Adjustments to TAL 100 Objective Cell

[John]

Can anyone shed any light on the adjustments that are possible on the objective cell that the TAL 100 refractor objective lens is housed in please ?

The image below was posted by AndyH in a thread on this topic sometime back. The red arrowed screws (marked by Andy) hold the lens cell onto the tube. It's the function of the 3 grub screws marked with a blue arrow and whatever is hidden under the paste / putty marked by the green arrow that I'm interested in.

I've played around the the ones marked with the blue arrow and the tilt of the objective seemed to be altered but I'm not really sure what they are doing exactly. The function of those hidden by the paste / putty by the  green arrow are currently a mystery.

Has anyone explored these further ?

Thanks.

[nightfisher]

John, the screws marked with blue press against the edge of the outer lens maybe allowing a little sideways movement, the putty is near impossible to remove, but may come out with a little determination but i would not like to say what they do, i would still say leave well alone

[Stu]

Hi John,

I have no direct experience of Tals, but have collimated a Televue Genesis before.

From reading another thread on the Tals, and looking at the location of the various screws, I would think the ones towards the front of the objective cell hold the two elements in place. I would suggest leaving these alone for now.

With the Televue, you basically loosen the three screws holding the whole cell onto the tube, and gently adjust it until you reach a point where the scope is collimated. On the Tal these look like the red screws.

One problem I had was that I got the cell in position and when I tightened it it would just pop back into its 'out of collimation' position. I suspect that there were dents formed in the ota which made small adjustments tricky. I ended up making a jig using guidescope rings to hold things in place whilst I tightened the screws up and this worked.

Does that make any sense?

EDIT This CN post has detail on the TV method, may give you some ideas

http://www.cloudynights.com/topic/292216-collimating-my-tv-np-101/

[Roy Challen]

Hi All, I agree with the labelling in the picture - this is exactly how my RS looks. My R though, was completely different. The entire cell had a thread that screwed onto the tube, with a tiny grub screw to lock the position, three more grub screws (marked by blue arrow in the pic). There were no other screws or putty or any other method of adjustment. The serial number for this scope was 0018, built in July 99.

I'll have a little dig around under the putty, see what's what and report my findings.

[philj]

They allow alignment of the objective elements, e.g. centreing, they are not collimation screws. There is no true collimation adjustment on a TAL cell, Tal rely on the collimateable diagonal for collimation which isnt the best method.

The red screws hold the cell onto the tube. If you want to collimate the objective you will have to loosen these screws, wobble the cell till you get collimation, then tighten and hope the collimation stays on. If the collimation shifts during tightening then you will have to drill and tap new holes.

Just another reason why I dont think Tals are the best scopes around, sorry but when they are good they are good but when they are bad they can be a pain to put right due to their basic construction.

[Peter Drew]

I've had hands on experience with collimating Tal 100's. As correctly mentioned by Philj there is no provision for collimating the cell and the forward cell screws perform lateral adjustment for the lens components. The lens design is very sensitive to lateral position, a small amount of misalignment causes severe coma which can easily be mistaken for collimation error of the cell. Very small adjustments of the screws are needed and rechecking once nipped up is essential. I suspect the "putty" is there to prevent the screws from coming loose. The lens adjustment procedure is difficult to do in the dark, I performed this by looking out from a lit room towards a fibre optic artificial star at a suitable distance outside.  

[John]

Thanks very much for all the feedback on this issue folks - much appreciated

I've got the opical axis of the focuser on the scope accurately aligned with the optical axis of the objective now. The diagonal is in pretty good collimation having tested it on my other refractors and a (collimated) laser put in the diagonal drawtube exits the objective pretty much spot on the centre.

Last night I played around with the small grub screws marked with the blue arrow on the photo above. The results were pretty much as Peter describes above though I was using a star test to check each small adjustment. What I took to be mis-collimation due to objective tilt was clearly coma due to a de-centered objective and it's now looking much better

Playing around with the relatively simple TAL 100 doubet and cell makes me wonder how folks would cope with the 6 element ( in 3 groups ) optics of the TAL Apolar 125 if they needed adjustment

I'm enjoying the tinkering but I have to say I'm glad not to have paid much money for the scope.

[spaceboy]

Sorry I can't offer any advice on how to collimate the 100R but I'm sure a man of your experience will get it sorted and when you do it will be another feather in your cap..

This is why I have always held off on buying a 100RS despite the praise of good optics. I sold my 200K for the concerns I had that one day it might get knocked and I would struggle to collimate it. I went to an event put on by Birmingham astronomy club at Clent hills once and a guy had a TAL1 Newtonian that was clearly out of collimation but neither of us could figure out how to collimate it as there was no visible screws at the rear of the mirror. In fact you couldn't even see the back of the mirror it was enclosed.

TAL make good scopes built like tanks but they really are not collimation friendly scopes and this is why I have always stuck with blue tube Evostars or Bresser messier refractors as they can be easily collimated. May not be as good optically as a Tal but I never have to worry about collimation being out which my guess is mediocre optics that are well collimated are better than good optics with bad collimation ??

[John]

Sorry I can't offer any advice on how to collimate the 100R but I'm sure a man of your experience will get it sorted and when you do it will be another feather in your cap..

This is why I have always held off on buying a 100RS despite the praise of good optics. I sold my 200K for the concerns I had that one day it might get knocked and I would struggle to collimate it. I went to an event put on by Birmingham astronomy club at Clent hills once and a guy had a TAL1 Newtonian that was clearly out of collimation but neither of us could figure out how to collimate it as there was no visible screws at the rear of the mirror. In fact you couldn't even see the back of the mirror it was enclosed.

