Someone else with collimation woes (with photos)

[wolfpaw]

Hello all

First post here and I'm afraid I need some advice re. collimation of my Skywatcher 250PX solid tube Dob. I've had it for a while now and have never been that happy with the collimation or convinced that I knew exactly what I was doing despite looking at loads of things on the internet. I've got a collimating eyepiece, a Cheshire and a laser collimator. A couple of things are bothering me re. the alignment of the secondary mirror.

I've taken a photo of the current alignment looking down the focuser without an EP:

I'm happy with the mirror clips evenly spaced out around the edge but the reflection of the secondary mirror in the primary looks all over the place to me. It's not very circular and doesn't seem to sit exactly in the centre of the spider vanes. The primary centre spot looks off in the photo too (this is after I collimated the whole scope using the laser collimator).

Also, in the reflection of the secondary there's like a black crescent to the right side shown in the detail below. Is this a longtitudinal error in the secondary mirror? I've tried moving the secondary further away from the primary but I can't seem to get rid of this black crescent. Is it supposed to be there?

Also, my laser collimator: I've checked it to make sure it's collimated and by turning the allen screws I can get it to within about 3mm at a distance of around 3 metres. Is this accurate enough? I've spent ages trying to get it perfectly accurate and ended up going around in circles.

Any advice would really be appreciated!

wolfpaw

[Twiggles]

(NOT AN EXPERT!)

Laser collimators themselves need to be collimated and should have adjustment screws to move the beam (which may be hidden under a silicon type gel). So If you have been relying only on your laser collimator and made adjustments based on what it says, then I would first start by checking the instrument. From what I can gather, laser collimators are not generally used to align the secondary mirror. I know you say you have checked the laser collimator, and I believe for accuracy you need to be checking at a distance of at least double your tubes focal length (to replicate the path the light has to take).

Personally (and I am ready to be corrected by those with more knowledge of such things) I would check the spacing of the clips by moving the focus tube out. From your top picture it appears to be cutting in to the picture. I know it may seem a trivial thing since you are looking at the clips not the focus tube, but it detracts from the image and can draw your eye in the wrong direction. To me it appears as though the clip at approximately 11 o'clock is slightly more eclipsed than the others. Since there is also a clip at 3 o'clock, then allwoing for equal spacing there should be one at 9 o'clock which is hidden by the reflection of the focus tube.

The best way to check its collimation would be with a star test. Find a star, defocus it then check and see if the rings that should form appear to be concentric. If they are then great! If not you can use the differance to try and adjust or take it back inside. I don't think every OTA lines up perfectly as you would expect when collimated, and if the centre spot was applied in the factory then it may not be exact. Astrobabys site offers a very good guide and explains how some 'fast' tubes differ when in view and properly collimated.

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[NGC 1502]

The 'black crescent' you mention looks like the secondary offset, due to the F4.7 focal ratio.

Your collimation looks good to me.

Now this can get a bit controversial I know  but if that scope were mine, I'd just fine tune the collimation on a defocused star at 150 - 200x, using only the primary adjusters.

Check with the defocused star in the centre of the field of view.

Others may say differently, and may be correct.

Regards, Ed.

[wolfpaw]

Thanks for the reply. Sorry about the focuser being in the way! I should've extended it right out before taking the pic and didn't notice until I was uploading it.

I'll take another photo tomorrow and upload it. I'm going to have another go at fiddling with the mirrors. Unfortunately the skies have been cloudy where I am for what seems like ages and I've really only had chance to play around with collimation during the day. When it's been clear I've tended to rush out and use the telescope, lol.  I'm always aware of the fact that a 10'' scope with a 1200mm tube really needs precise collimation and I tend to worry that it's not right.

I'm pretty much OK with getting the primary mirror clips visible and getting a nice circular shape for the secondary through the focuser.  The position of the spider vanes bother me a bit as does that weird black crescent that I mentioned in the OP.

