Star test advice

[CraigT82]

As today is pretty grey and rain and the better half is away I've got my new skymax 150 set up inside and have been trying to do some artificial star testing with it.

I've got one of the hubble optics artificial star torches set up down the hall 12m away from the scope, with the smallest pinhole uncovered (50 micron I believe). Although it may be too close the scope will focus on the star fine.

I've snapped these images of an intrafocal and extrafocal defocused star... I'm a bit worried by the line through the donut, it's a dark line at 12 o'clock on the extra, and a light line at 6 o'clock in the intra.

Could this be an artefact somehow from a flawed test or could it be something more sinister? 

Cheers

[Peter Drew]

Probably a temperature plume from radiation from the shade tube inside the OTA, a noticeable effect whilst observing an out of focus real star when the optics are still cooling. 

[CraigT82]

14 minutes ago, Peter Drew said:

Probably a temperature plume from radiation from the shade tube inside the OTA, a noticeable effect whilst observing an out of focus real star when the optics are still cooling. 

Thanks Peter, I've let it equalise for another half an hour and the effect seems to have gone completely so you're dead on with that explanation.  Phew!

Only the scope came in from outside into the warm flat, so needed to warm up!  Schoolboy error.

Here is the intra focal after a bit more 'warming', I also checked the collimation, seems spot on even after a cross country trip in a fedex van!

[Peter Drew]

Well done, should perform well!  

[AKB]

1 hour ago, CraigT82 said:

I've got one of the hubble optics artificial star torches

Can I ask where you got it?  Don't seem to find a UK supplier.

In the mean time, I've cobbled one together with a white LED and a pinhole in aluminium foil.  Fortunately, I have a 25m straight run indoors that I can use, but I'm pleased to see that 12m seems good enough for you.

[DirkSteele]

Do be aware that an artificial star while good for collimation is less suitable for star testing (to evaluate the quality of your optics) unless you have a really long base line, as at such a short distance the light waves will still be spherical (rather than plane wave) and can introduce (or mask depending on the figuring of your optics) spherical aberration that may or may not actually be present in the optical system.

[CraigT82]

2 hours ago, AKB said:

Can I ask where you got it?  Don't seem to find a UK supplier.

In the mean time, I've cobbled one together with a white LED and a pinhole in aluminium foil.  Fortunately, I have a 25m straight run indoors that I can use, but I'm pleased to see that 12m seems good enough for you.

I got it from the Hubble optics website, if I remember it actually came from hong kong, was a while ago though! 

[CraigT82]

2 hours ago, DirkSteele said:

Do be aware that an artificial star while good for collimation is less suitable for star testing (to evaluate the quality of your optics) unless you have a really long base line, as at such a short distance the light waves will still be spherical (rather than plane wave) and can introduce (or mask depending on the figuring of your optics) spherical aberration that may or may not actually be present in the optical system.

Oh don't worry I don't know anything about star testing and wouldn't attempt to assess oprcal quality! I actually only set it up to tweak my flip mirror and got sidetracked when I saw the lines in the image