Collimating a Newtonian...

[RichieJarvis]

Hi All,

Ok - I've been pulling my hair out recently trying to collimate my 6" Newt. Everytime I think I've got it right, things get worse.

I took it to Herstmonceux for advice, and thought I had the procedure licked, but now I've got it home, I am getting heart shaped stars. I am really at my wits end with this thing right now.

Is there anyone local to East Sussex (South Chailey) who would be willing to help me out with this? Just to collimate it, and show me where I am going wrong, and then mess it all up again so I can do it myself and get it right would be a big big help!

Thanks in advance, and sorry to be a pain

Cheers,

Richie

[rigel1]

Don't Worry. No matter how much you "mess" up your collimation, It can always be corrected.

There are plenty of ways to correct and loads of advise available on this forum that I'm sure are going to help you.

Firstly, without any eyepiece, look through your focusser, and you should see a reflection of your eye.

Centre your eye to the centre of the reflections and you will see how far out your collimation is. You should see reflections of all the mirrors in a concentric and equal manner. adjust so all circle reflections are concentric. Once achieved, your collimation wont be far out, but using(purchasing) a laser collimator will help tremendously.

Hope this simple advise helps

Rigel1

[RichieJarvis]

Hi Rigel,

Thanks for the reply. I've got a laser collimator, and a cheshire eyepiece. The Cheshire tells me things are fine. The laser tells me its not, as do my imaging results

Whenever I get things looking right in the Cheshire eyepiece, things look pretty much inline from the front, but the laser doesn't hit the centre doughnut on the primary. If I tweak the secondary to get the laser centred, and then get the bounce back directly into the centre of the laser by adjusting the primary, then the visual view from the front is skewed to the right, away from the focuser.

So I worked on the cheshire, getting everything as concentric as I could - the secondary mirror won't quite go high enough though, as its reached the end of its travel upwards towards the spiral.

Since playing with all this, and going mad with it, the heart shapes have appeared, which I presume must be due to the secondary being at a funny angle.

I've also downloaded and followed the Kendrick pamphlet on collimating as well.

I guess what I am looking for is an expert to look at me and tell me whether my scope is defective or not.... I am running towards that conclusion, however, I doubt that is true, because in my experience the problem is more likely to be with the operator (i.e. me!)

Thanks again for the help,

Cheers,

Richie

[Astronut]

You have to be methodical when collimating. Don't try to centre all the reflections at once.

Always start with the secondary. Ignore the reflection of the primary, which isn't easy. Regarding this, a good tip is to place a piece of blank card or paper between the secondary and primary so that it blanks out the primary reflection. Then all you'll see at the focuser is the secondary and because it's reflecting the light hitting the card or paper you'll be able to see it's shape and outline clearly.

Put the cheshire in and rack the focuser in or out so that you can see the outline of the secondary and a little bit of the space around it.

It also helps if you tape a different coloured piece of card or paper to the tube wall opposite the focuser behind the secondary so the space around the secondary is easily seen and contrasted with the secondary (which is reflecting the white card).

You may have to pull the cheshire slightly out of the focuser even when it's fully racked out to see the whole secondary but don't tighten the lock screw too much ie. just enough to hold it in position.

The aim here is to get the secondary outline circular and concentric with the outline of the bottom of the cheshire. You may have to adjust the secondary holder centre bolt to move the secondary up or down the tube in order to get the secondary centred under the cheshire outline. If the secondary top or bottom edge (as seen from the cheshire) is too low or high you may have to adjust the spider vanes to raise or lower the secondary. All you should see when the secondary is centred under the focuser is the outline of the secondary (circular) and a thin ring of 'dark' surrounding it (which is the tube wall opposite the focuser or the coloured card behind it if you used one). If you can get the cheshire postioned just right so that this ring surrounding the secondary outline is as thin as possible, you'll get more accurate centering.

I hope this is making sense :oops:

Once you've got it centred, tighten the 3 secondary holder adjustment screws (those little allen screws) just enough to stop the secondary moving and check to see that it's outline is still centred. When you're happy with that, remove the card or paper blank.

