Being the owner of a Celestron C102-HD (4 inch aperture / 1000mm FL) refractor, I came across articles that say this can be a very nice OTA... provided that one takes the time to tweak it. I started by measuring and trying to understand it better and the first things I learned are:
1. This OTAs is built based on a tube that has a smaller diameter than the objective aperture. Aperture = 102mm, ID of the tube = 97.5mm.
2. Whoever spray-painted the baffles did not take the time to make sure that they are properly covered; some areas are silver-ish with tiny black spots.
3. There are 3 baffles inside the tube. The third one has an inner diameter of 44mm and is placed in front of a 2" (51mm) focuser tube. Even with the draw-tube positioned as if a diagonal was used, the diameter of the light cone projected by this baffle on the front end of the draw tube is about 31mm.
4. If I project the entire aperture of the OTA on the focal plane, going through each baffle individually, the resulting diameter of the projection is negative for the first two baffles; meaning that those first two baffles are completely "choking" the outer edges of light cone, before they reach the focal plane.
5. If I try to connect the inner edges of the baffles with straight lines, the second baffle (the one in the middle) acts like a see-saw hinge, because it sticks into the light cone defined by the first and third baffles, therefore:
5.a. The light cone defined by baffles 1 and 2 (going front to back) has a diameter of about 97.5mm on the BOL and a diameter of 0.6mm in the focal plane.
5.b. The light cone defined by baffles 2 and 3 has a diameter of about 96mm on the BOL and a diameter of 3.2mm in the focal plane.
The way I would interpret these findings is:
a) Although this is being marketed as a 102mm aperture scope, the baffles effectively stop it down to 96mm and that is only valid for a very small area around the center of the focal plane (as shown in 5.a and 5.b).
b) Moving away from that center, the effective aperture keeps decreasing, causing a reduction in light throughput and resolution, compared to the center of the image; to the point where trying to take a picture with an APC sensor (about 26mm diagonal) would result in a 54% reduction in light throughput and 32% loss of resolution at the edge of the image, compared to its center.
c) Based on (a.) and (b.), it looks like trying to use larger aperture eyepieces (or larger imaging sensors) would result in significant light throughout variation across the FOV. Using 2" eyepieces without doing something about the baffles would be a total waste; unless the objective is stopped down to 50mm aperture, at which point the diameter of the projected disk in the focal plane becomes 39mm.
What I am considering to do is:
i. Reposition the baffles, allowing more light to reach the focal plane. That would involve eliminating the rear baffle and pushing back the front and middle baffles so that they allow for a 22 to 26mm diameter unobstructed light disk in the focal plane.
ii. Flock the inside of the lens shade, the draw tube and the first section of the OTA (up to the first baffle).
iii. Use ultra flat black spray-paint to darken the baffles and the inside of the OTA (while moving the baffles, the existing finish will get scratched).
Here are some questions for the more experienced community members:
Q1. Do my conclusions (a. through c.) make sense?
Q2. Do the "corrective actions" I am planning make sense?
Q3. Would it be worth trying to increase the effective aperture in the process? I was warned that sometimes manufacturers make baffles smaller on purpose, in order to reduce CA generated by the edges of the main lens, so I realize that there is a risk involved there.
Edited by Watch&Learn, 11 December 2020 - 11:16 PM.