Collimation woes - HELP!!

[F1Bird]

So yesterday afternoon we took the plunge, got the screwdrivers out and tried to collimate my scope. BIG mistake! It's now worse that it was when I started but I can't figure out why! When I started I looked down the focussing tube and saw the whole of the primary with a black circle to the top left quadrant. In this black circle was a reflection of my eye. The collimation guides that I had read indicated that the black circle was the secondary mirror and that I needed to adjust the primary to get it centred in the view. However, moving the primary did nothing but distort the view of the primary. The black circle with my eye in it stayed put in the upper left. So we started playing with the secondary screws and (eventually) got the black circle to move to the centre. At this point I could see the whole of the primary and the black circle with my eye in it was bang centre. I thought this was a good thing and took the scope outside to cool before checking out the fabulous clear view. How naïve was I?! Not only was the view not clear everything is now totally blurred. Before I started stars had small flares coming from the sides but Jupiter looked ok. Now I cant focus on anything! Any ideas?!

[NGC 1502]

The trick is to sort the secondary mirror first.   Looking into the focuser using a collimation cap, you need to get the secondary centered within the view of the end of the focuser, pointing at the primary mirror, with edge of the primary equal all round with respect to the visible edge of the secondary.  Most use the primary mirror retaining clips for that.

The important thing when doing this bit, is to IGNORE the reflection of the spider veins.

When the secondary is sorted, use the primary mirror adjusters to center the reflection of the spider.

A defocused star at medium to high power, and centred within the field of view can be used to fine tune the collimation, using the primary adjusters only.

All this can be a bit controversial I know, but that's what works for me.

Regards, Ed.

[Purplehayze104]

hi there

what sort of tool are you using to do the collimation i.e cheshire or a lazer ? i have a cheshire an did my first collimation the other week took me ages to finally get it sorted or as close as possible to what i thought & the misses for a second opinion.

[F1Bird]

hi there

what sort of tool are you using to do the collimation i.e cheshire or a lazer ? i have a cheshire an did my first collimation the other week took me ages to finally get it sorted or as close as possible to what i thought & the misses for a second opinion.

we just used a DIY collimation cap. I was looking to buy a Cheshire but they are out of stock on FLO at the minute. The thing that baffles me is that the circles of the primary, secondary and focuser all seem to be lined up and concentric now that I look down the focusser! Not sure what to adjust as everything looks ok, but it clearly isn't! I tried to take a photo to post but clearly optics are not my thing at the minute as the pic wouldn't come out properly! Is it possible to be so far out in terms of blurred images yet still seem to be lined up? Will using a Cheshire make a difference to just a collimation cap?

[itmo]

Use a piece of pape behind the secondary mirror to see what in the picture is a secondary mirror and what is a reflection. Also try putting it between the secondary and primary mirrors to see what happens. 

[umadog]

As Ed says, the thing is to break down the process into stages. Take a step back and think about it: all you need to do is adjust the tilt of the secondary, centre the secondary in the focuser, and adjust the tilt of the primary. Three things. The purpose of a collimation tool is to give you the visual feedback needed to make the adjustments. The collimation cap does a more or less acceptable job of the first two items and a pretty good job of the second (assuming that it's got a reflective inside surface and it's been made correctly). The combination tool (what is often called a Cheshire) is the way to go: the sight-tube component will do a better job of the first two and the Cheshire component will do the final adjustment (primary tilt).

The procedure is:

1. Point sight-tube cross hairs at centre spot using secondary tilt screws. When cross hairs point at spot, the tilt is good.

2. Check that secondary appears rounded and centred in focuser by framing it with the peep-hole of the tool. Adjust rotation of secondary as appropriate. If this has moved the cross-hairs away from the spot then cycle back and forth between this step and the previous until all is good.

3. Adjust primary tilt until centre spot is in the middle of the Cheshire reflection. If adjustment from 1 is now undone you should cycle back and forth between this step and 1 until both are good.

That's it. Details here: http://www.physiol.ox.ac.uk/~raac/collimationLinks.shtml Of those links, I recommend this one: http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm

If you don't yet have combination tool then you just have to do your best with the first step. Just try to visually place the centre spot in the middle of your field of view. Don't sweat it, the primary tilt is the important adjustment.

[Moonshane]

this might help

http://stargazerslounge.com/topic/197640-collimation-and-star-hopping/

[bomberbaz]

Whdre abouts sre u F1Bird, as you csn see from my sig I am from burnley but as I am in sales I get around so might be able to help if you are in my travdl area.

Ahave various collimation tools yhat may help

steve

[wookie1965]

Try this read all three pages and use the coloured card or whatever you have makes it much easier.

http://www.schlatter.org/Dad/Astronomy/collimate.htm

And same as above if your near me i can come and help let us know what area your in.

[F1Bird]

Thanks for all the offers of help! I'm in sunny Southport. I'm going to persevere and will see how I go. If I managed to find Saturn using the Astromaster comedy red dot finder then aligning some mirrors should be child's play......   

[wookie1965]

Thanks for all the offers of help! I'm in sunny Southport. I'm going to persevere and will see how I go. If I managed to find Saturn using the Astromaster comedy red dot finder then aligning some mirrors should be child's play......   

Well if you get stuck i dont mind a ride up i can spend the day theirs a good book shop i like going in.

