Collimation Problems?

[RA180]

I think I might be having a problem with the secondary mirror on my scope (250PDS).

During collimation, It seems to be centered under the draw tube, but it isn't in a position where I can see the whole primary mirror. Before I start fettling with the seconday mirror holder (it's a fairly new scope and I'm new to this!), does it matter that the whole primary is not visible? I'm guessing that the answer is 'yes', but want to check first, if anyone can help??!!??

Thanks

RA

[Freddie]

The answer is yes. Astrobabe has a great tutorial on her site. Worth a look.

[RA180]

Thanks, I've read the astrobabe guide (and in part did my collimation from it), That, and all the other guides I've read all say to have the mirror clips showing, but my secondary, from new, is a long long way away from that.

Is it normal for Skywatcher to have it so far out from factory, or could it be something other than the secondary position causing the problem?

[Starflyer]

It sounds like the secondary is almost correct, now you have to adjust the secondary tilt screws to get the three mirror clips showing.

While your adjusting the tilt you should keep checking that the secondary is still showing as a circle slap bang in the middle of the focuser draw tube - it's too easy to rotate it slightly and not spot it.

This should get the mirror clips in view, then it's just a matter of adjusting the primary.

Read and read and read again Astrobaby's guide - it really is excellent and helped me understand collimation after 3 years of blindly fumbling around with it

[RA180]

Hi Starflyer

I've probably not been clear about my problem - I can't see any of the mirror clips! The secondary seems pretty well centered under the focuser, but it's almost like the whole secondary mirror assembly is too far toward the back of the tube, so I can't see the whole primary mirror. Bringing it as far out as it will go makes no difference, and makes its position to the focuser hard to achieve.

If I adjust the secondary tilt screws, I can get one, or maybe two of the 6 clips in view, but that chucks the secondary mirror alignment out by a mile.

Any ideas??

[moriniboy]

I can't see my mirror clips either, its a 300 flextube

How does it look when you do a star test?

[umadog]

The idea is that you want to be capturing all the light from the mirror with the eyepiece. So all of the mirror should be visible from the focal plane. The mirror clip instruction is just a shorthand for getting you to do this. There's obviously nothing special about the clips themselves. Now on some scopes, the secondary is too small and will never show all of the primary. You lose a little light: not much, no big deal. Also this configuration masks the edges of the primary, which are often bad. If they're really bad, the masked view will be better.

EDIT:

The implication is that it should be fairly obvious to the eye if the primary reflection in the secondary is substantially larger than the secondary itself. If this is the case, then you will never see all three clips and you will never be able to fit the whole primary into the secondary. The correct thing to do under these circumstances is to center the primary in the secondary as best you can so that the edge of the primary isn't visible anywhere. If you like, it's possible to calculate exactly how much light you're losing and what secondary size would get it back. However, swapping the secondary isn't worthwhile unless you know your mirror edge is good. A bad edge (turned edge) can throw scattered light throughout the whole field of view and reduce contrast globally. Contrast is very important and you don't want to risk that. My suspicion is that the manufacturers know the edges are often bad so they mask them with a slightly small secondary.

[RA180]

For some reason, I'm not surprised that the manufacturer may deliberately put in too small a secondary to compensate for a poor primary.

It's hard to tell how much of the primary is out of view of the secondary, but something in the range of 10% of the surface area would be my guess. That would only have an effect of making it a 9.5" tube, rather than 10" (assuming the primary is a flat circle, rather than parabolic), so not that bad.

I was out with it last night (getting eaten alive my mosquito's!), but couldn't do an accurate star check as seeing conditions were quite poor. I'll try again over the weekend. In the mean time, I'll get the secondary as far toward the top of the tube as can.

[umadog]

No, don't do that. You want the secondary centred in the focuser and there's only one position in which you're going to achieve that. Moving it up the tube in an attempt to see more of the primary is a bad idea.

[Peter Drew]

Some manufacturers also deliberately place the secondary and the focuser nearer to the primary than the optimum in order to gain enough back focus for camera use. Adding an extension adaptor to reach an eyepiece focus makes matters worse, try getting the collimator as near to the secodary as possible. If you use your eye for checking rack the focuser in as far as possible.

[lukebl]

I'd be interested to hear how you get on with your problem. I have had the 250PDS for a couple of years now, and consider myself fairly experienced, having perfectly collimated my old 200P, but have never got my collimation quite right on the 250. It shows up on my efforts at imaging, by eggy stars to one corner indicating that the CCD is not square to the imaging plane.

If I get the focuser aligned to the tube, by carefully aligning it with a spot on the exact opposite side of the tube using a Cheshire, and then fix the secondary (in the exact centre of the tube), they are never lined up correctly. So, should I then try and line up the focuser with the secondary, or adjust the spider so that that secondary lines up with the focuser. If you get my drift! Two years on, and I still haven't got it right.

[umadog]

You don't need to square the focuser in that way. The main thing that matters is the relationship between the mirrors and the focuser. The tube is irrelevant to a large degree.

[Black Knight]

I'm having the same problem. 3 times now I have completely recollimated my focuser from scratch, and although things have improved from the pre-focuser collimation situation, they are not right and I still get eggy stars. I followed Dions video tutorial on Astronomy shed by the letter, and although the process was simple enough, it ain't right (my focuser collimation, not Dions tutorial!) . Does anyone know of another resource that tackles this issue? I've been battling with this for almost a year.

