This is a mystery to me. I am now using the OAG in the QSI. I have the flattener around 140mm from the CCD. That I am sure is incorrect. Tak owners might know that the metal back distance is 107mm, I thought it meant 107mm from the metal of the flattener to the back of the extension from the camera. However, is the metal back distance from the flattener face to the CCD? If so I am going to need an adapter from the QSI body to the flattener.
So the problem as I see it I have round stars through the central area of the image but flaring stars to greater degrees in the corners. The curvature on CCD inspector says that it is around 39-44%. With the stars in the corners, some stars are round and others are elongated. Why is this so? I am thinking some smaller stars cover the problem by have bloat, while the brighter ones just show the elongation.
Is this a flattener problem or is there more going on? Flexure should not be a problem. Collimation is around 7.1" depending on seeing conditions; but this is an accumulation where some images had 3.9" and some 4" etc. I have not seen any flop like I was getting before borrowing some extensions from a mate here that bought another 8" RC.
The bottom RHS of the image looks like the problem, most of the rest of the image looks OK to me. It could be a registration problem, what software do you use for registration?
If you can, put some subs up to a place like www.yousendit.com then post the link so we can have a play.
It could be a tilt in the camera plane or an alignment offset (primary/secondary) (collimation). In my experience field curvature make the stars on the edges blurrier but doesn't create coma. Your FF will only work to its full potential if your optics are aligned.
Stuart you can see an example of a sub. It is a red sub, which has been stretched so you can see the shapes better.
I have been using CCD stack to stack the images.
Marc, the collimation is fine really and I appreciate your comments. I visually checked collimation with the QSI. It looked fine to me. CCD inspector says it has 7.1" of miscollimation in the final image but all the subs vary quite widely in what collimation it found. The field flattener must surely be at the wrong distance though and this could be exacerbating any minor collimation errors or worse creating some curvature over different parts of the field. Wouldn't this be the case?
If the FF is tilted or slightly off-axis it can indeed make things much worse. If it is exactly on axis then the deformation would be uniform and consistent over the edges of the field.
Paul,
could be my old eyes, but the subs don't look too bad if you ask me. I know you are the ever perfectionist, and this is to be admired, but to me they look OK.
If Mr Tak says 107mm, then he normally means this and to quite a close tolerance, to the point of almost being anal.
I always too the described distance to be from the rear of the metal to the film plane/chip surface.
Gary
To me it looks like tilt. Rotate the focuser+flattener+camera 90 degrees, recalibrate PHD and take another shot. If the tilt rotates, then adjust the push-pull screws on the back of the scope.
Also have you checked the optical center of your flats with CCD inspector?
i might be well off the mark here but its something that may need to be thought of.
I have recently had a head smashing moment where no matter what i used to get colmination i could never ever get perfect focus (accross the field stars where not the same)(i used all sorts of methods, laser, barlow laser, cheshire and all high end equipment none of the gso varients) it ended up being that the tilt of the actual focuser was not inline with the ota axis (it is a newt and yours isnt) but even with your colmination being spot on you focuser could be slightly and i mean slightly tilted in which case your FF will magnify the problem as the mpcc magnified my focuser tilt problem.
how did i check? well quite simply actually ditch your laser and grab a cheshire that has a silvered cutout look at the rings/lines/circles that is being reflected back if you can see nice sharp well defined and concentric rings/lines/circles that means your all good remembering to move focus in and out during this as thats how your reflected image will differ! If it does differ, look at the tilt on your focuser then once everything looks the goods recolminate your scope.
Now i get multi coloured diffraction spikes and virtually spot on star images when using stars to collminate! furthermore the inside defocused star is virtually identical to the outside defocused star meaning that my tilt is now eliminated or reduced to a tolerable level.!
Like i said it could be truely useless/ful information but anything is worth a shot when your at stumps end.!
Moon, there should be no flexure as this is with an off axis guider.
The more I look at this issue, the more that the idea occurs to me that it is the position of the flattener. I have checked through a heap of subs and the stars in the center are round. The stars further out get elongated.
So I now have to get the flattener into the correct position. Some comments from Tak owners would help to confirm my thoughts I guess.
All this just goes to show that one can have everythign going just right and make a component change and have to get everything sorted again.
Put the focuser assembly on an old vinyl disc turntable. You'll be surprised. I did that with my GSO focuser and adjusted/shimmed it until it stopped doing the "belly dance". I'm now convinced that 90% of problems happen at the imaging end not within the scope.
Marc, that should not be the problem. I have a Feather Touch focusor which is supposed to be square to the optical path. It may well be part of the problem, but given that I have done 6 images with the RC and the 40D; managed to get a flat field and very tight round stars it must be something that has transpired since putting the new camera on.
Marc, that should not be the problem. I have a Feather Touch focusor which is supposed to be square to the optical path. It may well be part of the problem, but given that I have done 6 images with the RC and the 40D; managed to get a flat field and very tight round stars it must be something that has transpired since putting the new camera on.
Just double check Paul. Can't hurt. Trust me on that one. I took so may things for granted when aligning my C11 like the cut hole in the corrector glass being centered or the secondary being glued centered on its aluminium block. Both weren't. The corrector cut hole glass was off by no less than 3mm. Just check it again. Doesn't take long then you can take that out of the equation and work your way forward by elimination other wise you'll go in circle and get more confused
