Secondary mirror size recommendation

[Schro]

I recently undertook the task of baffling my 114 F4 reflector (Celestron Cometron 114)

for the purpose of this I did a scale ray-trace diagram to plan baffle size and location and 3D printed the baffles.

from my ray trace drawing I was able to measure the width of the light cone where it hits the secondary mirror at 34mm wide (I verified this with a bit of trigonometry)

the OEM secondary mirror is 34mm but because of defects from the beveled edge the flat useable surface is actually 31.5 to 32mm. This means that it is only utilizing about 86-90% of the aperture of the primary.

I would like to replace with a better mirror.

Available sizes are constrained. Should I go with a 35mm or 40mm? 

On a fast Newtonian like this will a limited secondary size make collimating more difficult?

Primarily visual use but I may experiment with EAA.

PS don’t mind the paw prints on the drawing. My daughter’s kitten found my berry smoothie.

[Mandy D]

You will only lose 12.3% of the light with a 40mm mirror and it might make your life easier On the other hand you will lose a bit more contrast. 

[bosun21]

I don't think that the diameter of the secondary will make the collimation anymore difficult/easier. It's the actual focal ratio of the telescope that determines this. F4 can be sensitive to any collimation errors.

[Captain Scarlet]

As bosun21 says, collimation won’t be affected. But illumination will be. With a secondary smaller than the principal (i.e. from straight ahead) light cone, some of the light striking the primary will miss the secondary on its way back up the tube, even at your maximum illumination point, the central point of the image. In other words, you are effectively seeing the image of a smaller mirror. As you go away from the central point of the image, with a tilted light-cone, more still gets missed by the mirror. The point of a so-called “oversized” (larger than the minimum necessary for the principal light-cone) is to fully capture at least some of the tilted off-axis cones too, allowing an area of “full illumination” at the centre of your image. So you should definitely go for the 40mm. As you go larger, you are trading loss-of-contrast-from-increasing-obstruction with increasing area of fully-illuminated central area of your image. Imagers tend to like more illumination; observers tend to prefer more contrast at the centre of the image.

By the way, I see you had a furry helper for your ray-trace renditions! 😁 🐈 

Cheers, Magnus

[Second Time Around]

For visual use a slightly undersized secondary won't make much difference at all.  It won't affect the brightness of anything other than at the edge of the field of view.  Even then it may well be too small for a human eye to notice. You won't lose any light in the centre of the image.  You can check the practical effect in the following easy to use calculator https://www.bbastrodesigns.com/NewtDesigner.html#diagonal

[CraigT82]

A reason for going oversize is to prevent the edges of the secondary mirror from being brought into play. The edges of the flat can be a bit dodgy and may introduce aberrations of they are fully illuminates by the primary. 

[Second Time Around]

1 minute ago, CraigT82 said:

A reason for going oversize is to prevent the edges of the secondary mirror from being brought into play. The edges of the flat can be a bit dodgy and may introduce aberrations of they are fully illuminates by the primary. 

Agreed, but many secondaries are secured by a lip rather than being just glued on.  I don't know what the score is with the Cometron but I suspect at the price point it'll be glued. 

[Captain Scarlet]

1 hour ago, CraigT82 said:

A reason for going oversize is to prevent the edges of the secondary mirror from being brought into play. The edges of the flat can be a bit dodgy and may introduce aberrations of they are fully illuminates by the primary. 

I'm not sure that first sentence is right. The purpose of an oversized secondary is to capture more of the off-axis light cone to produce a larger fully illuminated field. The edges of the secondary are always going to come into play as the light cone goes off-axis. Grinding away the edge or covering it with a lip is what helps avoid the edge-aberrations.

Edited August 8 by Captain Scarlet

[CraigT82]

9 hours ago, Captain Scarlet said:

I'm not sure that first sentence is right. The purpose of an oversized secondary is to capture more of the off-axis light cone to produce a larger fully illuminated field. The edges of the secondary are always going to come into play as the light cone goes off-axis. Grinding away the edge or covering it with a lip is what helps avoid the edge-aberrations.

