Success and a question

[Dujon]

Firstly the success. I managed to (I think - see later) collimate my Newtonian. As it happened the clouds of the last week or so cleared and allowed me to cart it and its mount on to my back balcony for a quick look around and a check of the optics.

Having drifted around Crux for a while I decided to wait until Jupiter had cleared the guttering of my house and have a dekko at it. Talk about bright, I nearly went to get my sunglasses. The seeing wasn't all that good and the Moon was climbing quickly after it - the equatorial bands were drifting in and out of sight - but three moons were easily visible. One had only just cleared Jupiter's limb; at first I thought it might have been a background star. After a while it crept away slowly from the planet but then started to disappear and, until the penny dropped, I thought the atmospheric conditions were conspiring against me. Then I realised that it might be being eclipsed. A quick trip back to the house (and blinding light) plus reference to the latest info. on Jupiter told me that it was indeed an eclipse of Ganymede. It was still visible but quite faint but I didn't stick around to see what would happen as it was getting close to kick-off time in the football (soccer).

Sorry about that, but it was so pleasing to have the new toy performing reasonably well. I won't post such stuff again . . . honest injun.

A couple of questions though which could well be related. I hope that some of you more experienced ladies and gentlemen might be able to help me out.

I wear glasses and, particularly in my right eye, suffer from astigmatism.

I use my left eye for viewing.

If I star test by defocussing each side of a reasonably bright star then on one side of the image I see a dark centre circle surrounded, not by rings but a series of radial lines; it's almost like an iris surrounding a pupil. On the other side of focus I see a three-lobed apparition - it almost looks like a round winged moth. That is, it has two major lobes and a shorter ovalish extension in one direction at right angles to the other two. If I move my head relative to the eyepiece it follows my eye around. I'm assuming, but am not too sure, that this is the result of my own astigmatism rather than that of the telescope's optics.

Possibly related is that when I collimated the 'scope I used a visual method in the first instance and then followed that up by using a laser collimator. The laser's red dot fits nicely into the centre of the return path in the collimating tool itself. However, when I then look down the focussing tube at the secondary and primary mirrors I see the primary mirror's three holding mounts and a single secondary mirror mount centred in the field of view as well as the four spiders of the secondary's mount. The reflection of my eye in the focussing tube though is off centre. By that I mean that it's like looking at the focussing tube from 'the other end' but at an angle.

I'm a bit confused. Can laser collimation give one a false reading? Do I need only to do a bit of tweaking? Have I lost the plot?

To those of you who view whilst wearing spectacles: Do you suffer from glare when viewing bright objects? Whilst viewing Jupiter I found it jolly annoying as it's like looking through a slightly misted pane of glass. Taking off my specs would clear the problem but, obviously, degrade the image.

Err, sorry, that's a bit of a long post.

[rmcpb (Rob)]

If I move my head relative to the eyepiece it follows my eye around. I'm assuming, but am not too sure, that this is the result of my own astigmatism rather than that of the telescope's optics.

Yep, if the problem rotates along with your eye then its your eye at fault not the scope.

I'm a bit confused. Can laser collimation give one a false reading? Do I need only to do a bit of tweaking? Have I lost the plot?

No, you haven't lost the plot. Lasers have to be collimated themselves, and to be honest, most reasonable ones are. If the laser is not aligned correctly then it will cause problems. If it is held in a cradle and pointed at a far wall then rotated the dot should not move.

With collimation ensure you work form the focuser out ie. secondary centred in the focuser, secondary pointed at the centre of the primary, primary pointed at the centre of the secondary.

If you are interested the Western Sydney Amateur Astronomy Group meet monthly and hold regular viewing nights at Linden, check the Calendar page for details. You would be able to get lots of help with collimation there.

Cheers

[astro_nutt]

I agree about collimating lasers...some of them can be harder to collimate than a scope!...I align mine over a distance of 5 metres using a V-block mount supplied with the laser about once a year..generally it's out by no more than 1 mm..it takes about 10-15 minutes

[Blue Skies (Jacquie)]

The word is....offset!!!

I wouldn't blame the laser, the problem is inherent in the Newtonian design. To get a correct collimation the secondary mirror actually has to be moved away and down from the focuser. If you use a a sight tube/ cheshire to visually collimate you fix the down shift automatically.

If you want to keep using the laser as a collimator do a little research on offset in Newtonians - I know there are have been some big discussions on it in this forum in the past.

As I've said before, lasers are great for big truss tube dobs, that actually have a measurable offset, but in a small closed-tube dob get yourself a cheshire and save yourself a heap of trouble. At worst, only use the laser for the secondary adjustment and use a cheshire for the primary mirror.

[Dujon]

Thanks, all, much appreciated and taken on board. I'll have another go (and search the site for 'offset' info).

[janoskiss (Steve H)]

Offset will take care of itself automatically, if you follow the usual collimation steps properly. Have a look at this video tutorial on collimation: http://www.andysshotglass.com/Collimating.html

Cheaper lasers can go out of collimation themselves and don't respond well to being recollimated (the keychain laser unit inside the housing tends to burn out quickly afterwards).

[Dujon]

Thanks, Steve. I don't think I'll be playing with the laser collimation.

A search of the site came up with LaFey's drawing of his problem, which (the left hand one) seems to equate to the effect that I am seeing. It seems that the general consensus of the time was that is what one should see when offset is taken into account. In my case the 'cross hairs' are the spider's vanes.

http://www.iceinspace.com.au/forum/s....php?t=9436#12

For the moment I think I'll just ensure that the primary mirror centre spot (ring binder washer) aligns with the centre of the reflection my eye, the spider is centred and the laser tool indicates the return reflection is also centred in its viewing window.

It seems that I was worrying without cause. Thanks again everyone.


John