Allan Gould sent me a PM the other day asking if I could come up with a collimation aid for imagers, and this is what I've come up with.
Al's Collimation Aid is a resizable transparent window with:
a small cross hair at the centre of the window
3 adjustable concentric circles
variable opacity of the circles and cross hairs
The concept is the "centre" circle can be adjusted to circle the centre spot of the Airy disc when it is visible, the "inner" circle is adjusted to match the inside diameter of the Airy disc, and the "outer" circle is adjusted to the diameter of the outside of the Airy disc. As the circles are concentric, any collimation error should be readily apparent.
The "centre" circle and cross hairs can also be used to centre on the star while in focus and then the inner and outer circles used to estimate the amount of mirror flop or misalignment of the focuser as the focus is adjusted.
If there's too many circles on the screen for you, just adjust them to zero radius.
As its cloudy here I can't exactly test it myself for real, but feel free to try it out and give me feedback.
This Airy Disc image was stolen from the web to illustrate how Al's Collimation Aid works. Unfortunately, the collimation is pretty good (perhaps a tiny bit of astigmatism)... a poorly collimated Airy disc would probably illustrate it better.
Perhaps just to explain...
Lets say we aligned the inner circle with the inside of the Airy disc... if the scope is out of collimation the centre dot will be offset and not in the centre of the centre circle or cross hairs, and the outer edge of the Airy disc will be offset the other way relative to the outer circle when its adjusted to the right size. Clear as mud?
Al.
Last edited by sheeny; 22-06-2010 at 09:01 PM.
Reason: Afterthought
That's brilliantly simple lateral thinking there Al & Al. It's this sort of idea that I admire the most. Well done, and I'm not sure why no-one has come up with it before... invention, necessity, mothers.. all that.
This is an example of an SCT and miscollimation. Its particularly good with an SCT as the central x-hairs that Al has put there enable you to place that on the small white central dot you get with these scopes and then place the circles on the image from your camera and it shows you where the eccentricity lies.
One nice little improvement for the dyslexic among us would be to place three numbers (moveable on the circumferance of the outer circle) so that you could rotate this to line up with your Bob's knobs or the collimation screws so that you knew which one to turn to bring the airy disk back to centre. Just a thought, Al.
But as it is at the moment its a ripper.
Allan
Last edited by allan gould; 23-06-2010 at 09:03 AM.
I'll certainly be using it to properly collimate my Mewlon. It'll be a real boon as once I have it tracking, I can fiddle with the Bobs Knobs at the front while staring at the monitor without having to wizz back around to the EP all the time. It can be done in real time, which is an absolute bonus.
Al, if I may, could I suggest that it may be useful if you put another set of static crosshairs inside a scalable box like the circles (optional, and maybe selectable via an on/off button) right in the middle of the screen. This could indicate that that we're looking at the airy disc on-axis? This would help with Dall Kirkhams such as the Mewlon as even slightly off axis the collimation is pointless. We could position the entire window centrally and resize the box to gauge whether we were sitting on centre within the sensor display area. Idea?
Al, if I may, could I suggest that it may be useful if you put another set of static crosshairs inside a scalable box like the circles (optional, and maybe selectable via an on/off button) right in the middle of the screen. This could indicate that that we're looking at the airy disc on-axis? This would help with Dall Kirkhams such as the Mewlon as even slightly off axis the collimation is pointless. We could position the entire window centrally and resize the box to gauge whether we were sitting on centre within the sensor display area. Idea?
I'm not entirely sure I follow the need for another set of cross hairs Chris.
You want the Airy disc on axis, so presumably the centre of the CCD is on the axis. The existing cross hairs are positioned on the centre of the window (same point the circles are concentric with), so wouldn't you just need to size the window so it fits and lines up with the capture image? Then move the Airy disc to the cross hairs???
I stand completely corrected. I didn't realise that your applet window was completely scalable, and that the crosshair dynamically centred itself depending on the size of the window. Very clever
That's brilliantly simple lateral thinking there Al & Al. It's this sort of idea that I admire the most. Well done, and I'm not sure why no-one has come up with it before... invention, necessity, mothers.. all that.
Mother of all evil? Mothers... Oh!... MOTHERS! Yeah, that's us! ...wait a minute... you mean mother of invention don't you...
Quote:
Originally Posted by allan gould
One nice little improvement for the dyslexic among us would be to place three numbers (moveable on the circumferance of the outer circle) so that you could rotate this to line up with your Bob's knobs or the collimation screws so that you knew which one to turn to bring the airy disk back to centre. Just a thought, Al.
But as it is at the moment its a ripper.
Allan
Let me think about that one a bit, Al. I've used the e.graphics draw commands to do the circles and cross hairs, so I'm not sure how to do the movable text in any sensible way yet... but I'll work on it.
And thanks for the out of collimation example image!
Quote:
Originally Posted by Omaroo
Hi Al
I stand completely corrected. I didn't realise that your applet window was completely scalable, and that the crosshair dynamically centred itself depending on the size of the window. Very clever
Don't mind me....
That's OK, Chris. I don't mind at all... I say and do dumb things increasingly more frequently these days...
It now has the numbers 1,2 and 3 able to be displayed around the outside of the outer circle as per Allan's example in post 12 to represent the screw positions. There is a checkbox to turn the numbers on or off, and a numerical updown control to change the orientation of the numbers through +/- 120° in 5° increments.
For those who might be interested, I've included a PDF of the code. VB.NET programming is so easy (compared to where it was heading when windows first started - that stopped me programming!) the biggest problem is just finding the commands you need among all those available. Fortunately, the web and other resources are excellent help.