Look at the reflection of the primary. Chances are it's not centred in the secondary (still viewing through the cheshire here).

Use the 3 secondary allen screws to adjust the secondary's tilt or angle to centre the outline of the reflection of the primary within the outline of the secondary. Concentrate on the outline of the primary reflection and try to ignore the reflection of the secondary in the primary's reflection.

You may have to rotate the secondary slightly if the primary reflection is too high or low (as seen from the cheshire).

I always find this is the most fiddly part of the collimation process. Those allen screws can be a real pain. Bob's Knobs replacements help - you don't need an allen key with these but it's still a fiddly job. When you've got the primary reflection centred it's time for the final stage - adjusting the primary's collimation screws.

The aim here is simply to get the primary's centre spot or doughnut centred under the cheshire's crosshairs. Loosen the primary collimation lockscrews and adjust one collimation screw at a time to move the centre spot so that it lines up with the cheshire's crosshairs. Use small adjustments on one collimation screw and look in the cheshire to see which way the centre spot moves. If it's going the wrong way, simply turn the screw the other way or use one of the other screws.

Lazers are OK if you're 100% sure the lazer is collimated and doesn't fit sloppily in the focuser. They're great for getting the secondary's angle/tilt set up by centering the beam in the primary centre spot doughnut. At this point the primary reflection should be centred in the secondary outline when using the cheshire. What they don't cater for is centering the secondary outline under the focuser. Personally, I don't trust 'em but that's just my opinion. I used to have one but I find the cheshire is more reliable and consistent.

Bottom line is don't let it frustrate you and I hope the above makes some sense.

Useful link:

http://web.telia.com/~u41105032/kolli/kolli.html

[MikeD]

Excellent description Astronut , this post should be made a sticky in this section.

Mike.

[Astro_Baby]

I am in the process of writing a huge collimation tome - I hope to have it online in a week or so. Other things ( flu ), chest infection, fitting a new focuser have all slowed me down.

Question for you ? - Heart shaped stars you mention - that sounds like it might be down to a pinched primary mirror which will cause the stars to take a triangular shape. Have you checked for that ?

Also I agree with Astronut above - I had a laser and found it was wildly inconsistent and always disagreed with the Cheshire. I'd trust the Cheshire myself as lasers seem to often cause more problems than they fix.

[supernova]

Astronut is the colli man :hello1: :hello1: he is very good.

If he was a lady I am sure somebody would buy him a bunch of colli-flowers

for his efforts at the Salisbury star party.

[Astronut]

........buy him a bunch of colli-flowers

for his efforts at the Salisbury star party.

groan Thanks Bernie. Under these damn clouds, collimation is all I get to do these days but it's all good fun

[supernova]

........buy him a bunch of colli-flowers

for his efforts at the Salisbury star party.

groan Thanks Bernie. Under these damn clouds, collimation is all I get to do these days but it's all good fun

I have sorted the mirror cell , thanks matey.

[rigel1]

Well there you go Richie.

Told ya there would be plenty of good advice on this excellent forum.

Well done Astronut and thankyou for a far better description than I could put together.

All the best all.

Rigel1

[phillc]

Thanks for the lesson Astronut, after reading your post even I'm going to check my scopes collimation.

Any chance it could be made a sticky so its easy to find for future reference

phillc :wave:

[RichieJarvis]

Thanks All!

I've got a few moonlit nights to get this licked!

One Q for you though - you say the mirror might be 'pinched' - what do you actually mean by that?

Cheers,

Richie

[MikeP]

It refers to the mirror being distorted from its parabolic shape by the clips that hold it in position being too tight.

Mike

[RichieJarvis]

Ahhh - gotcha...

I've just had the whole thing apart - secondary out, spider out, primary out - I've also discovered that I was distorting the tube by overtightening the spider adjustments. A ruler helped me sort that out and get the secondary mount central in the tube without distorting it. Could that be what caused the heart shapes as well?

