How do I collimate my 152mm (Achromat) refractor with the aid of a laser

[Leo.G]

I want to check the collimation of my old Skywatcher 152 Achro. I've removed the lens cell several times to clean it in the past and think it probably wouldn't be a bad idea to check it. The lens cell was marked before removal (cheap nail polish is great for the purpose) and always put back in it's original screw holes.
There are 3 adjustment grub screws on the cell.



I have a green laser and I'm intending to machine an adapter on the lathe to fit into a 2" imaging adapter I had made so I can point it directly at the lens from the rear of the 1200mm scope but how would I know if it was out of alignment?
I'll be putting the laser in the chuck of the lathe to check for alignment of it before I do anything.

Is it as simple as if the laser dot/beam is centred the scope is fine because with 3 adjustment screws on the lens cell I'm thinking it could still be central but the lens cell not square.
Does the laser deviate at an angle if the lens is not square?
I know, that's mirrors but I DO NOT have a clue.

Now I think I like reflectors despite always being fond of refractors.
Collimation is easy on my 8" Newt.

[fsphotography]

Get yourself a Cheshire eyepiece Leo, very cheap and easy to use.

Frank.

[Leo.G]

Thanks Frank, I have one but I don't know how it helps/how to use it with a refractor.

[doug mc]

Cut a piece of grease proof paper that fits neatly over the outside of your objective. Draw with a compas to get neat circle. Mark a centre dot where the compas point was. Place this paper over the objective lens. Remove diagonal and place a laser collimator where the diagonal was so that the beam point lands on the paper mask. Loosen the focuser screws that hold the focuser on the tube and manipulate the focuser till the laser spot lands directly on the centre spot of the paper mask. Tighten the screws to lock your focuser in place.Your focuser is now aligned with your objective lens. Seeing that your objective lens cell has collimation screws, star test out of focus star image and adjust to symmetrical pattern achieved.

[doug mc]

There is another method using a torch and a Cheshire collimator.

[fsphotography]

That is the method i use Doug, once you work out which screw to tweek
its easy. Its also a quick and easy way of conveniently checking collimation,
couple of minutes and your done.

Frank.

[big-blue]

Further to Doug's suggestions,after the focus tube is aligned with the centre spot, I would be looking for faint laser reflection from the lens centre, that should come back into the laser itself. (much fainter than a mirror reflection : after all, you are looking at an anti-reflection coating ;-) Maybe it is perfect : and the objective assembly is already perfectly square : in which case tilt the laser ever so slightly to find the reflection.
Some refractors have push-pull screws around the objective to enable better alignment if it is off.
Of course the laser itself must be perfectly collimated. A lathe chuck could be used as suggested, though I prefer to slowly rotate the laser in a v-block and point it at a target some meters away, to check and adjust until there is minimal movement of the laser spot.
Good luck with it,
regards
Gerhard

[Leo.G]

Thank you kindly gentlemen!


I'm not sure on the focuser screws to align it, I'll have to check that part, I have a Williams Optics DDG Synta focuser on the big refractor.


Sorry, not the best included image and I'm a little busy at the moment to go take another image.

[rbronca]

I am shocked that most of the above didn't actually read the OP's post. The answers are at best wrong. It isn't a Newtonian reflector, but is a 152mm refractor.
From what I have read, for more than two lens surfaces it is impossible to collimate a refractor without a very expensive optical test bench.
OP, I strongly recommend not touching this unless you need an excuse to buy a new telescope... :-)

[Saturnine]

Hello Leo, hope you are well. There seems to be a few differing opinions on collimating your refractor and apart from using a laser to align the focuser using the method described by doug mc, you don't need the laser to collimate the lens cell. I am assuming that the 2 lens elements are a snug fit in the cell and the spacers between the elements are all the same thickness and are equally spaced around the lens circumference and the retaining ring is allowing just a little movement. If you gently shake the cell you should be able to hear a slight movement of the lens in the cell, meaning the ring is not overtightened and pinching the lens'.
All you need now is an cheshire eyepiece inserted into the focuser drawtube , with a light shining on the reflecting surface of the cheshire, leaving the dew cap on the dew shield, look through the cheshire and you should see a series of faint concentric reflections, all being well. If the reflections aren't concentric, gently adjust the 3 sets of collimation screws around the outside of the lens cell until they are concentric again. I own several refractors, all the achros are home made and have used the described method to collimate them after having removed the lens for cleaning, the pollution around here tends to leave an oily film on the outside lens surface and needs to be cleaned ocassionally. Hope this helps, it's not too different from collimating newts, just have to get all the reflections as concentric as possible.