Yes that is true… but wouldn’t you want the secondary large enough to capture all the on-axis light cone without it encountering the edges of the mirror (ground, covered or not) to maintain the sharpest on-axis image? 

[Schro]

This is obviously a discussion about compromises.😁

I have done quite a bit of work on the optical tube in an attempt to improve contrast.

i live in a light polluted area so my thought was to do my best to minimize any reflections and stray light (hence the baffles. I think they turned out well 

the internal reflections are so significantly reduced now that it is difficult to see all 9 baffles down the tube (musou black paint on the baffles and flocking on the tube wall helped with that)

the stock focuser was a floppy piece of crap that made it difficult to collimate, so I’ve replaced with an Antares low profile helical focuser

if I hadn’t already done so much work I would have found a way to lengthen the tube to make the stock secondary work more effectively. The low profile focuser gives me some leeway so that ‘was’ a viable option . But I’m not prepared to re-do all of the baffles at this point.

Given the small aperture my intuition says minimizing the central obstruction would be best for my use case, so a 35mm mirror if I can find one without a “dodgy” edge, after all, the math says it should work but I recognize that theory doesn’t always coincide with experience, and my intuition could be wrong, and since this is my first telescope I have nothing to compare it against 

I will check out https://www.bbastrodesigns.com/NewtDesigner.html#diagonal thanks for the reference.

[Captain Scarlet]

11 hours ago, CraigT82 said:

Yes that is true… but wouldn’t you want the secondary large enough to capture all the on-axis light cone without it encountering the edges of the mirror (ground, covered or not) to maintain the sharpest on-axis image? 

Yes totally I agree with that. Designing a secondary that’s smaller than the minimum to cover the principal cone is a waste of primary mirror size and a waste of weight. I bought a 200p with the same defect, the first thing I did was pull the primary as far back as it could go within its range, and put a Steeltrack onto it which also allows longitudinal movement: I needed it all.

M

[CraigT82]

1 hour ago, Captain Scarlet said:

Yes totally I agree with that. Designing a secondary that’s smaller than the minimum to cover the principal cone is a waste of primary mirror size and a waste of weight. I bought a 200p with the same defect, the first thing I did was pull the primary as far back as it could go within its range, and put a Steeltrack onto it which also allows longitudinal movement: I needed it all.

M

Oh we do like to fettle our newts! 

[CraigT82]

12 hours ago, Schro said:

This is obviously a discussion about compromises.😁

I have done quite a bit of work on the optical tube in an attempt to improve contrast.

i live in a light polluted area so my thought was to do my best to minimize any reflections and stray light (hence the baffles. I think they turned out well 

the internal reflections are so significantly reduced now that it is difficult to see all 9 baffles down the tube (musou black paint on the baffles and flocking on the tube wall helped with that)

the stock focuser was a floppy piece of crap that made it difficult to collimate, so I’ve replaced with an Antares low profile helical focuser

if I hadn’t already done so much work I would have found a way to lengthen the tube to make the stock secondary work more effectively. The low profile focuser gives me some leeway so that ‘was’ a viable option . But I’m not prepared to re-do all of the baffles at this point.

Given the small aperture my intuition says minimizing the central obstruction would be best for my use case, so a 35mm mirror if I can find one without a “dodgy” edge, after all, the math says it should work but I recognize that theory doesn’t always coincide with experience, and my intuition could be wrong, and since this is my first telescope I have nothing to compare it against 

I will check out https://www.bbastrodesigns.com/NewtDesigner.html#diagonal thanks for the reference.

I like that baffle sheet opposite the focuser, can I ask where you got it from? 
 

[Schro]

The baffle sheet is 3D printed in black PLA

each of the vanes are angled on the back side so with the end that you are looking at being oriented at the top from the build plate I was able to print without supports

(Ender3, budget PLA, 0.2mm layer height, about 5 1/2hours print time)

i primed it with a rattle can of paint for plastic to help with adhesion and airbrush sprayed Musou black