
Eventually reaching a point where the star was generally a point - but not always, and not always in the same place.

 

In less than a second, it would go from point to blob and back. A problem of unstable air.
It would also move a little in the eyepiece. Which could argued as a mount problem, or unstable air.
Obviously you need an eyeball, not a camera (unless it has a fast shutter) to see this effect.

 

The important thing is that the test told me the scope was performing optically.
Can you try something similar?

If (on a good night) you can't shrink a blob to a point, or if you get comet shpes, etc. you can look for causes.
Unless you have paid big big money for eyepieces, use only the centre part of the field for this test.

 

HTH, David.

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Hi David -- in general, it does seem like it is hard to get the brighter stars down to a point though I don't recall any weird shapes.  I'll have to wait a little while til our skies clear up and frost is out of the picture and I'll check it out paying particular attention to that.   I think my next accessory will have to be a heater - the dew shield works nicely up to a point.

 

For this test, are you suggesting highest magnification?   My eye pieces are the plössls that came in the celestron eyepiece kit, so nothing fancy.

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Hi Adam. It is difficult to give absolutes.

 

On the basis of aperture, your scope should produce decent results up to magnification 400. Beyond that things go a bit fuzzy or lose contrast. Of course on many nights the sky won't take this sort of magnification.

 

Are you able to borrow any 'better' eyepieces for comparison. For example a friend had a Celestron newt that he thought was fine with the standard kit eyepieces. I lent him a Baader Hyperion and he discovered contrasty views.
 

If you want to do some daylight trials, you can. I am fortunate to have a horizon at 2KM with a brick chimney, electricity pylon and associated cables. All good for assessing scope performance.

 

Can I resolve bricks and certain other small parts of the chimney? That is indicating resolution and contrast.

Do I see colour fringing on the pylon structure? Chromatic aberration, either refractor objective, or any eyepiece.

Do I see a sharp focus cable all the way across the field of view. Eyepiece issues, etc.

 

With another scope, I did daylight resolution trials in poor weather. Scope in the garage looking at a steel rule about 25M up the drive. A Rabone Chesterman rule with graduations to 1/100". Resolvable!
So how far away would your (3mm dia?) LED star need to be? Commercial artificial stars have a much smaller emitter.
By doing a short distance test, air stability and transparency could be ignored.

 

Another useful comparison is to try different items in the optical train.
I once tried a Skywatcher Startravel and William Optics refractor side by side.
Yes a big price difference. The views were foggy day to clear day.
Though until I looked thorugh the expensive scope, I thought the Skywatcher was OK.😁
Swapping eyepieces and diagonals one at a time, then comparing performance, demonstrated that the objective, diagonal and eyepiece all had their part to play.

 

It is only by trying different combinations of conditions and optical components that you can get a measure of the scope performance and possibly identify significant weak links.

 

HTH, David.

 

 

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