Flat field

[Peter Ward]

I've been tinkering with my camera and RHA set-up for some time now, but finally decided to nail down the camera orthogonality over the last few nights.

It just took a bit of math, plus knowing the step increment of my focuser.

Mating various adapters, Filter wheel etc. basically meant the system had a residual tilt of 0.012mm. Some shim stock did the rest.

Have to say I'm pretty pleased with the results.

[Joshua Bunn]

Looking good Peter :thumbsup: how did the star field look?

Gee, the tolerances must be small with fast focal ratios. what did you do to find the residual 12 micron tilt?

[Paul Haese]

Still a bit to take out I reckon.;)

In all seriousness I had not noticed any tilt in your previous images. Looks pretty flat across the field too.

[multiweb]

Bummer... here goes my 0.012mm every time I pack and unscrew my camera. :P

[Peter Ward]

Used CCD inspector to get the direction of the tilt, which thankfully was pretty much along the Y axis.

I focused on a star at the top edge of the frame, then refocused on the lower edge. The difference was 144 counts on the Atlas focuser or 0.012 mm....
(each atlas step is 0.08 micron)

[Peter Ward]

:lol: seems trivial, I know, but you could actually see the difference from top to lower edges.

[multiweb]

Ha... that's more like it. Hard to repeat for a nomad astronomer though. Will keep that in mind when/if I ever get an obs.

[multiweb]

Oh I totally agree. Theoretical tolerances are extremely tight. In practice it's hard to get (and stay) there. I have no doubt about the quality of field improvement. I've 'nailed it' a handful of occasion by luck but would be incapable of reproducing it. I remember reading in the book "Celestron - the early days" a paragraph written by Ceravolo when he was working there saying he got an SCT to 1/20th of a wave on a rig by centering the corrector on a test rig. Then went on saying "of course, 4 cork shims aren't going to keep you there".

[leon]

Looks impressive, just wish I knew what you were talking about :shrug: :lol:

Leon :thumbsup:

[Joshua Bunn]

Ahh yes, thats an effective and simple method. in you're situation this works a treat because you dont have to collimate you're scope. With scopes that have adjustable collimation, this is not so easy because if the collimation is out a little, the focus distances will be a little different on oposite sides of the chip - regardless of tilt. this has been my experience.

[Peter Ward]

Indeed I'd suspect you be chasing your tail if collimation had to be done as well.

By the way...real world result is here (http://www.atscope.com.au/BRO/gallery291.html)

I've also found that when you get down to these levels that, not just the star field used with CCD inspector, but also differing exposures of the same field show small tilt variations.

[Joshua Bunn]

True, thats why the final test for me is always the star test at high resolution, of which yours is looking the goods :thumbsup:

[h0ughy]

what a fine result - impressive, and not even a bee's XXXX in it ;)

[Peter Ward]

Ta Dave. I should point out the diffraction spike in bright star in the top tight corner was not due to any problem with the optics....is was due some tallish black-bamboo next to my Obs that got in the way during the exposure... :doh:

[ericwbenson]

That's right, and that's also why large aperture AO is limited in it's use. The isoplanatic patch (the area of the sky that has undergone the same atmospheric aberration) at most sites is pretty small, so large chips pickup focus differences from top-left to bottom-right etc. pretty easily (the atmosphere moves the star around on your chip AND causes defocus at the same time)

Best,
EB

[gregbradley]

That's very impressive.
I find the CDK17 is very good in this regard. But that's F6.8 which is a lot easier.

With the reducer it could do with a bit of this type of work but overall not bad.

Those tilt/tip adapters from Teleskop Service would be handy to have in the imaging train to make it repeatable for those who remove their cameras routinely.

Greg.

[Joshua Bunn]

I realized this method can be used with scopes that require collimation too. Instead of moving the mount so the star is on the opposite side of the chip, you rotate the camera (providing there is no run out in the rotation). Now the star is on the other side of the chip and the collimation has no effect.