TAL make good scopes built like tanks but they really are not collimation friendly scopes and this is why I have always stuck with blue tube Evostars or Bresser messier refractors as they can be easily collimated. May not be as good optically as a Tal but I never have to worry about collimation being out which my guess is mediocre optics that are well collimated are better than good optics with bad collimation ??

I get a bit OCD about refractor collimation

I can tell that the TAL has a nicely figured objective as it shows a really nicely defined airey disk and diffraction rings which look very similar inside and outside of sharp focus. The optics seem to handle high power well too. Up to now the frustration has been the slightly displaced diffraction rings, due, as I now realise, to coma. Despite this the scope was showing nice detail on Jupiter and Saturn, Venus without excessive fringing and has spilt double stars to 1.5 arc seconds separation.

TAL's have often been described as "quirky" and it's a very appropriate description

I've often fancied a TAL 200K but I don't think I'm brave enough to face the prospect of collimating it !

[nightfisher]

I would hazard a guess that the collimation is out on my 100rs but it can stay that way as it still gives superb views

[spaceboy]

I would hazard a guess that the collimation is out on my 100rs but it can stay that way as it still gives superb views

Do you not in the back of your mind think, if the Tal gives me such superb views with the collimation slightly out, what would be the views like if everything was perfect ??

[sloz1664]

I get a bit OCD about refractor collimation

I can tell that the TAL has a nicely figured objective as it shows a really nicely defined airey disk and diffraction rings which look very similar inside and outside of sharp focus. The optics seem to handle high power well too. Up to now the frustration has been the slightly displaced diffraction rings, due, as I now realise, to coma. Despite this the scope was showing nice detail on Jupiter and Saturn, Venus without excessive fringing and has spilt double stars to 1.5 arc seconds separation.

TAL's have often been described as "quirky" and it's a very appropriate description

I've often fancied a TAL 200K but I don't think I'm brave enough to face the prospect of collimating it !

Hi John,

Had my Tal 200K for four years. Built like a tank, never needed to collimate it.

Steve

[John]

Hi John,

Had my Tal 200K for four years. Built like a tank, never needed to collimate it.

Steve

Thats great Steve. Having read up on the procedure for 200K's I hope it stays that way for you

[John]

Just a quick update on my TAL objective lens.

As I had some time to spare today I decided to have a closer look at the objective lens of my old TAL 100 and the cell it's mounted in, to see if I could sort out the mild coma that I've been experiencing.

The foil lens spacers that TAL use are very small compared with other air spaced doublets I've looked at. On close examination I could see that one spacer was actually missing leaving just the tiny mark on the edge of the lens where it had been. So I decided to remove the objective from the lens cell, split the 2 elements and replace the missing spacer. Removing the front lens retaining ring and releasing the 3 uppper lens element retaining screws around the cell (the ones marked with the blue arrow in the photo in my 1st post in this thread) I was then, with a little effort, able to slide the lens cell off the objective lens.

The objective edges were blackened (both elements) and were marked as having been made in 2001 which tallies with the date in the manual. The spacers though very small were made from quite thick metal foil - maybe .25mm ?. The closest match I could find was a (cleaned !) foil pie dish so I cut a tiny replacement for the missing one from that. I did check the inside edges of the lens cell first to see if the original spacer had dropped out and was lodged somewhere but drew a blank. It may have been missing from new - the coma was not at all obvious unless star testing and I'm not sure how much previous owners of the scope actually used it.

I've now replaced the lens elements in the cell, gently tightened the 3 upper element retaining screws and replaced the lens retaining ring. The objective and the cell are now back on the scope and the collimation looks good with a cheshire eyepiece. I'll be interested to star test it at the first opportunity.

Incidently the elongated holes marked with the green arrow in my original photo don't house any screws or other adjusters at all. The putty that is in them seems to be used to hold the top lens element in place in a central position. Star testing will tell if I've managed to get it central again now. Once I'm happy with it I can put some blu-tak in the holes to keep the dust out.

A fun thing to do on a quiet Saturday although it's a bit fiddly !

[AndyH]

Hi John.

I asked one of the designers, a while back, the spacing on the 100rs lens. 0.3mm was the answer i got, so sounds like you're there with the pie container ☺

Andy.

ps: was the pie nice?

[John]

Hi John.

I asked one of the designers, a while back, the spacing on the 100rs lens. 0.3mm was the answer i got, so sounds like you're there with the pie container ☺

Andy.

ps: was the pie nice?

Thanks Andy - I wasn't far out with my estimate. They were thicker than I was expecting. The ones on my Vixen 102 are about 5x larger than the ones on the Tal. I've read that keen imagers reduce the size of spacers in some scopes to stop them interfering with the image.

I can't comment on the pie - my son got to it before I did !

I just got the foil dish ......

[russ.will]

I get a bit OCD about refractor collimation

Having experienced the before/after results of a professional collimation of both a Achro doublet and an Apo triplet, I would not say you are being paranoid.

I'd say refractor collimation was mandatory, but I would also include the diagonal as part of that requirement. In either case, that which comes out of the box, may be close at best, but the magic is in the optimisation of the complete system, accounting for simple things like compression rings not quite gripping orthogonally.

Russell

[nightfisher]

John,

You are a brave man, when i stripped my 100rs objective, i bottled out from removing the rear lens. But i salute you as you have solved the mystery of the putty