Laser collimation on fast scopes

[Bart]

Hi All,

I always have a sneaky feeling I'm not quite there with my F5 150mm Newt and the Cheshire collimation. I get nice diffraction defocussing one way and slightly off centre defocussing the other way. Would a laser unit help here and remove doubt, and messing around? Thanks for help

Bart

[Nova]

id say its difficult to answer that for certain. only real way to find out is by trying but you should be able to acheive collimation with just your cheshire. a laser can show correct collimation even if your secondary is out. also collimation results with a laser will depend on how well calibrated the laser is in the first. imo stick with the cheshire. I was recently pointed towards this guide

John Reed Home Page Collimation - Cheshire

When I found (via my recently purchased cheshire) that my optics were off.

[TheThing]

I use a laser on a fast (f5) newt and it is simplicity itself. Done in seconds and always spot on.

Having said that, with care, my newts hold collimation for a good long while.

TheThing.

Sent from my iPhone using Tapatalk

[Damo636]

I would stick with the Cheshire also. I sold my Baader unit a few weeks ago as i found i got a different result almost every time i inserted it in the focuser even though the Cheshire showed alignment to be spot on!

[Bart]

thanks Gents, I'll stick with the cheshire for the moment and 'perfect ' my technique, ahem ..

[661-pete]

I find that the laser is spot-on for collimating the secondary, provided that you have accurately centre-spotted the primary in the first place. But then for collimating the primary I switch to the cheshire - more accurate. The problem with the laser is that over the length of the optical axis, twice over, the beam spreads quite a bit.

[John]

I've become quite a fan of the barlowed-laser technique for adjusting the primary mirror. There is a good description of the technique here:

Astromart Articles - Barlowed Laser Collimation Made Easy

It works well with my 10" newtonian which is F/4.8

[acey]

The star is the most accurate test. If you see a slightly asymmetrical pattern on a star at the centre of FOV, try moving it off-centre until the pattern looks symmetrical. Then adjust the primary to bring the star to the centre. In practice this is difficult, of course, if the scope is not driven and the star is drifting at high power. In fact, if the collimation is only very slightly out then it might be only when star-testing that you ever notice the effects: in ordinary viewing it probably makes no difference. Near enough is better than trying to reach perfection.

[umadog]

I use an autocollimator. It's the most accurate tool out there and supplies a wealth of information if you know how to read what it's telling you. More informative than a laser. Cheaper, too.

I don't know what would cause what you see but I'd look into these: You're sure the secondary is correctly centred and not rotated? Are you sure the centre spot is correctly positioned?

EDIT:

Look around on Cloudy Nights. There are some old threads there (see sticky on Newt forum) which go into a lot of detail on these topics.

[TheThing]

I'd say look on this forum rather than CN! Much more friendly.

I wouldn't be without my laser. Makes the job so easy and spot on everytime, plus they can be picked up cheap 2nd hand, like most Astro equip.

TheThing.

Sent from my iPhone using Tapatalk

[Bart]

"I don't know what would cause what you see but I'd look into these: You're sure the secondary is correctly centred and not rotated? Are you sure the centre spot is correctly positioned?"

Well, Umadog, I thought I had it centred and never really checked the centre spot as it came like that and never thought to check it.

If I am that close what difference will it make at mag x180 ?? Thanks

Bart

PS Thanks to all for the advice

[umadog]

It's often said that the star test is very sensitive and will reveal stuff that isn't a problem in the real world. This really is an endless discussion topic. It depends exactly what you saw and how large the effect was, etc. I'm not clear what you saw. What does "off centre" mean? Are the diffraction rings not circular or are they simply not concentric? How far are you defocusing? For the star test you don't have to defocus very far. I've read that stuff which looks bad well outside of focus is probably not an issue.

I have noticed that when I'm aligned and looking through the autocollimator, the reflection patterns are stacked when I in-focus but come apart slightly at a particular point as I out-focus. Small glitch in the focuser, I would imagine. This sort of asymmetry could be what you're seeing (wild guess). It is for this reason that you should collimate with you focuser racked to the focal point: it minimises issues of this sort.

[Bart]

Interesting Umadog. yes I am defocussing a fair bit out.Saying that, I could see Jupiters bands reasonably well last night, with it just at Alt 20 degrees (ish). Had a great night, got another 3 Messiers and Andomeda and associated galaxies were spectacular. Milky way was magnificent. North American Neb still eludes me, even with UHC filter. Cheers Umadog

[umadog]

If the images look good, then you're probably ok. f/5 is still relatively forgiving. you should be accurate but you probably don't need to obsess about it.

As for the N. America nebula, it ought to be visible where you are. There are some fairly dark skies around Cork. My bet is that you've not realised how big it is. It's 2 degrees across so it'll fill the field of view of even a low power eyepiece. Try with binoculars first. Go looking for a very extended nebulosity with the UHC filter. Check out this great article: http://media.skyandtelescope.com/documents/DSW_Oct2004.pdf

On Saturday I saw it for the first time with the big Dob. I used a narrow-band LP filter and it was pretty spectacular. At 60x the Gulf of Mexico regions fills the whole field. Really wonderful!

[Bart]

You're making me jealous now. That is the one that seem to elude me. I know its big and therefore know I'm lloking at it, I just can't isolate it from the rest of the starfield in the milky way.

Bart

[John]

....That is the one that seem to elude me. I know its big and therefore know I'm lloking at it....

The NA Neb has eluded me for years as well. I eventually managed to glimpse it a couple of months back with my 4" refractor at 30x with an O-III filter. Even then it was very indistinct and I could only make out parts of what is seen in the photos.

To put it into context I could see both the east and west portions of the Veil Nebula with the same scope and filter really clearly that night but the NA Nebula was nowhere near as obvious.

I've not had a clear dark night to try it again since

[umadog]

Bart, the first time I saw it in a 12" I also had that sensation. I knew it was there in binos but through the 12" I was really confused as to what I was seeing. Spend some time on it: figure out where the different bits ought to be using an atlas or that article. Take it one step at a time. Once you've made sense out of it you should see it (conditions willing, of course).