[BritAngler]

It isn't very often that you will need to adjust the secondary mirror, usually it's the primary mirror that is out of kilter. First align the primary, then the secondary if required, then fine adjust the primary. I've collimated my scopes a few times and only needed to adjust the secondary a handful of times.

Full instructions can be found here http://www.garyseronik.com/?q=node/169

Buy a Cheshire eyepiece as well as a collimating cap, the former is far more help as you can use it for alligning both the primary and secondary; the collimating cap cannot really do the secondary mirror easily.

[F1Bird]

As my primary doesn't have a centre spot marked is there anything I can do other than take the primary out and stick one on?

[wookie1965]

It will take a lot of time and patience to collimate it using unfocused star test other than that no it would be better to put a centre spot on it.

[F1Bird]

And one other thing I can't get my head around is if you can't see the centre spot when you're observing because it's in the shadow of the secondary, how come you can see it when you're collimating?!

[Moonshane]

off the top of my head, you don't see the spot because it's not near the point of focus, not because it's in the shadow of the secondary. you can see it when collimating as you are working with mirrors of course. I think being in the shadow of the secondary is more relevant to light gathering and e.g. damage to the mirror - i.e. if you damaged the mirror or put a larger than normal spot on the centre of the mirror it would have little or no effect upon the light gathered overall.

[cantab]

And one other thing I can't get my head around is if you can't see the centre spot when you're observing because it's in the shadow of the secondary, how come you can see it when you're collimating?!

The inner side of my collimation cap is white and slightly shiny. I believe this allows light to enter through the front of the scope, reflect off the primary, off the secondary, then be diffusely reflected off the inside of the cap, back off the secondary, and illuminate the centre of the primary. That's not a light path that will occur with a properly-blackened eyepiece.

[Jason D]

And one other thing I can't get my head around is if you can't see the centre spot when you're observing because it's in the shadow of the secondary, how come you can see it when you're collimating?!

As others have mentioned:

1- When you are observing at night, there is not light striking the center spot. That is, the center spot will not be illuminated to see it. But when you are at home there is plenty of light to illuminate the center spot.

2- Your eye is focusing at infinity to see the stars. Near objects will be blurred.

Actually, your question should be: Why you do not see the whole secondary mirror when observing but you can see its shadow clearly though the collimation cap (see my avatar)

Jason

[LeeWilky]

A lot of good advice already on here but just incase heres a great tutorial that i used http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Its a pain doing collimation for the first time and like you i messed mine up real good and took me a long time to fix it.

Good luck!

[cantab]

Astro Baby's guide was the one I used too. Putting the card in the tube is a bit fiddly but really helps you see what you're looking at.

[bomberbaz]

You can collimate a 130 or lolwer reflector fairly accurately without a centre spot. You just need a good eye and judgement and a laser collimator.

Not done it myself but I have seen it done and a star test showed it to be very good.

[Tonys]

A lot of good advice already on here but just incase heres a great tutorial that i used http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

Its a pain doing collimation for the first time and like you i messed mine up real good and took me a long time to fix it.

Good luck!

I've just looked at astro-baby's tutorial. One of the best I'd say.

[F1Bird]

You can collimate a 130 or lolwer reflector fairly accurately without a centre spot. You just need a good eye and judgement and a laser collimator.

Not done it myself but I have seen it done and a star test showed it to be very good.

I have just got my Cheshire so I think I'll have another go using that but without the centre spot. The thought of sticking things on the mirror is terrifying so if I can get away with it I would rather not!! Someone mentioned that I wouldn't be able to use a laser collimator because of the Bird-Jones lens that it built in to my scope. Still not entirely sure what this is or what purpose it serves? When I look down the focuser in good light I can see glass at the end of the tube with a small maybe 5mm wide "lens" kind of looking thing in the middle. Is this the offending object?

[cantab]

Yes. It's a corrector lens, intended to fix the spherical aberration of the main mirror and also increase the focal length. Along the same lines as the corrector plate in an SCT, only much smaller. The design is a bit notorious because it's hard to get right - that lens may be little but it's got to be precisely shaped - and lots of scopes are sold that really don't get it right.

[umadog]

I have just got my Cheshire so I think I'll have another go using that but without the centre spot. The thought of sticking things on the mirror is terrifying so if I can get away with it I would rather not!!

To honest, you always need the centre spot to collimate a Newtonian. Collimation is less critical for large focal ratio reflectors. The focal ratio is the critical number, not the mirror diameter: there are plenty of f/5 130 mm EQ Newtonians out there and they will have the same collimation tolerances as a typical 10" or 12" Dobsonian. You have a bird-jones, though, and that makes things different. The alignment of the secondary is likely to matter much more than with a regular Newt, for the same reasons that it also matters more when using a coma-corrector on a regular Newt. Look at the first graph in this PDF: http://www.catseyecollimation.com/Newtonian%20Axial%20Tolerances.pdf The first graph is secondary tilt tolerance with and without a coma-corrector. If I'm right, your situation is equivalent to a coma corrector. So you have to nail the tilt alignment on the secondary. Perhaps the primary being spherical means that this alignment matters less. I did a Google search and came up with this link: http://www.cloudynights.com/ubbthreads/showflat.php/Number/4004900 It seems that might be the case, but I didn't read the stuff there in detail.