I really got the laser collimator to collimate the primary mirror as I just couldn't do it by walking backwards and forwards from one end of the scope to the other. I'm just not sure if the laser collimator has to be pinpoint accurate when shone and rotated against a wall.

(NOT AN EXPERT!)

Laser collimators themselves need to be collimated and should have adjustment screws to move the beam (which may be hidden under a silicon type gel). So If you have been relying only on your laser collimator and made adjustments based on what it says, then I would first start by checking the instrument. From what I can gather, laser collimators are not generally used to align the secondary mirror. I know you say you have checked the laser collimator, and I believe for accuracy you need to be checking at a distance of at least double your tubes focal length (to replicate the path the light has to take).

Personally (and I am ready to be corrected by those with more knowledge of such things) I would check the spacing of the clips by moving the focus tube out. From your top picture it appears to be cutting in to the picture. I know it may seem a trivial thing since you are looking at the clips not the focus tube, but it detracts from the image and can draw your eye in the wrong direction. To me it appears as though the clip at approximately 11 o'clock is slightly more eclipsed than the others. Since there is also a clip at 3 o'clock, then allwoing for equal spacing there should be one at 9 o'clock which is hidden by the reflection of the focus tube.

The best way to check its collimation would be with a star test. Find a star, defocus it then check and see if the rings that should form appear to be concentric. If they are then great! If not you can use the differance to try and adjust or take it back inside. I don't think every OTA lines up perfectly as you would expect when collimated, and if the centre spot was applied in the factory then it may not be exact. Astrobabys site offers a very good guide and explains how some 'fast' tubes differ when in view and properly collimated.

http://www.astro-baby.com/collimation/astro%20babys%20collimation%20guide.htm

[wolfpaw]

The 'black crescent' you mention looks like the secondary offset, due to the F4.7 focal ratio.

Your collimation looks good to me.

Now this can get a bit controversial I know  but if that scope were mine, I'd just fine tune the collimation on a defocused star at 150 - 200x, using only the primary adjusters.

Check with the defocused star in the centre of the field of view.

Others may say differently, and may be correct.

Regards, Ed.

Hi Ed.

So the black crescent might be normal? The thing that made me wonder is an illustration on this webpage:

http://starizona.com/acb/basics/using_collimating_newt.aspx

About halfway down the page it shows a 'longitudinal error' as a black crescent on the right side. I assumed my telescope had the same error.

[NGC 1502]

You could drive yourself nuts with this http://www.cloudynights.com/item.php?item_id=2677 or do the easy thing and just scroll down to picture 5 

Regards, Ed.

Edit - well that's a lot to take in, but scrolling to the bottom of your link, shows collimation with no secondary offset. Usually, with a 'fast' scope like yours, when all is correct, you can see the offset secondary, towards the primary.  So yours looks ok to me.  As I said, I'd just fine tune on a defocused star, just using the primary adjusters.  I may be wrong of course, but that's how I do it, and earlier this evening, was getting great views of Jupiter, loads of belt structure, and a pin sharp shadow transit of Ganymede.

But I'm willing to be corrected, Ed.

[Mr Spock]

What you really need to do is collimate the scope using a star test. Get that right and you will get every last bit of performance out of the scope.

[ardbeg74]

Standing ready to be corrected here but it its possible for the secondary holder to be at an angle but as long as the secondary itself is correctly angled then there should be no problem. I guess it would cut out a fraction more light but dont think it would be noticeable to the eye.

Here's a few pics of my before and after collimation shots, i use card to mask the reflections then gimp to draw circles so that i can tell if i have any rotational / tilt error on the primary, and also check the centre position.

Even in my after shots i was a little out, but I couldnt tell any difference on a star test. Also the focuser tube on my PL has a lot of slop in it too so my main reference point with the cheshire is around the focal points of the EP's i use the most.

