5 replies to this topic

[Watch&Learn]

Greetings,

 

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.

 

Thank you.

Edited by Watch&Learn, 11 December 2020 - 11:16 PM.

[Rutilus]

No harm in re-painting the baffles and inner tube. As you can see the paintwork on my Sky-watcher 150mm f/8

was rather lacking.  Plus the factory paint seems to go an off grey over time. I've ended up repainting the inner tubes

on all my Sky-watcher achromat scopes. 

Attached Thumbnails

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[LU1AR]

Greetings,

 

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 is 102, the ID of  the tube is 97.5.

 

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. The third baffle has an inner diameter of 44mm and is placed in front of a 2" (51mm) focuser tube.  Even with the draw-tube positioned as it would be if a diagonal was used, the diameter of the light cone projected by this baffle on the entrance of the draw tube is about 31mm.  Doesn't this defeat the purpose of having a 2" focuser?

 

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 light cone, before it reaches the focal plane.

 

5. If I try to simply 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 1mm in the focal plane.

5.b. The light cone defined by baffles 2 and 3 (going front to back) has a diameter of about 93mm on the BOL and a diameter of 9mm in the focal plane.

 

 

The way I interpret these findings is:

a) Although this is being marketed as a 102mm aperture scope, the way the baffles are placed inside the OTA I have effectively stops it down to 97.5mm and that is only valid for the very center of the focal plane (as shown in 5.a).

b) At the outer edge of a 9mm disk in the focal plane, the effective aperture becomes 93mm, which translates into a 10% reduction in light input and resolution, compared to the center of the image; and that keeps going, to the point where trying to take a picture with an APC sensor (about 26mm diagonal) would result in a 50% reduction of light input and resolution at the edge of the image, compared to its center.

c) Based on (a.) and (b.), it looks like trying to use wide angle eyepieces or larger imaging sensors would result in a less than desirable result and using 2" eyepieces without doing something about the baffles would be a total waste.

 

What I am considering to do is:

i. Flock the inside of the lens shade, the draw tube and the first section of the OTA (up to the first baffle).

ii. Reposition the existing baffles, to allow more light to reach the focal plane. I am thinking to eliminate the rear baffle and push back the front and middle baffles so that they allow an unobstructed light disk with 22 to 26mm diameter in the focal plane.

iii. Re-paint the baffles with ultra flat black and also re-paint the inside of the OTA (as moving the baffles would damage the existing finish).

 

My questions to the community members would be:

 

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?  I was warned that sometimes manufacturers make baffles smaller on purpose, in order to reduce the CA that may be generated by the edges of the main lens, so I realize that there is a factor of risk there.

 

Thank you.

I think that the baffles thus arranged, are an elegant way to reduce the aperture in a sneaky way, to improve the chroma.
My generic Chinese 90/900 refractor improved a lot when I painted the inside with "Enriched" blackboard paint with laser printer toner.
I also painted in black, the inner chrome of the focus tube.
The lens shade was opaqued, rubbing with sandpaper the interior in a circular motion. First with a coarse sandpaper and then with a finer one. That leaves a thin micro-ring finish that helps improve contrast.
Since I bought the used OTA; when cleaning the lenses, I adopted to install an internal black cardboard separation washer, to reduce the aperture by 5mm. It is in that part of the lens where the greatest imperfections are. Just as with the mirrors; which improve by masking 1/2 "of the diameter.
THE EDGES OF BOTH LENSES OF THE OBJECTIVE WERE PAINTED BLACK WITH FIBER. That greatly improved the contrast when looking at the Moon.
But without a doubt what improved the quality of the telescope the most, was to center the projection of the focuser tube with the center of the objective.
To do this, use a laser collimator, but it can also be done; loosening the three screws that support the focuser assembly (It is advisable to replace them with flat head screws) and after observing the Airi Disc with moderate magnification, I improved it with much magnification, giving tapping the focus head and then, adjusting the screws.
I have not seen a cheap telescope, with the image quality of my "10 dollars OTA".
Hope this helps you. Regards.
Edgardo

[Watch&Learn]

Thank you Edgardo. I understand the idea of "sacrificing" the edges of the lens; the part that I don't get is why squish the disk projected in the focal plane to such a tiny size? (I found a flaw in my initial calculations, the actual values are even lower, as shown in the edited "original" post.)  I would rather attach a permanent stopper to the lens itself, like you did, than resort to the baffle trick (which you are very kindly calling "elegant").  For comparison, I also "ran the numbers" on my Vixen made C-80 refractor: the baffles are also "stopping" the edges of the lens a little, but they are allowing a much more generous (26mm in diameter) unobstructed projection disk in the focal plane.  Probably that's why the C-80 provides a better viewing experience than the C-4, despite the smaller aperture?

I like the idea of "enriching" blackboard paint with laser printer toner. Do you mind if I ask what kind of paint did you use and how did you apply it to the inside of the tube?  Did you use a special roller?  Once again, thank you.

Edited by Watch&Learn, 12 December 2020 - 05:31 PM.

[LU1AR]

Thank you Edgardo. I understand the idea of "sacrificing" the edges of the lens; the part that I don't get is why squish the disk projected in the focal plane to such a tiny size? (I found a flaw in my initial calculations, the actual values are even lower, as shown in the edited "original" post.)  I would rather attach a permanent stopper to the lens itself, like you did, than resort to the baffle trick (which you are very kindly calling "elegant").  For comparison, I also "ran the numbers" on my Vixen made C-80 refractor: the baffles are also "stopping" the edges of the lens a little, but they are allowing a much more generous (26mm in diameter) unobstructed projection disk in the focal plane.  Probably that's why the C-80 provides a better viewing experience than the C-4, despite the smaller aperture?

I like the idea of "enriching" blackboard paint with laser printer toner. Do you mind if I ask what kind of paint did you use and how did you apply it to the inside of the tube?  Did you use a special roller?  Once again, thank you.

I used a cheap plastic toilet brush. Since I added baffles.
I also made a reducing mask that supports a 72mm UV filter that greatly improves the chroma. An advantage that on this filter you can put others, such as yellow # 8 or FLC (It was used to remove that bluish -Soviet- tint from film photos with fluorescent light).
I have two Mak and a Vixen F / 5, but I enjoy showing the quality that can be obtained, with this cheap refractor.
Regards

[Watch&Learn]

For the time being, I took a conservative approach and repositioned the existing baffles. First, moved the one closest to the focuser, bringing it as far back as possible (2 - 3mm away from the inner end of the drawtube, when rolled all the way in). Then pushed back the middle baffle, to make its edge line up with the other two. This way, the unobstructed disk in the focal plane went up to a little over 8mm (at 97.5mm aperture), which is still less than ideal, but a lot better than it was initially. There is no noticeable increase in CA after this change and the views are better. To go any further, I would have to start enlarging the openings in the baffles, which I am not ready to do.

Edited by Watch&Learn, 21 February 2021 - 12:52 PM.