Distance for artificial star.

[cfranks]

Hi,

I'm having a great deal of difficulty in collimating my DSI RC10C scope using the stars. I don't have any eyepieces so have to use a camera which adds to the difficulty. I have an Artificial star that I can use but can't find it in the scope. Does anyone know what distance (approx) would I need to place it to be able to focus on it. The scope is 1854 fl, aperture 250 with the secondary obstruction 50%. and the focusing is done by moving the secondary. I will have to go 'mobile' somewhere but would like a suitable distance before I go.

Thanks
Charles

[Merlin66]

Suiter, "Star Testing Astronomical Telescopes", p85 recommends x20 the focal length.

[multiweb]

With 600mm up to 800mm I find that I can do this indoors. You only need a 9m line of sight or so.
With the C11 I have to do that outside. Set up in the garage and look across the road in the reserve at probably 20m away.

This won't be far enough in any case to judge optics because the focus position will be a fair way out from infinity so depending on the scope you'll get spherical aberrations in and out of focus patterns but it is more than enough to collimate and center the rings in the airy pattern.

To find the guide star you need to have a finder and set it up during day time if you're outdoors so you match what you see in the scope with the finder. Everytime you collimate the star will move off center so you'll have to re-adjust that all the time.

[casstony]

I can collimate a C8 indoors in a long passage but larger scopes need to be done outside on an overcast day - a cloudy, calm day lets me push the magnification up to get accurate collimation. For collimation it only matters that you're far enough away to achieve focus.

[cfranks]

Thanks Guys!

Ken, I have Suiter's book, but of course, had forgotten about it! He says those distances won't work where the focusing mechanism damages the optical correction. RC's do that. Anyway, you've all given me some good points. I had tried before but found I couldn't even focus on a hill 1Km away. Maybe I was doing something wrong.

Thanks again.
Charles

[Astro_Bot]

22 metres - for a Meade LX200 10" - focal length 2500mm. I do it indoors since I have a long corridor that opens into the lounge at one end and garage at the other, so I can use the full length of the house, less space for me to get behind the mount/OTA. That setup is also useful for centring a polar scope reticle.

I find indoor collimation works just fine as I rarely have to adjust the collimation outdoors and, even if I do, it's the tiniest of adjustments.

[tlgerdes]

20M across the road :lol: your kidding me. It would be 35-40M. From your garage.

[multiweb]

26m to the bottom of the fence.

[Barrykgerdes]

This is only my theory so I could be wrong However

The use of an artificial star for fine collimation is to get around the tracking problem with a real star.

The object of a fine collimation is to make all the rays that form the images on the focal plane to arrive in phase from all parts of the mirror. This means extremely fine adjustment (micro distances - angstrom units).

The way we do it on a SCT is to adjust the secondary mirror to produce uniform Airy discs caused by diffraction of the light around the central obstruction. It is the phase difference between the diffracted rays and the direct rays that cause the Airy discs.

To get defined Airy discs from an artificial star the light from the artificial star needs to be a point source that subtends an angle (size in arc seconds) that is less than the real resolution of the telescope otherwise the disks will be blurred beyond recognition.

The distance will therefore need to be suitable for this to happen at a focal range that will allow the main mirror to be locked in position.

Barry

[cfranks]

Thanks Barry, that sounds sensible. I've got ~ 50 metres as the longest distance in my garden and my Astrozap artificial star is said to have an aperture of 100µ. Would that distance be sufficient? I'm having great difficulty trying to collimate using stars. :(

Charles

[Barrykgerdes]

At 50 metres that would be less than half an arc second so I would expect it to be satisfactory.

Barry

[rodmollise]

That is necessary when testing for optical quality to prevent the introduction of false spherical aberration. For collimation? No...all you need is the artificial star far enough away so that you can get it into focus--or almost into focus. ;)

Uncle Rod

[PeterHA]

Beside sthe capability of the scope to focus on near objects, the size of the artificial start is also a factor in determining the distance.

To get a cheap and very small star, here a simple low cost solution.

take a black cardborad or paper >250gr/ sqm large enough to cover the front of a flashlight, best a round white LED flashlight.
punch a 4-6mm hole in the middle of the cardborad
stick a piece of kitchen aluminium foil to one side of the cardboard, size of aluminium should be flashlight diameter.
Place the cardbord/ foil composit with the aluminium facing down on a hard surface, e.g. a glass plate
take the thinest most pointy needle from a sewing kit and press it once on the middle of the foil, this will leave a extremely small hole.
stick the cardbord to the flashlight with the aluminim foil facing the LED, is seal of with black sticky tape so no light comes out between the cardboard edge and the flashlight
Done.
I use that in my house with about 8-12m distance to the telescope and the image is absolutely perfect.

[AndrewJ]

Gday Peter



Or use an acupuncture needle press :P
I made up the gizmo below by gluing a std hypodermic needle into a block of jarrah so it sat just above the base line.
Then i got the finest acupuncture needle i could find and fed it through the hypodermic.
This allows you to easily align, then twist/push the acupuncture needle through the foil without the acupuncture needle bending or getting off course.

Andrew