I think its all too easy to get obsessive about collimation, but if it looks ok on a star test then its good enough The atmosphere is always going to impart more loss of detail than tiny collimation errors IMHO. But as i said standing ready to be corrected

Mark

[ardbeg74]

sorry that should read:

so i can tell if i have any rotational / tilt error on the secondary.

Dunno why i cant edit my posts...

[wolfpaw]

People say you shouldn't necessarily use a laser to collimate the secondary by looking down the tube and getting the red laser point to line up with the centre spot on the primary. Why is this? I've not used the telescope since I got the laser collimator but I love how easy it is (apparently!) to align both mirrors with it (especially the primary). In principle at least they are a fantastic idea.

[ardbeg74]

Hi Wolfpaw,

For the secondary there is a good explanation here, at around 6:10

Though the whole video is pretty cool.

HTH

Mark

[Jason D]

As others have mentioned, it is normal for the secondary shadow to look shifted. See my avatar and I am including a photo of my well-collimated scope.

If you center the secondary shadow then your secondary will look too high up the OTA.

That is not really a secondary shadow but rather a reflection of the area around the focuser area. Maybe the following two photos will help explain the shadow. I placed a template at the bottom of the focuser then took a photo through the empty focuser.

Jason

[Jason D]

Few diagrams that explain the secondary offset. The last is little complex.

[Jason D]

What you really need to do is collimate the scope using a star test. Get that right and you will get every last bit of performance out of the scope.

The OP is asking about the secondary mirror alignment. The star test can only be used to collimate the primary mirror.

Jason

[itmo]

If your cheshire and (collimated) laser agree, then your scope is collimated, if they dont you probably have a rotational+tilt error in your secondary. The way to start collimating the secondary is to turn the tube vertical, loosen all secondary collimation screws and turn it (looking down the tube, clockwise or counterclockwise) until it looks to be exactly towards the focuser tube. One way i do this is to put a laser collimator in the focuser and look at the red dot moving on the primary mirror , you can see it moving in a curve. I believe that when the point is closest to the focuser in its curve, the mirror is pretty much pointed (rotationally) at the focuser. Assuming ofcourse no tilt. Then you carefully tighten the tilt screws until they just  touch. Then you start the normal secondary mirror collimation to adjust it to point to the primary. Then you check with cheshire AND laser and iterate until they both agree. 

[wolfpaw]

Thanks for the replies. If I get the opportunity today I'm going to spend an hour or so playing around with the collimation. I'm a lot less nervous about adjusting things now compared with when I first got the telescope (when I was almost too scared to touch the thing!).

Just to clarify, that off-set 'black crescent' I mentioned is entirely normal, right? I appreciate that some off-set is natural in a fast telescope creating a bulging effect to the right, but is that the same thing as the black crescent?

Also, can anyone have a go at my question re. the laser collimator: how accurately collimated does the laser collimator itself have to be?

[wolfpaw]

Hello again

I had another attempt this afternoon:

I didn't use the laser collimator at all, just the cap and the Cheshire. I'm pretty happy with the results but as others have said, I need to take it outside and do a star test (if the rain ever stops!)

A question though: what causes the white ring around the primary mirror's centre spot? I've highlighted it with red arrows in the photo. The centre spot isn't in the centre of the white circle and I'm not sure what I should adjust in order to correct it. Is it a primary mirror error? Or an issue with the position of the secondary? (the infamous 'black crescent' shows up very well in this photo!)

Also, when the Cheshire is in the focuser there's quite a lot of slop. Do people tighten the the nuts at the side to fix it into place or use some sort of tape around it to make for a tighter fit? I'm a little concerned that each time I put the Cheshire in it's not in quite the same place as it was previously (for the laser collimator I used that white plumbing tape wrapped around the end and then rotated it into the focuser. Is this a better idea than securing it into place with the two little screws?)

I'm beginning to see how people can get obsessed with collimating.

[Jason D]

Sounds like my earlier post with photos did not make sense to you. Let me try again.

