Barlowed laser collimation

[Pixies]

Hi,

I have a collimation cap and Cheshire and have no problem collimating with them. Star tests are ok too. However, with a big dobsonian it's impossible to adjust the primary while looking down the Cheshire. So I bought a laser collimator - a cheap Starguider (<£20 new). It is adjustable and I've collimated it. I've been comparing the results using the laser with a Cheshire, and it's perfect.

However, I thought I'd try the barlowed laser method, so bought a cheap 2nd-hand SW barlow (no expense spared with me!). However, I'm having a problem with it. When I stick the laser in the barlow, I get a reflection thrown back at the collimator surface that's so bright, it's stopping me see the shadow of the doughnut on the primary mirror. Even when out of the focuser, you can see it (see pic).

Is it just the cheap barlow and non-coated lens surface? If I unscrew the barlow lens, turn it over and hold it back up to the end of the barlow tube, I don't get the reflection.

 

[Ricochet]

What if you turn the brightness down on the laser? Does the reflection ever drop to a level where you can see the reflected beam instead?

If that doesn't help then another barlow is probably the way to go. The barlow you've got looks like the one sold by FLO under their Astro essentials brand. I've got one of that type and I don't recall it having that issue when I tried it.

[Pixies]

5 hours ago, Ricochet said:

What if you turn the brightness down on the laser? Does the reflection ever drop to a level where you can see the reflected beam instead?

If that doesn't help then another barlow is probably the way to go. The barlow you've got looks like the one sold by FLO under their Astro essentials brand. I've got one of that type and I don't recall it having that issue when I tried it.

Unfortunately, if I turn the brightness down, the reflection of the doughnut falls too. I'll probably need another Barlow

[Stardaze]

Have you noticed anything different between the Cheshire and lasered collimation? 

[John]

When I've used the laser collimation method, the shadow of the "donut" has been a bit indistinct. I found reducing the lighting in the room helped to see it more clearly. My laser collimator does not have an adjustable brightness - it's either off or on !

My method, when I use it, is similar to this one:

http://www.smartavtweaks.com/RVBL.html

Some barlows work better than others and some not at all. Cheap ones seem to do quite well !

 

[Pixies]

After I collimated the laser collimator, they agreed perfectly! The reason I fancied having a shot with the laser, was to be able to watch my progress while I was actually turning the primary adjusters (my arms aren't that long!). It was interesting seeing the effect of the locking screws - more like a fine tuning.

I'll use the Cheshire as the main method and the laser for a quick check in the dark - as long as it proved to remain reliable against the Cheshire.

The laser collimator cost <£20 from Sky's The Limit. The collimating jig cost a little less (see pics)

[John]

Those simple laser collimation rigs work well. If the beam stays on the same spot at about 10 metres for a full 360 degree rotation of the collimator you are good to go !

My understanding is that the main advantege of the laser method is that it is less prone to errors introduced by sloppy eyepiece fittings etc.

[Stardaze]

34 minutes ago, John said:

Those simple laser collimation rigs work well. If the beam stays on the same spot at about 10 metres for a full 360 degree rotation of the collimator you are good to go !

My understanding is that the main advantege of the laser method is that it is less prone to errors introduced by sloppy eyepiece fittings etc.

I'm hoping the click lock system when I do buy one at the end of the month will take the sloppy fitting out of the equation too.

[Pixies]

7 minutes ago, Stardaze said:

I'm hoping the click lock system when I do buy one at the end of the month will take the sloppy fitting out of the equation too.

What did you decide to go with at the end? There were questions about needing an extension piece.

[John]

1 hour ago, Stardaze said:

I'm hoping the click lock system when I do buy one at the end of the month will take the sloppy fitting out of the equation too.

Are you going for click lock on all the connections ?

 

[Louis D]

Back when I was regularly using my 15" truss Dob, I would use the laser to do rough alignment by getting the return laser beam to coincide as closely as possible with the outgoing laser beam on the secondary mirror (which is plainly visible from the back of the scope) while at the back of the scope.  I would then use the Rigel Aline to tweak the collimation incrementally going back forth between the focuser and rear of the scope.  This method obviously won't work for a solid tube telescope, though.

Edited July 3, 2020 by Louis D

[Stardaze]

3 hours ago, Pixies said:

What did you decide to go with at the end? There were questions about needing an extension piece.

I need a 20mm extender too which is £43 on it's own, so adds up. I measured the focal point of all my EP's. Both of my 2" EP's focus at 32 and 34" on the focuser currently, (tops out around 10mm more) whereas the 1.25" were around 18-21". The click lock gives 43.5mm and so add the 20mm extender = 63.5mm. The measured distance currently from the extender to the top of the 2" EP holder is 58mm. 

It's £140 outlay, which will have to wait till next month.

2 hours ago, John said:

Are you going for click lock on all the connections ?

 

Yup