What your laser collimator can do while you aren't looking

[GeoffW1]

Hi,

I recently bought a 300mm Bintel Dob from Zuts, and am very pleased with it (many thanks to a friendly colleague). I thought that with such a monster OTA the inverse law must apply, and so I should put a bit extra effort into collimation.

Accordingly I got out my laser collimator (Bintel model) and went ahead. I like Barlowed collimation, and used that method. Everything was smooth, a star test was good, and I was able to enjoy the increased aperture.

Tonight I was keen for a repeat, and put the scope outside early. In between BBQing, I thought I would collimate again to get a feel for the behemoth. What ?? Miles out!! The laser spot was on the edge of the primary mirror!! It can't vary that much in one day surely.

So I took the laser collimator into the garage to find out what was going on. I have a little wooden jig (previously shown here) to check the laser alignment and on first rotation it was enormously out, all over the garage door. Had something gone loose? No, it had not. What then?

I then found something we need to know about these devices. You can align these laser collimators all you like, but if you then give the laser on/off screw an extra turn or 2, it can ruin everything! I don't claim they are all like this, but mine is.

Now colleagues, this might not be news to you, but it was to me. Turn it on too much, and you are, um, politely put, not where you thought. Argh.

Thus enlightened (was that a pun?) I think the only repeatable, reliable thing to do if you have a laser collimator, is to turn it on just a smidgen, no more than just enough. How many times was I kidding myself I wonder?

I also found another interesting wrinkle on barlowed collimation which I will submit as an article, because it needs a few photos.

Cheers

GeoffW1

[bkm2304]

If turning too much on the on screw can throw your laser out, how do you bring it back to true - or close to it?

[GeoffW1]

Hi,

Since posting on this I have run many many trials, and have found the following to be true of my Bintel laser collimator:

- if you use the same amount of turn-on each time, it is quite repeatable over a number of trials, that is, it will display the same alignment each time if you take care to not scew in the on/off screw too much

- it must be realigned after replacing batteries (3 x LR44, and not really a surprise when you think about it)

- the 3 alignment grub screws are a 1.5 mm hex socket, and are quite sensitive, so that alignment proceeds by 1/4 turns or less

- the on/off screw has an effect like a 4th alignment screw, since it too pushes on the internal laser cartridge (which is just an plain ordinary key-ring sort of laser emitter)

- the best thing is to turn it on so that it is just on, maybe 1/8 turn, no more, then align it with the 3 grub screws with whatever v-block support you like

- every so often during this process, turn it off, then on again, by the same small amount on the on/off screw

- once aligned, as I mentioned above, it will be quite stable and repeatable if turned on by the same 1/8 turn from contact, each time

********

My main concern was to find out if the design of the gizmo was unstable and variable in repetition, and apparently it is not, so goodoh.

If all this sounds somewhat paranoid and finicky, the amount of misalignment that I saw appear, with one extra turn of the on-off screw, was quite startling.

Cheers

GeoffW1 (or Geoff #7 or something)

[CoombellKid]

Man that must be a real pain in the bum!!! mine a good old no name varity
has never gone out of alignment by turning it off and on. Although I dont
use it anymore and prefer my Chesire <-typo> thingy eyepiece these days

regards,CS

[bkm2304]

Thanks for the info. One more question. Exactly HOW do I align it if it is out? And how do know when it is aligned? (Actually that's 2 questions isn't it!).

[Rodstar]

You took the words from my mouth. It is all very well having consistency in the alignment of the laser collimator from time to time, but if it is out to begin with, the scope will consistently be out of alignment.

This all comes back to some of the comments by Don Pensack in his detailed article on collimation on the articles section of this forum, which I must admit I am yet to read from beginning to end.

[erick]

My Bintel laser collimator doesn't show this problem. It was slightly out of collimation, but tweaked up easily - when 1) I finally found the right hex key at the bottom of one of my toolboxes, and 2) managed to get the batteries out and replace with fresh set. The last battery was jammed in place - I had to drill a hole in it, insert a screw and pull. :scared:.

I've paid no particular attention to how far I've screwed the on/off screw in, but on checking the laser's collimation recently, it was still spot on. My 8" has been a dream to collimate with the laser, particularly after I pulled out those Phillips head tilt screws on the secondary and replaced them with "Eric's Secondary Tilt Knobs"! Rough prototype at present - I'll polish them up soon - get the edges out of the light path :doh:.

The problem I have is that the laser throws a "sausage" image through the Barlow, so won't cover the primary centre spot well enough to use the "barlowed laser" technique. It's not the Barlow lens - I can shine my green laser through it and get a nice big diffuse circle of light. I'm stumped - maybe need to fit a different cheapie red laser - after I check that it shows a good diffuse circle through the Barlow. :shrug:

[GeoffW1]

Hi,

There are 3 socket head alignment grubscrews around the barrel (on my Bintel unit they are 1.5 mm hex) and you tweak these with an allen key. They are quite sensitive on mine.

I use a wooden jig (illustrated) and shine the laser on the garage door 5 m away. Rotate the laser in something like this, and if it is misaligned you will see the laser dot wander round in a circle.

Now if we say that the best practical visual alignment for the laser spot is within a 2mm circle over a 5m baseline (the laser dot itself is 1-2mm) this is a best alignment of + or - 0.0002 rad, or 41 arcsec. Over say a 1500 mm focal length this is a circle 0.6 mm in dia.

Now N.O. Carlin mentions that the diameter of the "sweet spot" around the focal point in a f/4.5 scope is about 2mm, and for best results aim for 1/2 that, so, with a base length of 5m we are in the ballpark.

GeoffW1