Star testing

[mik]

Hello without this turning into a major colliminating thread i would like some help and advise on star testing, i am trying to get used to my XLT150 f5 newt

and have recently purchased a cheshire(thanks Steve/FLO) and with the help of Astro-Babys superb primer (Thanks) have managed to colliminate the scope

now i understand that to fine tune the scope it is advised to carry out a star test after the tube has cooled (how long to cool ??) and also im lead to beleive that a star test can only be done in good seeing and at higher power

(What power for my scope ??) and lastly how do i know what primary mirror

Collimination knob to turn and how much.

Many thanks Mick :?

[Astro_Baby]

Okies for a smallish newt I would suggest 40 minutes at least BUT for absolute guarantee I;d take the scope out once its cool in the evening and then just leave it there until its dark so a couple of hours.

Star testing can only really be done with good seeing. My guide has some pics of what it looks like if the seeing is bad. My rule of thumb ( and I stand to be corrected ) is if you can see the Airy disk conditions are ok. If not try again but dont assume you have a major problem.

High power - I was told go to the scopes max power if possible - ie 2x for every mm of aperture which would be x300 for your scope. So you;d need either a 2.5mm EP or a 5mm with a x2 Barlow. Personally I find such things a bit presecriptice and I have found star testing can be carried out with much lower powers. I would use about 10mm on mine giving a magnification of 100 when its theoretical max is 400. In a nutshell you only want enough magnification so that when you go out of focus you get a decent sized disk in view.

Which knob to turn - well thats a tough one. I just start with whichever one is nearest - give it a tweak and see what happens visiually. If its the wrong one I set it back to where it was and try the next one. YOu'll very quickly see whats happening with a star test. I mean there wont be much doubt as you'll see the dark centre of the disk start to shift.

I think you'll find that if the Cheshire is showing good I'd doubt there woulkd be much wrong with star testing. To be honest I have done it a couple of times only because the star test is always so close to perfect after using the Cheshire any imprefcetions have been so small as to be not worth messing about to correct.

Hope thats of some help

[narrowbandpaul]

you can purchase an artificial star or picostar, for when its cloudy. It does the same job as a real star, but with less atmosphere to be adultered by. Again, do it at night so that you can see the airy disc. Collimation is achieved when the rings are concentric

Paul

[mik]

Thanks for the pointers next clear night i will get out and have a look cheers

Mick

[lunator]

Hi Mick

If you can get your scope really well collimated you will be amazed at what can be resolved.

What you will probably see when you go up to x300 is a star surrounded by a couple of rings that will be a bit brighter on one side.

When you make adjustments move the screws my a tiny amount as at this level small adjustments make large changes in the view.

I hope this helps.

Cheers

Ian

[Andrew*]

Sorry, I don't get it. :oops:

Do you go way out of focus so you see the secondary shadow? Or just a tiny bit? If only a tiny bit, your eyes will anyway adjust the focus, so it will go back into focus...

I've never seen any rings around a star - are they bright?

Also, if you move the mirror, you will also move the position of the star in the field, and at 300x won't you lose it?

Finally, if you do this with a big dob, you have to go to the back of the tube, then back to the front every time... :insects: How do others do this on their own?

Sounds like an awfully tricky process to me.

Andrew

[mik]

Thanks Ian when you say move the screws a tiny amount are we talking 1/4 of a turn or less here ?

Mick

[Astro_Baby]

Mik - less - mine will jump out of collimation in about 1/8th of a turn. Have a look using a Cheshire and turn one of the screws a little bit - that will give you an idea.

Atropethean - take a look at a bright star and defocus slowly - you'll see a dark centre spot emerge surrounded by rings - looks kind of like a polo mint.

[narrowbandpaul]

i take it sensitivity to collimation depends on focal ratio, much like for focussing?

[GazOC]

Yep, scopes get a lot more fussy the faster you go.

[narrowbandpaul]

cheers gaz, thought as much

[supernova]

Yep, scopes get a lot more fussy the faster you go.

Well collimate slowly then :scratch:

[narrowbandpaul]

lmao

[mik]

Thanks for all the help with this ive never been a big fan of newts but as i have one i thought i would get used to how they behave and i must say i am getting quite attached i am going to stick with it over the winter months and if i convert i will be looking at the Orion optics europa 150 f11 1/8pv ota for my planetary/lunar and double star work,things are looking good so far.

Mick.

[lunator]

Thanks Ian when you say move the screws a tiny amount are we talking 1/4 of a turn or less here ?

Mick

Hi Mik

When I collimated my scope at x400 mag I had to adjust the screws by mm's. Once you are that close to perfect collimation you only need to make tiny adjustments. If you adjust to much the star will exit the field of view.................I have done this more than once...

[Astro_Baby]

ive never been a big fan of newts but as i have one i thought i would get used to how they behave and i must say i am getting quite attached

A convert to the clan of the newt owners Come over to the dark side

The nice thing with a newt is if its cloudy you can always potter about perfecting your collimation

[mik]

Well its been a few weeks but i managed to get out this morning 02.00 to see how i had done with the collimination there was a lot of whispy cloud about so a star test was out of the question but after the scope had cooled since midnight on the patio i thought i would at least have a look about, M45 was the first target with the 20mm swan i was unable to fit it all into the fov but focused images seemed satisfactory, next target was Capella focus was good with a nice set of thin crisp diffraction spikes cloud was starting to gather so time to call it a night but happy that my collimination is good just need a nice steady night and more power to give it the final tweak if needed.

Astro-Baby yes see what you mean about fiddling, now to make a dewshield and sort myself out with some new eyepeices but thats another story

Mick..