I’ve been experimenting with an f/4 newt for imaging, as a bit of a project. Given the lockdown I’ve finally had a little time to play with it.
Having collimated the scope in situ with Cats Eye autocollimator (secondary) and Barlowed laser (primary) in an iterative manner, if I turn the scope to a bright-ish star for star testing I find that the shadow of the secondary shifts position between roughly equivalent amounts of defocus intra and extra.
So far I’ve failed to get completely round stars when in focus...I always find something wrong with them. Night before last I got really close, where the smaller stars looked round but the larger ones appeared fat-bottomed. Not that there’s anything wrong with that, I just prefer my stars round
Can anyone shed any light on what might be happening?
I suspect you probably know more on this topic than I do, but here goes...
How confident are you about your laser's collimation? Even if you're confident about that, does it always register the same in your focuser? I find that mine is a poor fit for my focuser and so it's basically useless for collimation. Good for doing things like tracking down collimation shift though...
Are you 100% sure that you have the autocollimator at the focal plane? Apparently you know you're "close enough" if the reflections in the AC are all the same size.
Based on the secondary shadow shifting either side of focus either your collimation is out as you've already guessed, or your focuser has slop... but re the fat-bottomed stars, are you sure your scope had reached thermal equilibrium?
I’m not going to profess I know enough, given that I haven’t nailed it yet The degrees of freedom with these things is mind boggling...
Good call...I should check the laser. It’s a 2” and fits well into the focuser, but I also should check for any shift on securing the focus ring.
I typically rack the focuser out such that the reflections roughly match in size, I find it easier to be sure my collimation is good that way.
Also, not convinced about thermal equilibrium...I’ve been struggling to nail the collimation with somewhat wooly seeing this week, which doesn’t help when the Poisson point jumps about when the star is slightly defocused. But thermal disequilibrium might also look like wooly seeing
All things said, I’m encouraged by the images I’ve gotten with the maligned scope to make me want to get it that final step over the line to round stars. I’ll post an image shortly...
Hi Dunk, I can suggest one thing else to check out. You maybe spot on with your static collimation, but collimation can shift in differernt sky orientations likely due to primary movement. GSO newts, Dobs, are well known for having weak primary springs, and this can affect other brands as well. It is pretty easy to test, do your static collimation on your mount, then pick a range of various star targets at different altitudes to the east and west, checking the collimation by defocusing each star, if your springs are weak you will see definite shift. The fix is pretty easy and available at any Bunnings store, replacement springs. Remove one of your primary springs and go along to the Cabinet parts aisle at Bunnings, find the display where the little yellow labelled spring packs are hanging. Find one suitable size, but with greater compression strength than your stock springs. Don't worry if they are not exactly the right length, in fact slightly longer is better, and even if too long you can cut them down. I have done this mod to every newt and Dob that I have owned in Australia.
I'd recommend you consider loading up a high res image to Astrobin or Flikr so we can have a proper look - to see if you're experiencing a collimation or tilt issue. These things can be challenging.
Do you have CCD inspector? Some folks have found that useful in diagnosis of these matters.
Cheers
Andy
Looks pretty good to me. I'm working on same target at present. That star looks slightly flared in mine too, like the other larger stars nearby (just out of the zoomed in shot you have shown). Mind you, I'm using a newtonian also - and its my first time collimating with a catseye
Noooo, it's a conspiracy, the stars are fat-bottomed
Maybe I've been stuck in the house too long
Thanks for your feedback, everyone
I should probably add that the image was taken with an ASI183MM, so 2.4 micron pixels...I knew I was pushing it, especially given the woolly seeing we've been having lately, was just thankful for the clear nights around New Moon.
Naturally, I pulled it all apart yesterday on the search for a problem. I did actually find one...pretty much on day one with the scope, I replaced the focuser tube to 2" nozzle, as it had a screw to secure the coma corrector in the barrel which I wasn't having any of. The replacement - a Baader M54x1 click-lock 2"...shifts whatever is in the focuser depending on the position of unlock. So far, I haven't found it to be reproducible, at least not to any level of what I'd call accuracy. Having spent hours box diving yesterday, I can't find the (unjustly maligned?) original
Dunk, relax, . Keep up your search. How about trying some images from across the meridian at the same altitude at the Hamburger, to see if you shift the direction of the star bulge. That might confirm sag/ collimation shift.
