Collimation issue

[johnnyringo]

Hi Guys

Some how I have introduced a misalignment into my optics that I can't seem to correct . I was hoping that someone would recognise the symptoms and point me in the right direction.

I removed and cleaned the primary mirror on my 10" newt, a task I have carried out many times over the years with no issues. I replaced and collimated as usual using a lazer to check and all seems aligned. Once I started imaging again it was obvious that something was very wrong, the previously pin point stars are now elongated. at first I thought it was a tracking issue, but soon eliminated this by taking exposures of varying lengths which resulted in elongations of exactly the same size. I then slewed to a bright star to check the focus using the diffraction spikes as a guide. This is normally very simple, focus until eight spikes merge into just four. This time however, as I focus it is impossible to merge all four, as two combine the other two open up and visa versa?

This driving me mad, particularly because there are clear skies tonight!! typical

I would appreciate any advice.

Thanks in advance

John

[Tinker1947]

Have you checked the laser collimation...

[johnnyringo]

Hi Tinker1947

Thanks for the the quick reply. Yes I have many times, which is what is confusing me. The laser hits exactly the centre of the pre-marked mirror and again exactly down the hole in the viewing window of the collimator. It doesn't make sense.

I can't work out if the symptoms suggest an issue with the primary or the secondary.

[johnnyringo]

No suggestions at all..... I'm really surprised!?!

Oh well, I guess I'll figure it out on my own.

[Parus major]

The laser can't account if the secondary is out of whack, though. As long as the secondary plane at the correct angle the laser dot will come back so the secondary coule be too far in the tube and a little bit to the side so the laser will come to the point. I'd check with a cheshire to get the secondary correct.

[johnnyringo]

Thanks Parus Major

Yep that makes sense... I have a cheshire somewhere, I'll give it a try in the morning.

The scope has seen some pretty severe temperatures over the past few nights, and has been completely frosted over. I wonder whether this may have something to do with it, it is a steel tube after all.

I'll report back tomorrow! :

[devilgas]

when tinker says check the laser collimation, he's referring to checking that the laser itself is collimated. when you rotate the laser in the EP holder, does the beam describe a circle on the primary? if it does, then the scope collimation will always be out because the laser beam is not perfectly parallel to the laser collimator housing.

there's various articles on here / google on how to do this.

astrobabe has created a great article on collimation which discusses getting the secondary aligned etc.

i had the same symptoms with my scope. my laser was collimated, but the secondary was out of kilter.

hth.

[johnnyringo]

Hi Devilgas

Thanks for the suggestions. To be completely honest, I've heard mention many times about collimating the collimator, and its one of those jobs I keep promising my self I will do one day I guess the time to get it done is here.

I've been using reflectors for years now..... but for visual use. Now I'm imaging I suppose even the slightest misalignment will show up!

Well, I've dug out the cheshire, brought the OTA in from the observatory and cleared this evening from distraction. I will check the collimation of the laser and get this problem sorted.

Thanks again for your help.

[johnnyringo]

Hi Guys

Just a quick update for your info.

After a full week of pulling my hair out trying to solve this issue.... I've cracked it!!!

I was literally on the verge of giving up and resigning to the fact that I'd killed my scope, when....... bingo!

Believe it or not it was nothing to do will collimation (well sort of actually). And the culprit....... Over tigtened primary mirror clips!

Who'd have thought it!!! Oh well lesson learnt and at least now I'm a little wiser..... with slightly less hair .

Thanks all

John

[umadog]

Glad you fixed it!