The other thing I am wondering is whether i over-tightened the lock-screws on the primary. There are 6 screws - 3 on springs to tighen it down, and 3 which push against the flanges in the tube. I wonder whether over-tightening those might have contributed to the heart-shaped stars as well?

Its all back together now, and everything appears to line up in the cheshire now. Looking down the tube, I think it will need a small tweak when i get it on a star - hopefully tonight. I'll report back later assuming I am able to see any stars tonight! I just KNOW its gonna be tonight though!

Thanks for all the help and advice guys - appreciate it... Astronut - I'll definately sort you out a colli-flower if we ever meet - or maybe a :occasion5: would be more to sir's liking??

Cheers,

Richie

[Astro_Baby]

The pinched mirror MAY be cause by the collimatiom screws being overtightend as well as the mirror clip screws being overtightened.

[Starflyer]

Astronut is the colli man :hello1: :hello1: he is very good.

I totally agree Bernie, Ian did a fantastic job on my collimation at Salisbury

Thanks for writing up the procedure Ian, I'll give it a go in the near future. It's definitely the reflection of the primary that distracts me, I'll have to get me a piece of card and try blocking the reflection for the first bit.

Cheers,

Ian

[The Warthog]

Don't Worry. No matter how much you "mess" up your collimation, It can always be corrected.

There are plenty of ways to correct and loads of advise available on this forum that I'm sure are going to help you.

Firstly, without any eyepiece, look through your focusser, and you should see a reflection of your eye.

Centre your eye to the centre of the reflections and you will see how far out your collimation is. You should see reflections of all the mirrors in a concentric and equal manner. adjust so all circle reflections are concentric. Once achieved, your collimation wont be far out, but using(purchasing) a laser collimator will help tremendously.

Hope this simple advise helps

Rigel1

Right there is probably the best description of what a collimated system should look like, that I've ever seen. Good show!

I still like my Cheshire collimator. I don't think I'll ever bother with a laser.

[The Warthog]

A ruler helped me sort that out and get the secondary mount central in the tube without distorting it. Could that be what caused the heart shapes as well?

Probably not, but I should just mention that the secondary should be slightly off centre towards the back of the tube. It's a matter of millimeters, and there are websites about, and probably some people on this forum, who can tell you how many millimeters.

The other thing I am wondering is whether i over-tightened the lock-screws on the primary. There are 6 screws - 3 on springs to tighen it down, and 3 which push against the flanges in the tube. I wonder whether over-tightening those might have contributed to the heart-shaped stars as well?

Unlikely but possible. They might spoil your careful collimation. The three unspringed screws are to act as keepers to make everything behave once you've got it collimated. They should just lightly contact the back of the mirror cell.

[RichieJarvis]

Well, it wasn't exactly clear tonight, but Stephen Green came over tonight with his new SXV-M25, so we had a go on some bright stars with both his M25 and my SXV-H9. The newt is still not quite perfectly collimated, but is miles and miles better than it was!

Warthog - thanks for the advice - I think I was seriously overtightening those primary screws! The heart shaped stars are seriously reduced - learning the hard way how to collimate, but then, as those who know me know, thats the only way I ever learn! :mrgreen:

Visually, it looks a tad out, and I mean a tad. Through the camera its a bit more pronounced - next time I actually do some proper imaging, I intend to give it a tweak (and will probably mess up all my hard work so far!) and see if I can improve it.

A few questions for you all though on collimating using a CCD - is this a good way to do it for a start? Also, as its a newtonian, I have an Coma corrector inline with the camera - I assume thats ok, or should I remove it for collimating with the CCD?

I wish we could have got something to show for a nights playing though - damn clouds again

I think I owe quite a few folks from SGL a beer or 3 now.... I'll be at the IoW Star Party, Herstmonceux and Salisbury next year, so come and say hello if you get chance!

