MCT Secondary Baffle Mod
Posted 14 August 2013 - 11:18 PM
Posted 21 November 2013 - 10:19 AM
I have these same archs in both my Celestrons. Thing is, ones the C90 and the other the C6. They have completely different curves on the correctors, one being MCT and the other SCT. Because the curves are so different but the archs are the same, do you still think this would be from the corrector? Both of the Meade SCTs do not have this issue at all.
Posted 21 November 2013 - 10:40 AM
Posted 21 November 2013 - 12:07 PM
Posted 21 November 2013 - 08:01 PM
If this info does not help, I can setup and shoot video of a star panning across the field.
Posted 22 November 2013 - 03:13 AM
Posted 22 November 2013 - 04:15 PM
Posted 22 November 2013 - 07:39 PM
Doing the images I found something strange. In my 32mm 67 degree eyepiece, it shows as I had posted before. When I started doing the images, I did not see this. I moved the star to the bottom left corner and it just starts to show. The problem must start at APS-C sensor size and shows more as you get farther out. I don't have a larger sensor to take more images. I hope this gives you some kind of idea. I took the visual back off and looked right into the primary baffle as far off center as you can get you eye. You can not see any of the correct at all, just the inside of the secondary baffle and this is right at the back cell where a visual back would be connected. Attached in the link below is a ZIP file with the pictures I took.
Posted 23 November 2013 - 12:55 AM
Posted 23 November 2013 - 01:18 PM
If I make the images small enough to post you can't see the stars but I have some new information. When you see this arch, if you defocus the star the bright arch does not change. Even if you defocus the star to the point you almost can not see the star, the arch is still bright and in focus and the same size. When you first see the arch it is small and points toward the center of the field. As you pan away from the center the arch gets bigger. Changing the focus at any position of the star when the arch is showing, never changes the arch size or the focus of the arch. Does this help any?
Posted 23 November 2013 - 01:52 PM
And what bothers me is that it's visible when the light source (star) is well inside the field of view.
Is this ever seen visually, or only when a camera is present?
If the latter, it would seem the camera is the culprit. My provisional 'analysis' , based on the camera theory, is that the CCD array is causing a conical dispersion, which is reflected back by the camera's window.
But then, how to explain the fact of the unchanging aspect of the reflection with defocus? Does the camera's chip have microlenses?
I'm still floundering about in something of a vacuum. If there are details which can further restrict the range of conditions when this reflection is present, it would help.
Visual, imaging, or both?
Camera at prime focus or through eyepiece?
Any other optics/filters present in the train?
Posted 23 November 2013 - 04:18 PM
It does this with the camera or the eyepieces. It does this with a 40, 32, 24.5mm swa. Using an 11mm or shorter FL eyepiece it does not show. With the 24.5mm 67 degree, you have to move the star completely out past the field stop before you see it. With the 32mm 67 degree it shows from about 60 percent of the field all the way to the edge but it is getting very dim at the field stop of the 32mm. I have used this same diag and eyepiece on the 8 inch sct and see no hint of this issue. I also see it on the C6 with a 1.25 inch diag. This has to be some reflection off the primary baffle to the secondary for it to stay in focus. I have looked into the tube with a bright light but do not see anything that should cause this. I think I will just pull the unit apart and see what I can find.
Posted 23 November 2013 - 10:37 PM
Posted 24 November 2013 - 04:37 AM
It seems a reasonsble source of this very geometric reflection. And it's in line with my much earlier supposition that it resulted from a reflection off the corrector.
If the onset of this reflection occurs at a *fixed* angle off axis, it would support your thesis, whereby the light source must lie at a certain off-axis angle so that its reflected light can get past the blockage of the secondary baffle.
A test could be devised involving a beam splitter, an illuminated pinhole and a camera or eyepiece. The beam splitter is mounted immediately in front of the camera/eyepiece, the latter 'looking' into the scope as normal. The illuminated pinhole shines into the beam splitter, it's location being at or near the focal surface, and adjustable in lateral position with respect to the optical axis. The front of the scope is covered, so as to provide a dark field. If the same oval reflection is seen, mystery solved.
Posted 26 November 2013 - 12:07 AM
- DocFinance likes this
Posted 26 November 2013 - 05:50 AM
Posted 26 November 2013 - 11:25 AM
Thanks in advance.
Posted 26 November 2013 - 11:54 AM
Posted 26 November 2013 - 12:49 PM
Posted 26 November 2013 - 01:19 PM
Posted 26 November 2013 - 06:20 PM
1) flocking the inside of the secondary baffle
2) flat blackening the end of the secondary baffle (facing the primary)
3) flocking the inside of the OTA from mirror to corrector
4) flocking the outside of the primary baffle
5) flat blackening the end of the primary baffles (inner and outer)
6) flocking the ring at the base of the primary baffle that pressed against the primary to hold it down.
7) flocking the inside of the primary baffle
8) flocking the inside of the insertion end of the 2" star diagonal
9) flat blackening the inside of the star diagonal
10) blackening the bottom of the eyepieces, as seen from the front of the scope.
Note that a well placed series of baffles could eliminate the need to blacken the bottom of the eyepiece or the inside of the primary baffle. Meade did a much better job of that kind of baffling in their 14" LX200, for example.
One could also flat blacken the edge of the secondary and primary mirrors and the corrector, but I did not do so. If the scope being worked on is a Gregory-Maksutov Cassegrain, there is no "edge" to the secondary mirror.
I should also have flat-blacked the background behind the secondary and primary mirrors, but did not do that either.
Prior to the above changes, the scope had a gray background to the images.
Afterwards, the background was velvety black.
Posted 26 November 2013 - 08:06 PM
Posted 27 November 2013 - 12:03 PM
Posted 27 November 2013 - 12:15 PM
Posted 28 November 2013 - 08:15 PM