My scopes star test.

[Garyh]

Hi folks,
Well have had some time to kill at night since the digital drive is out for the moment. So decided to setup a artifical star and catch some avi`s with the toucam.
These images were done with the smallest pinhole I could do in foil on my LED torch at a distance of 50m. I found capturing clean airy disk extremely difficult and even with the star defocused by around 8 wl still the air currents etc distorted everything. The spider vanes show up very clearly as well in the patterns.
Here is a image of a single frame inside and outside focus with my 8" f/4.6.
My collimation is still out a little so might do this again more carefully.

What is your verdict?
I think it shows slight overcorrection but I might be totally wrong!

Also what effects would the GSO style mirror cell have on the figure as these cells have support right on the very edge of the mirror via 3 thin cork pads?
Tried to find some info on this on the web but come up with nothing.
Surely this would not be the best way to support a mirror.

Any imput appreicated!

cheers Gary

[gbeal]

Oh man, get that drive going again, and stop looking too deep with the gear, LOL. Your images speak for themselves they are awesome, so be happy with the ability to produce them, and the gear to do it.
In as much as primary support. I like to use three equally spaced blobs of silicone an nothing else. I normally use a "base" of alloy for this, but the GSO primaries seem to be thick enough, or over-engineered to the extent that I doubt they will "twist". The three cork pads would be just as good.
Gary

[Merlin66 (Ken)]

Do the same test with the Genesis....
You should get "perfect" disks for comparison...
That way you can eliminate the seeing artifacts and be able to get a better analysis.
A Rochi grating should also show the over/ under correction.
The Roddier freeware software can also be used to estimate the accuracy of the optics based on webcam images.

[bmitchell82 (Brendan)]

so making a fake star, how far away should i put it to make it useful. (if i was using the super bright led and fibre optic technique?)

[Merlin66 (Ken)]

In Souter's Book " Star Testing Astronomical Telescopes", p82 he quotes Welford who recommended 20 times the focal length as a minimum distance to the pinhole.
He also gives this formulae:
N(ft) = 28 *(D/F)(D/F) (D in inches) where D is the aperture and F is the focal ratio.
To achieve a good Airy Disk, the size of the pinhole when viewed by the telescope should be close to the Airy limit, for that scope.
ie for a 200mm" f5 he gives a pinhole of 0.14mm diameter.
I've recently converted my old Foucault tester and fitted a 50micron fibre; this gives a very good starlike image. I didn't use the LED solution as I can use the fibre with various lamps ( flouro, incandecent and Neon) to help in testing spectroscopes.
Hope this helps.

[bmitchell82 (Brendan)]

that almost looks like len's law. should be good thank you for your reply. The reason why i want to do it is im using a sw newtonian and using it for astro photography.
once again thanks

[Garyh]

Originally Posted by Merlin66

Do the same test with the Genesis....
You should get "perfect" disks for comparison...
That way you can eliminate the seeing artifacts and be able to get a better analysis.
A Rochi grating should also show the over/ under correction.
The Roddier freeware software can also be used to estimate the accuracy of the optics based on webcam images.

Thanks Merlin66 for that!
Downloaded Winroddier (was fun trying to find it) but it is all in french and you can only bring in fit files. Might need some help with this one!
But looks like a decent bit of freeware to have!

Here is the link if anyone else is interested..
http://www.astrosurf.com/tests/visuel/visuel.htm

Also that formula you gave, would that give you a certain amount of overcorrection? The further away the artifical star the better as accuracy?

Thanks Gazz, but it`s nice to know exactly how good one`s system is!
Still no harm in improving the scope! so I can push it even further..

Might redo this to get a better result and stack a few hundred to help smooth out the turbulence.

cheers Gary

[bmitchell82 (Brendan)]

Just thinking I have a massive park (2 baseball fields wide) over the road, would that be the best place to put it? i think it would be a good 50 meters and over grass let the evening sky cool it down a wee bit there shouldn't be too much in the way of haze

[Garyh]

Hi Brendan,
Over grass would be the best if possible and let everything cool down for a hour or more.
Have done some research and have heard that you should double the minimum distance to get a more accurate result. I have found that the slightest breeze here really upsets the rings. Have to wait for a very calm night and work out this program!

[Merlin66 (Ken)]

The version V2.2 has been translated into English and is available from the Roddier Yahoo forum.
The further to the pinhole the better.
The formulae given was a minimum based on better then 1/4 wave residual spherical abberation when a paraboloid is tested.

[Satchmo]

Your mirror seems to have a slightly over corrected center and a little undercorrected edge. It is certainly better than 1/4 wave wavefront. Your mirror will be amply good for the job you made it for.

[Garyh]

Originally Posted by Merlin66

The version V2.2 has been translated into English and is available from the Roddier Yahoo forum.
The further to the pinhole the better.
The formulae given was a minimum based on better then 1/4 wave residual spherical abberation when a paraboloid is tested.

Thanks for that, I shall get the English version! and have a play..

Originally Posted by Satchmo

Your mirror seems to have a slightly over corrected center and a little undercorrected edge. It is certainly better than 1/4 wave wavefront. Your mirror will be amply good for the job you made it for.

Thanks Mark for that... Tried getting some images with the toucam as the seeing looked steady and couldn`t do much else anyway, but still not very good, those internal refections from the uv/ir filter and even some from the powermate seem to superinpose themselves over the defocused star.....I don`t know if you would get a acurate result also the effects from the spider vanes....

But I did a Ronchi star test which did show nice straight lines with just a hint of the outer 5mm or so being slightly under corrected but seems very minor. So I am happy with what it showed.

cheers and thanks...