Jeff

[doug mc]

As Jeff recommends. That method is the one I used also.

[Leo.G]

Thank you yet again!


rbronca I've seen a lot written on extremely complex means of doing this and that's why I asked here, I didn't understand most of it. I have several refractors and several cats and no matter how I store my telescopes and I vacuum daily cat hairs find their way into the optics. For purely visual an occasional cat hair in the optics is fine, if I'm imaging they are not and I take great care with my Nikon optics, particularly the sensor when changing lenses.
They are all Achromats so nothing hugely expensive and I'd be less prepared to strip the lens out of something worth a lot of money but they are what I can afford I'd just like to do a quick check particularly on the `152 mm unit.


In regards to the mention of using my laser in a V block to check the accuracy, I have a couple I have no idea where I've put them.
I tried the lathe, the 3 jaw chuck has minimal run out but too much to give me accuracy to check whether the lasers are square. One laser does have two collimation adjustments I haven't played with.
I have a 4 jaw chuck if I could be bothered fitting it, the thing is heavy and last time it dropped on my fingers I remembered for quite a while. The 4 jaw and dial gauge will give me a high level of accuracy over a few feet (the length of the bed) but yes, a V block would be better if I can find the things and use a distant target. I can't find my aluminium 7075 40mm rod to machine an adapter anyway but I've contacted a company in Minto a local engineer buys his alloy from to see if I can get a few shorter pieces cheap enough for what I have to machine up. I have steel but I prefer the alloy weight factor playing with telescopes.

[Joshua Bunn]

Leo, the lathe will be the best tool for collimation, bore out a price of Ali, then without removing it from the 3 jaw chuck, slip in your laser, job done, perfectly concentric with the spindle axis.

[Leo.G]

Thanks Joshua!
I have a length of 40mm 7075 somewhere, I just don't know where. A lathe, milling machine (still in it's box till I weld up a stand, 12 tonne work press, MIG, TIG and arc welding equipment (plus gas cylinders and consumables, grinding equipment and a whole host of other tools including a good cross slide table for the milling machine and vice doesn't leave a great deal of room in a 10x10 garden shed and I misplace things easily in there.
As mentioned I can set it up in a 200mm 4 jaw chuck with the dial gauge for acciracy.


Swing Over Bed 300 (mm)
Distance Between Centres700 (mm)
Spindle Bore 38

Swing over cross slide:190mm

2HP motor

This is the milling machine:


MODEL BF-20AV Digital Readout Optional Type Vertical Column Type Dovetail Spindle Taper - Vertical 3MT (mm)Vertical Travel (Z-Axis) 280 Type of Slides Dovetail (mm)Spindle Travel / Diameter 50 / 60 (mm)Throat Depth (max.) 185 Tilting Head (Left ~ Right) ±90º Tilting Head (Forward ~ Back) Fixed (mm)Drilling Capacity 16 (mm)End Milling Capacity 20 (mm)Face Milling Capacity 63 (No./ rpm)Spindle Speeds Vertical Variable (90 - 3000) (kW / hp)Motor Power - Vertical Spindle 0.85 / 1.13 (V / amp)Voltage / Amperage 240 / 10 (kg)Nett Weight 103

[Sunfish]

Build one of these GMK collimators yourself if you have that lathe equipment and an old three or four led torch. If the result shows almost perfect , leave it alone.

http://www.fernrohr-service.de/4.html?fbclid=IwAR18mAKTzmOMn0TveyY mmFP_K2Ba09J7ZSzuq4zI_NtgJuIMlmAqtK t2EjM

[Sunfish]

https://www.iceinspace.com.au/forum/showthread.php?t=184999

[Leo.G]

Thank you very much Ray, after I get an answer about some alloy supplies I'm hoping to get a price on I'll have the materials.
Alloy is cheaper in long lengths I can't afford.