Cheshire collimation question..

[tonel]

Hello,

Hoping someome might be able to help, have a question about collimating a Skywatcher 200P Dob using a Cheshire collimator. I've had a go at collimating using Astro Baby's guide which was very useful, but am a bit confused regarding the final part where the Cheshire is used to check the primary alignment. So far I've used a collimating cap to centre the secondary and check it's alignment which seems okay. But when looking through the Cheshire although everything initially seemed to look okay (can see primary clips, secondary still central and can see reflection of Cheshire crosshairs in reflection which are aligned with the primary's centre spot) the actual crosshairs of the Cheshire are adrift (to the left) of everything.

Sorry this is a bit long-winded but my main question is should the crosshairs of the Cheshire perfectly overlap the spidervanes holding the secondary? Am quite confused as I thought I had it but seems I might be quite a way off, especially the couple of occasions I've used the scope recently the image quality has been poor (although could be down to insufficient cooldown time I guess). Apologies if it's a daft question, any help gratefully received

[Craig20264]

Ignore the cheshire crosshairs. I had the same problem with the same scope and was given this advice. I can't get them to line up with spider vanes. As long as your secondary holder is central you're ok. (I did this by measurring as per Astrobaby's guide)

[sunshine185]

Hi, take no notice of the crosshairs not being central, it its the reflection of the crosshairs that matter. I heard that some people actually remove the crosshairs from the Cheshire completely.

[RikM]

I ignore the spider vanes and mirror clips and tweak the secondary until the Cheshire cross-hairs line up over the dark dot, then tweak the primary to make the doughnut line up with the dark dot and Cheshire cross-hairs (the reflection of the Cheshire cross-hairs will line up too). You may have to repeat a few times and it will get closer each time.

[RikM]

http://www.skyandtelescope.com/howto/diy/3306876.html

Edit: sorry, wrong link now corrected.

[Jason D]

should the crosshairs of the Cheshire perfectly overlap the spidervanes holding the secondary? Am quite confused as I thought I had it but seems I might be quite a way off, especially the couple of occasions I've used the scope recently the image quality has been poor (although could be down to insufficient cooldown time I guess). Apologies if it's a daft question, any help gratefully received

As other members have suggested, ignore the spider vanes. Also ignore the secondary mirror shadow. See attachments.

Jason

[tonel]

Thanks for all the advice - think I know where I've been going wrong, going to give it another go. Just hoping we get some half-decent weather to do some observing sometime soon, been terrible here recently. Thanks again!

[Moonshane]

for me the main steps of collimation (of a newtonian) are:

1) check that the spider vanes are all the same length - this ensures the secondary assembly is central in the tube.

2) looking down the empty focuser (preferably with a collimation cap) check that the coated surface of the secondary is central with the focuser (you can check that the focuser is square etc but I don't bother generally). this ensures the secondary is far enough down the tube.

3) check that the coated surface of the secondary looks round when looking down the empty focuser.

4) check that the reflected donut on the primary (but reflected in the secondary) is central to the crosshairs on the Cheshire.

Generally step 1) needs to be done once ever. Steps 2-4) need to be checked each time but will not often need adjustment.

5) Check that the black dot of the reflected Cheshire is central to the reflected donut on the primary and that this adjustment has not affected 4).

Preferably steps 2-5) but at least Step 5) should be done each time you observe.

it sounds hard but once you understand it, you'll do it quickly and easily.

this is one of those things that if someone shows you rather than you trying to fudge your way through instructions, it's soooooo much easier!

[RikM]

for me the main steps of collimation (of a newtonian) are:

1) check that the spider vanes are all the same length - this ensures the secondary assembly is central in the tube.

2) looking down the empty focuser (preferably with a collimation cap) check that the coated surface of the secondary is central with the focuser (you can check that the focuser is square etc but I don't bother generally). this ensures the secondary is far enough down the tube.

3) check that the coated surface of the secondary looks round when looking down the empty focuser.

4) check that the reflected donut on the primary (but reflected in the secondary) is central to the crosshairs on the Cheshire.

Generally step 1) needs to be done once ever. Steps 2-4) need to be checked each time but will not often need adjustment.

5) Check that the black dot of the reflected Cheshire is central to the reflected donut on the primary and that this adjustment has not affected 4).

Preferably steps 2-5) but at least Step 5) should be done each time you observe.

it sound shard but once you understand it, you'll do it quickly and easily.

this is one of those things that if someone shows you rather than you trying to fudge your way through instructions, it's soooooo much easier!

That's exactly the way I do it, only explained much more clearly