Refer to the following annotated photos. I attached a template at the bottom of my focuser then took a photo through an empty focuser. You can see the crescent which is really the reflection of the focuser edge but since the secondary mirror is lower in the OTA (as it should be) a crescent illusion is created.

[Jason D]

updated the first photo in the previous post

[moffer]

Your collimation looks fine to me - I'd be surprised if a star test was less than good.

I have to tweak my primary almost every time I travel out to my local darkish site and have yet to get a poor star test when I check - air turbulence allowing of course 

Can't really help with the sloppy cheshire as I  just use a colli cap to collimate, which is a snug fit in the focuser.  Maybe some tape around the cheshire would be the best method.

[mitchelln]

I can highly recommend using a barlowed laser to align the primary. The barlow spreads the laser light much in the same way as normal light is passed. You get a laser light image of the primary's centre dot on the laser collimator's angled face. This method is much less susceptible to movement and position than just using the laser by itself. It also doesn't matter if the laser is not perfectly aligned itself.

Primary advantage over a Cheshire (we're talking for primary adjustment here) is you can see the dot image whilst you are adjusting the mirror screws. With the Cheshire you are working blind and end up hopping from one end of the scope to the other. Disadvantage is you can't do this in full daylight as the dot reflection is fainter than the raw laser spot.

I've checked with a Cheshire a few times after doing this barlowed laser collimation and always found it to be spot on. Saves an awful lot of agro. You can collimate in a couple of minutes.

Here are some pictures:

In this picture you can see the primary's center dot doughnut reflected in the 10 o'clock position. So the primary mirror needs adjusting.

Now with the reflected center dot "in the hole". You also get a nice even ring of light around the edge of the collimator face. Scope's primary is aligned

[itmo]

I used to use barlowed laser to align my primary, but nowadays I use a cheshire lit by a red led. Since collimation is always anyways "almost on" and a cheshire is probably more accurate and easier to read. Only difference is that a barlowed laser works in the dark but that was solved by a 2 euro red led light. 

[itmo]

One note though, for long scopes like mine , barlowed laser does have the advantage that it can be read from behind the scope when adjusting the collimation screws. The problem for me is that if when the batteries are used for a while the laser dims so much that the image loses some of it's contrast and it is difficult to read. With a lit cheshire i have to go up and down a few times but the end result is quite the same and i dont have to wonder if I got it right.. 

[wolfpaw]

Sounds like my earlier post with photos did not make sense to you. Let me try again.

Refer to the following annotated photos. I attached a template at the bottom of my focuser then took a photo through an empty focuser. You can see the crescent which is really the reflection of the focuser edge but since the secondary mirror is lower in the OTA (as it should be) a crescent illusion is created.

I think I pretty much understand what I'm seeing now. Thanks!

[wolfpaw]

I can highly recommend using a barlowed laser to align the primary. The barlow spreads the laser light much in the same way as normal light is passed. You get a laser light image of the primary's centre dot on the laser collimator's angled face. This method is much less susceptible to movement and position than just using the laser by itself. It also doesn't matter if the laser is not perfectly aligned itself.

Primary advantage over a Cheshire (we're talking for primary adjustment here) is you can see the dot image whilst you are adjusting the mirror screws. With the Cheshire you are working blind and end up hopping from one end of the scope to the other. Disadvantage is you can't do this in full daylight as the dot reflection is fainter than the raw laser spot.

I've checked with a Cheshire a few times after doing this barlowed laser collimation and always found it to be spot on. Saves an awful lot of agro. You can collimate in a couple of minutes.

Here are some pictures:

In this picture you can see the primary's center dot doughnut reflected in the 10 o'clock position. So the primary mirror needs adjusting.

Barlowed_Laser1.jpg

Now with the reflected center dot "in the hole". You also get a nice even ring of light around the edge of the collimator face. Scope's primary is aligned

Barlowed_Laser2.jpg

I'm going to give the barlowed laser idea a try this evening. I've heard about it before but never actually given it a go.