Reflector Collimation - How close is close enough?

[erick (Eric)]

Now I own an 8" Bintel reflector and have started attempts at collimating, I'm reading all that I can find.

I'm interested in how detailed an approach to this can be, eg:-

http://www.iceinspace.com.au/forum/s...hlight=catseye

and I've pulled, off the "catseye" site, a major article on collimation:-

http://www.catseyecollimation.com/mccluney.html

and I'll read this soon.

But, my first question is "How close is close enough"? I know the answer will be - "It depends". OK, so currently I'm interested in visual rather than photography (with the reflector, anyway). I'm at the very beginning with a scope and my targets won't be super ambitious.

Proposal - set up the scope in the observing location, cooling fan on primary for about an hour. Check that the secondary mirror seems well centered under the focuser (shouldn't change with normal handling) using my "sight tube" - 35mm film cannister with 1/16" hole in bottom); drop in the laser, don't do up the eyepiece lock screw, check laser is sitting flush against the 2"-->1.25" adapter (2" focuser) and adjust the secondary so that the dot falls in the centre of the primary (I have a donut centre "spot" with a generous centre hole that the laser fits inside). Adjust the primary mirror until the laser returns through the hole in the collimator. Is that good enough? What is the relationship between the mm the laser dot is off centre of the primary or the centre of eyepiece, and drop-off in viewing quality - rapid, slow??

My second question is about the mechanics. The mechanical design of the primary adjustment, I think I understand. It seems to me that I can adjust the three collimation screws independently - ie. I don't have to screw the other two out to compensate for having screwed one in (unless I was really worried about my focal length to the nearest mm or fraction thereof?? On thinking about it, any change in distance of the primary mirror from the secondary is probably compensated for in the focuser?). But I don't understand the mechanical design of the secondary (we are talking about a Bintel premium 8" reflector, vintage Nov 2006). I will pull it apart at some stage to get rid of the screws (replace them with threads with wingnuts I can operate by hand - yes, I may have to "soup up" the dust cap by adding a raised section to accommodate ) but, in the meantime, do I need to move the three screws in conjunction with each other, rather than independent of each other? Will I bend or break something if I didn't move them together? I notice that they are relatively tight to adjust, which concerns me a little. Tight enough so that, as I turn them, the spider flexes then relaxes - I don't like that at all.

Thanks for your help.

[ving (David)]

How close is close enough?

yeah well you have answered that yourself. for visual get it as close as you can. I mean is there a way of actually measuring it so you can say give it a number or something? not really... visually using a cheshire and getting everything collimated with that is enough. if you wanted to take pics then i am guessing that it isnt... some barlow i believe...

my headspace is feeling a bit soupy at the moment to read question 2 though
best of luck
most importantly though, dont stress over it... it will distract from actually enjoying the view

[wavelandscott (Scott)]

The easy answer is that you should get collimation as good as you can...

The better your collimation the better your views...period.

How critical is collimation? Different question...

Good collimation is always important to get the "best" views visually or other...

The "faster" your scope the more critical it is...

I've read a comparison somewhere about how accurate is accurate enough (tolerance for sloppiness) but, I can't remember what the numbers are...

I think it was over on the Cloudy Nights website...and it gave some examples of needed accuracy for different f (ratio) scopes...

Anyway, practice makes perfect!

[astro_nutt]

Hi Eric..yep!..I can remember the first time I collimated my Saxon 8" inch dob and straight away I knew I need some special tools..a set of of those watch maker screwdriver sets..particulary the ones with the allen/hex heads to adjust the secondary mirror...the grub screw size being 4mm x 20mm (which I replaced with a set from the reputible bolt supplier)..means careful tightening..no more than 2 fingers turning the allen/hex driver!
The central screw on the back of the secondary mirror would be the first to loosen off then just nip it up while adjusting the collimation screws..
When the laser precisely returns to it's source with the focuser fully in and out..that's pretty close!!
Cheers!

[taminga16 (Greg)]

Eric, check out Andy's Shot glass web site, he has a great tut that inspires confidence and encourages boldness in collimation, the more you do it the better you get , or get it, if we were younger. On the mechanics of the primary, I have similar concerns, so why not wind the adjustments to a central position and start from there., If nothing else it may provide peace of mind. Regards Greg.

[taminga16 (Greg)]

Eric, Andy provides several points of intrest, but one truly wonderful insight, KEEP YOUR OTA HORIZONTAL WHILST YOU ARE MESSING WITH STUFF.

P.S. I really enjoy no BS stuff, and subscribe whole-heartedly to the KISS systems. Greg.

