Thank you all for replying. After reading Don's comment above about being almost impossible to have the optical center very far from the geometric center, I began to doubt my previous collimation. So, reset everything and started from scratch. After positioning the secondary, I presume it's normal to begin focuser axial alignment by initially not having the center of the primary centered. And the cross hairs are not on the center spot. Achieving that in step 1 would be a miracle. However, using tilt "only" I simply cannot keep the reflection of the primary centered because the secondary does rotate when the collimation screws are adjusted. I do not want to "cheat" and tilt the secondary by hand to center the cross hairs. I may be adding too much additional tilt error instead of collimating the focuser axis. I went through 4 hours if iterations between the two and gave up.
The location of the center marker in your annotated image in post #11 is difficult to accept...
you seem to be pushing your scope to its theoretical limits...and of course, expertise when making the alignment read.
Yes, this is why I am trying to get the scope within the high magnification tolerance. The entire FOV should be diffraction limited for coma, but it does not appear to be so. Up this high magnification is where I do see some residual coma in part of the field. And I believe some off axis astigmatism. (I went through Lockwood's article to remove as many potential sources of astigmatism as possible). The star images are otherwise "perfect" (coma coma astigmatism free) in the eastern part of the field, but they degrade a bit as the star image drifts toward the west. I am sure I am losing a tiny bit of critical performance in the small FOV. This increasing coma with the westerly star drift is what lead me to believe the colllimation is s tiny bit off. Adjusting the primary tended to improve the image to some degree.
...has this scope's image performance ever met your expectations?
I am doing some high magnification study of Saturn's rings. I am still pretty impressed with the image at 0.3mm exit pupil to within the limits of the descent seeing conditions. The mirror is at least descent and so are the images. I conducted a Foucault knife edge, a Ronchi tests at CoC and (probably too insensitive) on a star, as well as a visual star test and shadow break out. It's slightly under corrected and pretty good parabolic figure to the best I can make it out. The Knife edge nulls out very well on the star test and the Foucault knife edge shows the mirror has the expected parabolic figure (but no measurements were made, just looking at the shadows along the caustic at CoC). It snaps pretty well on the moon and Saturn. So, I'm confident the mirror is fine. If it is fine, then the optical center should be well placed, I suppose, but being mass produced I began to wonder about that. And if it is fine, then I have to assume my collimation is somehow off.
Is this the alignment you see through your combo tool after aligning on a star?
Yes. But when I look closer, I see some rotational error in that image. You can also see (the reflection of the secondary is offset from the red circles) where I adjusted the secondary laterally from step 1 to center the secondary and square under the focuser. I am wondering if these two adjustments are related somehow and require further refinement of the primary collimation to center the coma free point in the FOV. If so, this is terrible collimation.
The glue blob at the intersection of the sight tube cross hairs is getting in the way of the Cheshire alignment read. I can almost visualize how you have to move your eye around to see the primary mirror center marker.
You know what's interesting, and possibly wrong, but interesting? With astigmatism I see blurry cross hairs and try to extrapolate the blur across the center mark. That's difficult. But, I can tell the glue blob is on the center mark because the center mark dims a little as the blob passes over it blocking the light it reflects. This is pretty easy to see. I also tried using the camera to be my "eyes" by taking pictures of each step and each iteration to see how close I am getting. That's pretty frustrating. I can keep my eye pretty much centered, but the camera and my eye do not agree. So, I trust my eye instead of fussing with camera registration. Glasses get in the way, plus my astigmatism is not that bad. I can see everything else.
I'm also guessing that your Cheshire is 1.25-inch.
It's a 2" Light Pipe suitable for f/6. Yes, it registers very well using the 2" adapter. In fact, it's so tight I had to tap it back into round after dropping it just to get it to fit into the focuser. I managed that, but noticed one of the cross hairs was no longer centered. So, I measured, marked and glued the cross hair into position. Upon checking with a ruler across the site tube end with the cross hairs visually centered over the pupil, one cross hair seems dead on, the other measurement from center to the edge is off by about 0.5mm (which is really about 0.25mm error at the cross hair). That should be very good. I am thinking maybe I need a collimation tool that is easier to see well and use visually, but I still have to use the site tube to center the secondary and align the focuser axis without introducing unwanted tilt. With the site tube we can see any induced secondary rotation visually. I am not sure a laser will allow us to that. So, I am undecided. The Cheshire portion seems just fine.
You should only use the apex when you're trying to optimize the secondary mirror placement/offset. Don't use the apex when you're assessing/correcting the axial alignments. You should always assess/correct the axial alignments with the pupil near the focal plane.
Yes, after collimation earlier, I racked the focuser (not all the way) out to the Apex to check the reflection of the primary in the secondary. I had it very nice at one point. Clean primary reflection all around, a thin dark line separating the reflection of the primary and the actual secondary edge, and all three clips easily seen and the primary center mark dead center in the Cheshire. The cross hairs were on the center mark within 6mm tolerance from top dead center. That should have been darn near prefect. Still, I needed some primary mirror adjustment at very high magnification.
One difficulty of collimation, first it's pretty dark in there, so its hard to see the secondary holder to ensure it is centered under the focuser. I found if I tilt the OTA a little and get some better lighting, I can illuminate and see the secondary holder and better estimate longitudinal placement of the secondary along the tubes longitudinal axis. So, I did that and got the great collimation above. Last night I resorted to a yellow background reflection in the secondary (so not to see the primary mirror) to contrast it with the white background behind the secondary on the far side of the OTA. I can get that darn near perfect. It does seem to require a little "lateral" adjustment of the secondary spider vanes to pull the secondary to center of the focuser. No problem, that goes well.
However, and as expected, when removing the yellow background the primary reflection is not centered nor are the cross hairs. That's normal, I believe, so I proceed to collimate the focuser axis without adding unnecessary tilt. I guess we want to correct any tilt induced by the secondary collimation screws, but not *USE* tilt to "cheat" the centering of the cross hairs. I think this is where I am running into trouble mixing tilt with corrections to induced tilt during collimation. So, I added a milk jug washer last night to see if it helps reduce induced tilt on the secondary. The results were not good. Absolutely frustrating going through iterations of steps 1 and 2 and not making any progress. I cannot get collimation anywhere close to being close.
And remember, when you're aligning on a star at very high magnification, the star needs to be carefully centered in the field of view and any defocusing should be at most one or two (more) diffraction rings.
Correct. Of course we have to deal with stars drifting or use an artificial star. I do either or both. But, we can watch a star as it crosses the center of the FOV, and we can see it at the edges. In focus, it's not hard to see differing levels of coma when it's present. This is what is driving me nuts.
Okay, enough for now...coffee is cold. I will hit it again today, starting with step 1. Again. Maybe it is my collimation and not the optical center. The image you annotated above does not look good to me. If that's what the camera sees and if that is my collimation. Yuk! Something is not right.
Edit: OUCH! This was a long reply. I noticed it when I saved it, sorry. No wonder my coffee is cold.
Edited by Asbytec, 10 October 2019 - 07:57 PM.