Collimation in the dark

[PorkyB]

I have used my new (to me) 200p-EQ5 a few times now. The first time it was all a bit fuzzy so i checked the collimation at home and it was quite a bit out. Just the primary, so the whole process was a painless, five minute task.

The next time I used it the image was pin-sharp. I did notice that the third time I used it, by the end of the session, it was a little bit out again, but not enough to spoil my enjoyment. I suspect that whilst spinning the tube in the rings (the curse of EQ mount users) I have knocked the mirror cell.

Really i should at least check the collimation every time I setup. The scope will have been carried from the house and driven to a dark site, so it's not unreasonable that it would lose collimation a little.

The thing is I have only ever done this at home in the light - how feasable is it to collimate in the dark? I mean really dark, by the time I get to site twilight will have been and gone. I am using a Cheshire and a home-made collimation cap (made from a spare focusser blanking cover, who has 35mm film canisters these days?)

[Peter Drew]

Collimating in the dark can be a bit challenging due to seeing the adjusting screws. You would need either a laser collimator so that you can see the effects of the adjustment or use a real star, which is the definitive test anyway, for this I would recommend Polaris as it won't keep moving in the field of view.

[umadog]

The problem with the star is that it won't tell you if your secondary mirror is out. I use a Cheshire and auto-collimator in the dark by clipping a bright red LED to the spider vanes. Works perfectly well and is just as easy to do as during the day. The only thing that's hard is rounding the secondary, since you can't see its edges without a blasting a tonne of light everywhere.

[acey]

I collimate my 12" flextube at my dark site at the start of every session. I use a Cheshire (essential), a small light source (essential) and a laser collimator (optional). First I put in the laser collimator, look down the open tube with my red head-torch and see if the spot is illuminating the primary's centre ring. If not, I adjust the secondary until it is. I have "Bob's knobs" on the secondary and these are easily adjusted with gloved hands. Next I replace the laser with the Cheshire, and illuminate it by shining a red light onto the Cheshire's angled reflective face (being careful not to shine this light straight into my eye, which would spoil my adaptation). If, when looking through the Cheshire, the spot and ring are not aligned, I reach for the bolts at the rear of the primary and adjust accordingly. From practice I know which bolt to turn, and in which direction, in order to move the image of the ring in the required direction, so it is no problem that I need to take my eye away from the Cheshire in order to adjust the primary (thougth I can remain comfortably seated). Once the ring and spot are aligned, I note whether the cross-hairs of the Cheshire are aligned with their own reflection (i.e. do I see single cross or double cross). If not (i.e. double cross), this indicates some rotation of the secondary. If I'm feeling really fussy and the moon still isn't down then I'll give the secondary a twist and repeat the whole procedure. Most times I'm not really fussy (any rotation results only in a very small, practically negligible light loss at the eyepiece), and in any case I find that the secondary's position with respect to rotation rarely needs adjusting - once it's done right it tends to stay right. In general it takes me a couple of minutes to collimate the scope at the start of each session. I then adjust the finder (generally using Polaris since it stays conveniently still in the sky) and start observing. I don't find any need for further collimation adjustment through the course of a session.

[acey]

Edit: you might wonder why I don't just use the laser collimator for everything. The answer is that once I've collimated perfectly using the Cheshire, if I put the laser back in and examine its angled face, the laser spot is often off-centre. This is because although my laser is adequately collimated, my focuser is subject to slop. So I trust the Cheshire (and a star test if necessary), not the laser.

[acey]

Edit 2: PorkyB, I note that you have an 8" solid tube. In that case the secondary should hardly ever need adjusting and your Cheshire alone will be perfectly adequate for collimating the primary at the start of a session if need be, though in fact you may find that the primary also rarely needs adjusting - I used to take an 8" f6 to my dark site and I only collimated it every so often at home in daylight. The faster, heavier primary of my 12" f4.9 needs far more attention.

[PorkyB]

So shine a red torch onto the Cheshire's angled face. I'll have a go next time out, thanks a lot.

[iamjulian]

The torch onto the angled Cheshire face makes it much easier to see, even in daylight. Do what I did though - practice in a dark garage rather than waiting until you next get to your dark site. That way it doesn't matter how long it takes you, and you don't lose as much observing time. If you get into the wilds and make a pigs ear of it you can't just switch on the light and sort it out