[Geoff45 (Geoff)]

For a start it depends on your focal ratio. An f5 scope has to be collimated much more carefully than an f10 scope. Near the optical axis there is a small zone where the image is pretty much perfect--a "sweet spot." You have to be sure that your eyepiece takes in this sweet spot, and that is what collimation is all about. If you miss the sweet spot, you get coma, and if badly out, astigmatism. The size of the sweet spot is given in this table.
f4--1.4mm
f4.5--2.0mm
f5--2.8mm
f6--4.8mm
f8--11mm
f10--22mm
f12--38mm
So for an f4, be very exact. For an f12, you need hardly bother.
Geoff

[wavelandscott (Scott)]

Originally Posted by ghsmith45

For a start it depends on your focal ratio. An f5 scope has to be collimated much more carefully than an f10 scope. Near the optical axis there is a small zone where the image is pretty much perfect--a "sweet spot." You have to be sure that your eyepiece takes in this sweet spot, and that is what collimation is all about. If you miss the sweet spot, you get coma, and if badly out, astigmatism. The size of the sweet spot is given in this table.
f4--1.4mm
f4.5--2.0mm
f5--2.8mm
f6--4.8mm
f8--11mm
f10--22mm
f12--38mm
So for an f4, be very exact. For an f12, you need hardly bother.
Geoff

Thanks for posting this...it is exactly what I was refering to (but could not find) in my earlier post!

[Geoff45 (Geoff)]

It comes from here http://web.telia.com/~u41105032/kolli/kolli.html
Geoff

[erick (Eric)]

Thanks all, really useful information. Eric

[Satchmo]

The 'sweet spot' charted in Nils' article refers to the area around the optical axis where the coma is limited to 1/4 wave wavefront, I guess the idea being that if you are a planetary observer you want to keep the planet within this area. Charts his like this are always a good opportunity to sing the praises of the televue Paracor, again

As coma is linear increasing across the field so even at high power once you leave the diffraction limited area at the centre of the field stars will be increasing in size. Use of a Paracor will tighten up the star images right across the field not just at the edge, and hence improve contrast in deep sky objects.

At the edge of a 20mm diameter field ( and assuming you are using an astigmatism correcting EP like a Nagler star images will be 3 X smaller in an F4 Newt with Paracor than an F5 scope without. Even at high power the results are noticeable. Peter Bobroff reported a Paracor improvement with a 6mm Radian on his 16" F4. The Paracor aberration chart shows that images even at the edge of a 6mm Radian EP are tightened by a factor of 3.5 .

A $500 Paracor is probably the best turbocharge you can give a Newt because it will improve the view with most eyepieces and even give F3.5 scopes superior field sharpness over an F5 without it. I think as the use of Paracors becomes more widespread large fast Newts will be looked upon as an evolution from their longer `cousins' rather than some kind of 'performance vs. conveniance' compromise.

[AstroJunk (Jonathan)]

Here's a top tip:

When collimating, only ever use two of the three screws. Mark one, and never touch it!

Why - because by fiddling with all three, you actually end up moving the mirror up and down the tube! Not so important for the Primary, that just changes the focus position, but very important for the secondary.

If your secondary has 4 screws like mine, well the same applies, use 3 fix 1, it's just a little more fiddly.

Oh and buy a paracorr NOW!

[Starkler (Geoff)]

I have read that when you get a paracorr, the collimation tolerance tightens considerably and become more critical. I dont know the theory behind it but someone else might

[erick (Eric)]

The paracor would cost more than I paid for the scope!

[AstroJunk (Jonathan)]

Originally Posted by erick

The paracor would cost more than I paid for the scope!

Maybe getting a little carried away

Originally Posted by Starkler

I have read that when you get a paracorr, the collimation tolerance tightens considerably and become more critical. I dont know the theory behind it but someone else might

I don't find that to be the case - but I align with a top quality barlowed laser to within a gnatts whisker of perfection every outing anyway and re-check mid session too! (one of the biggest benefits of a big truss dob is the ease of alignment - you can watch the donut on the barlowed laser through it's reflection on the diagonal whilst 'doing the business' at the blunt end)

[erick (Eric)]

Originally Posted by AstroJunk

......(one of the biggest benefits of a big truss dob is the ease of alignment - you can watch the donut on the barlowed laser through it's reflection on the diagonal whilst 'doing the business' at the blunt end)

That's interesting. If I understand correctly what you are doing, it seems to me that a strategically placed "diagonal viewing window" in an OTA might be useful?

[AstroJunk (Jonathan)]

The fun bit would be getting a shallow enough angle without cutting half the tube out! I did have a newt with an oversized tube which let me squint between the tube and mirror. I didn't plan it that way - but it worked out well!