Artificial Star

[A40farinagolf]

I've made an artificial star based on Olly's suggestions and experience by using a 3mm diameter ball bearing taken from a roller skate.

My primary objective is to focus my Baarder LVI Smartguider camera by reducing the number of pixels illuminated on the hand controller to a minimum.

The camera is looking through my 80mm guidescope (with a 328mm FL) so @ 10m away will equate to 30x the FL. and the ball bearing is glued to a piece of matt black vinyl that will be mounted onto the washing line post.

As soon as it stops raining I'll give it a go and will feedback my findings.

[ollypenrice]

I visited Dany Cardoen's optical shop last year. He has mirrors at Paranal and did the mirror for the 0.8 metre Ritchey Chrétien at the Observatoire des Baronnies - along with his own 1 metre RC. He uses a ballbearing.

Olly

[spaceboy]

I made myself one out of a simple led pocket torch. Get some black plastic from a container of some sort (I used the plastic case my PC DDR3 memory came in) and put the smallest pin hole you can in it then simply tape over the end of the torch.

I have to have it some way away from my scope but it works showing out of focus diffraction rings.

[ollypenrice]

Ball bearings and sunlight work well in sunlight for visual adjustment but are too bright for normal CCD cameras, even with a fraction of a second exposure.  For CCD use, I have a Maglight LED torch, which I diffuse with a tissue and place at a distance so that it subtends 0.5 degrees (as the sun does).

Now here is the fun part - for RCT collimation, you need a central and peripheral stars for star testing the primary and secondary mirror alignments.  I bought 9 balls and glued them to a 500-mm square black planter base (from a garden center). (One center ball and the others in a circle.) I place this at 25x the focal length of the RCT and it fills my 8300-based CCD just right for a good view of the balance of aberrations.

If you can, take a look at Harold Suiter's book on star testing. It has a lot of useful information.

regards

Chris Woodhouse

How about a narrowband filter?

Or do the test in the dark with a controllable illumination of the ballbearing? I've never used the BB trick using sunlight, only a torch beam.

Olly

[Macavity]

One I made earlier.

http://stargazerslounge.com/uploads/monthly_08_2013/post-539-0-25390800-1376986947.jpg

The challenge is to drill a small enough hole or incorporate fibre optics somehow? OR

you can make use of "optics" to create a reduced image of a easily drilled larger hole!

The above was adopted from a rather clever: http://observatory.mvastro.org/library/Star_Test/ArtStar.html

My version is based on 2" PVC pipe with a 2" --> 1.25" adapter whacked(!) into one end.

I then made a little assembly out of MDF (cut into) disks and M4 screws to hold an ultra

bright white LED. The beam is created via a small hole in Aluminium cut from the end a

tin can! A miniature potentiometer is used to control the brightness... etc. etc.  

By changing eyepieces you can get a variety of effective angular size etc. I even used a

Baader solar continuum filter to get obtain "monochromatic" light for testing an achromat.

To be perfectly honest I don't get a lot of use out of it. My garden just isn't LONG enough.

(Racking out the eyepiece / camera sufficiently to obtain focus is not the "normal" setup!)

[Ajohn]

Some people have made them out of inkjet cartridges - hasn't worked out on the ones I have looked at - no signs of  a hole. Maybe they or it were blocked.

If people want to get light down a fibre optic more will come out if they drill down into the LED as close as they dare to the chip, add some oil and then put the fibre in. Bit like oiled objectives not needing to be polished as the oil nulls the surfaces out.

Personally I think a ball bearing is best. It's even possible to view diffraction rings off them if they are small. Only problem on say a SCT is it's best to have the scope pointing up rather than horizontal as there may be a tiny bit of mirror tip.

John

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