Cheers,

Richie

[Astro_Baby]

Richie if the heart effect is still showing check the primary mirror clips. They should be in contact but not tight with the mirror. The rubber gasket or blocks should should be in contact but no more. They are only there to stiop the mirror falling out when the tube moves - they dont have to be tight at all.

The collimation screws should be about the same - in contact but only lightly finger tight. Same for the lock screws. I use a small dab of loctite on my lock screws which makes them they where they are and not loosen under vibration.

[Mel]

Hi Richie

Glad you're well on the way to curing your 'heart-ache'! d'0h!!

Any prizes for the worst jokes on this forum?

cheers mate

:wave:

Iain

[glennbech]

Thank you. Did all that tonight and the newt is as good as It has ever been. It feels very good to finally have my newtonian collimated properly. The collimation process is one of the reasons why I feel it's a tad (just a tad) unwise to suggest a fast newtonain for a first scope. I managed to follow the procedure as described tonight; but Im not sure I would have been able to do that without the experience I have gained so far in my first year?!

Some comments on the text from a collimation virgin

l so helps if you tape a different colored piece of card or paper to the tube wall opposite the focuser behind the secondary so the space around the secondary is easily seen and contrasted with the secondary (which is reflecting the white card).

Isn't most newtonians black on the inside? I found no real use for a black card as the inside of the tube was black I card between the primary and the secondary is very very useful.

If the secondary top or bottom edge (as seen from the cheshire) is too low or high you may have to adjust the spider vanes to raise or lower the secondary.

That depends on the direction of the newtonian when you do the collimation (I mean; what "up" is). I adjusted the secondary with the newtonian standing. Using one spider vane moves the secondary left/right, the other has no effect (closer/further away). For real dummies (like me) the perfect text should say something like "with the OTA lying down..."

Look at the reflection of the primary. Chances are it's not centred in the secondary (still viewing through the cheshire here).

Use the 3 secondary allen screws to adjust the secondary's tilt or angle to centre the outline of the reflection of the primary within the outline of the secondary.

I did this by centering the crosshair the the chesshire in the primary's center marker. With the secondary already aligned with the focuser, this should now be a correct approach?

Loosen the primary collimation lockscrews and adjust one collimation screw at a time to move the centre spot so that it lines up with the cheshire's crosshairs. Use small adjustments on one collimation screw and look in the cheshire to see which way the centre spot moves. If it's going the wrong way, simply turn the screw the other way or use one of the other screws.

On my SW explorer 200 I actually find it easier to loosen the collimation screws, adjust the lockscrews and re-tighten the collimation screws. I wonder if this is bad karma?! The collimation screws are a bit fragile and sometimes my wrench get stuck in there etc... probably just me

[timetraveller]

I can one million percent confirm Bernie's ability in collimation . He is the collimation genius of the forum just take his advice and then take your time satisfaction guaranteed regards Pete

[Starflyer]

l so helps if you tape a different colored piece of card or paper to the tube wall opposite the focuser behind the secondary so the space around the secondary is easily seen and contrasted with the secondary (which is reflecting the white card).

Isn't most newtonians black on the inside? I found no real use for a black card as the inside of the tube was black I card between the primary and the secondary is very very useful.

Try a piece of orange or yellow card, something that makes the secondary stand out from the background.

If the secondary top or bottom edge (as seen from the cheshire) is too low or high you may have to adjust the spider vanes to raise or lower the secondary.

That depends on the direction of the newtonian when you do the collimation (I mean; what "up" is). I adjusted the secondary with the newtonian standing. Using one spider vane moves the secondary left/right, the other has no effect (closer/further away). For real dummies (like me) the perfect text should say something like "with the OTA lying down..."

You should really adjust the secondary with the 'scope horizontal; if your secondary somehow became detached from the spider it would make a real mess or your primary :shock:

Cheers,

Ian

[glennbech]

You should really adjust the secondary with the 'scope horizontal; if your secondary somehow became detached from the spider it would make a real mess or your primary

The collimation and the guide made yet another cloudy night a happy night .-)