[8kids]

You probably need to have you're collimation cap or cheshire further in to see the clips. I can't use my cheshire to see the clips I have a collimation cap with the focus tube all the way in.......I hope that makes sense.....

Gary.

[WaveSoarer]

I had the same issue with my 200p when I used the Cheshire. I just couldn't get all of the clips in view even with the focuser all the way in. It turns out that you need a collimation cap for this part as your eye is then closer to the secondary. I made one up from a paper circle with a hole in the middle and then all clips came clearly into view. It turns out that if you just want to use the Cheshire you need to adjust the tilt on the secondary until the Cheshire cross-hairs are lined up on the little black doughnut in the centre of the primary (if you've aligned the clips with a colly cap to start with then the Cheshire cross-hairs will be lined up on the doughnut already and the Cheshire confirms you've got it right). Once the secondary tilt is sorted out then you tweak the alignment of the primary until the reflection of the Cheshire target (the shiny bit at 45 degrees on the Cheshire) is aligned with the cross hair and centre doughnut. If the primary is quite a bit off, the act of tilting it may shift the centre doughnut slightly and you may need to tweak the secondary again before readjusting the primary for a final time. That should be it. The three-clips thing isn't ideal anyway as this assumes that all three clips protrude the same amount onto the mirror. This isn't quite the case on my 200p. I do wish that this point about alignment with the Cheshire was made more clear as, at least with my experience with the 200p, the clips are only visible with a colly cap and the cheshire cross-hair allows you to align the secondary without seeing them.

[Allerley]

I struggled for a while with my Quattro 10" but the astrobaby site helped me sort it. Also agree on getting the colli cap well down the focuser to see the clips.

Check this site it's gives you the best explanation I've ever seen and helped me to work out my collimation problems

http://www.propermotion.com/jwreed/ATM/Collimate/Chesire.htm

[umadog]

the clips are only visible with a colly cap and the cheshire cross-hair allows you to align the secondary without seeing them.

Exactly. If the secondary is rounded and centered in the focuser then sight-tube cross hairs allow you to centre the primary in the secondary. This will automatically do the mirror clip thing, should they be visible. So if you do it this way, you don't need to worry about the mirror clips or go to any lengths to seem them.

[lukebl]

You don't need to square the focuser in that way. The main thing that matters is the relationship between the mirrors and the focuser. The tube is irrelevant to a large degree.

...but most guides tell you to start with the focuser. If the focuser isn't aligned correctly, then nothing will be. So surely, you have to somehow ensure that it is square to the tube, and the way to do that is to align it to a spot directly opposite?

[moriniboy]

I check alignment of my focuser to a spot opposite every time

[Black Knight]

What process do you guys use to determine the point directly opposite the focuser?

I used the technique from Dion's video: wrap a piece of card around the inside of the tube, in both directions from the focuser, marking the end points on the tube, and halving the distance between those marks. I probably didn't explain that very well .

Sent from my mobile using Tapatalk, so please excuse the speeling and granma!

[moriniboy]

Thats the one, and I invariably find its in line with the opposite spider vane but its worth checking just in case

[lukebl]

....I used the technique from Dion's video: wrap a piece of card around the inside of the tube, in both directions from the focuser, marking the end points on the tube, and halving the distance between those marks. I probably didn't explain that very well...

You explained that very well! That's how I do it. Not that I'm any closer to sorting out the problem, though. The problem is that the small sensor of the CCD is very unforgiving, and will show up the slightest error in collimation, which you might not notice visually. My stars are eggy on one corner of the frame, but round on another corner, showing that it must be non-orthogonality, but I just can't get to the bottom of it, as the collimator says that collimation is good and I get good airy disks when viewing by eye.

[Black Knight]

Thats the one, and I invariably find its in line with the opposite spider vane but its worth checking just in case

Ok, I just wondered if there was another process I could try, to try and get my focuser collimation right.

Sent from my mobile using Tapatalk, so please excuse the speeling and granma!

[Black Knight]

You explained that very well! That's how I do it. Not that I'm any closer to sorting out the problem, though. The problem is that the small sensor of the CCD is very unforgiving, and will show up the slightest error in collimation, which you might not notice visually. My stars are eggy on one corner of the frame, but round on another corner, showing that it must be non-orthogonality, but I just can't get to the bottom of it, as the collimator says that collimation is good and I get good airy disks when viewing by eye.

Hehehe, you've just described my problem exactly! . Could it be focuser droop, as opposed to collimation as such?

Sent from my mobile using Tapatalk, so please excuse the speeling and granma!

[umadog]

The purpose of adjusting the primary and secondary mirrors is to align the optical axis of the instrument with the focuser. The focuser is stationary and the optics are adjusted around it. Unless your focuser is in some extreme position that it is precluding this from happening then you don't need to touch it. You do not, therefore, need to accurately square the focuser in the tube. It's irrelevant. Lowriders have focusers that aren't square to the tube but they're collimatable just fine (http://www.reinervogel.net/lowrider/lowrider_e.html). That's because the optics are square WRT to the focuser. In fact, if you think you're having focuser issues then you can adjust the focuser tilt WRT to the secondary using sighttube and the primary mirror spot. A while ago I was having trouble achieving a round secondary and centred primary. In the end, I centered the primary in the secondary and adjusted focuser tilt instead of secondary tilt. Now everything looks right. No need to remove the secondary and mess about with the spots on the OTA that aren't related to the optical train.