Decreasing Telescope aperture - Forums

Topic starter

IOTD Staff

Jun 14, 2019

Hello everyone ..

I heard from a friend that if you decrease the aperture of the primary mirror of the telescope with a circular shield ,
for example.. a circular cardboard to make 6 inch telescope a 5.5 inch telescope ..and you will get a sharper image especially in the corners..
did any one try that ..

Thanks
Ali

Roman Pierce

Jun 14, 2019

I have not tried this so I will be following with interest. With that, I would add that the math suggest two characteristics may prevail should one try this? First: A 6 inch OTA collects 40 percent more light than a 5 inch therefore one may see a darker image. Counter to that theory is that the amount of light collected through a 6 inch aperture would be concentrated onto a smaller mirror thus imitating the way that a focal reducer affects an image. Second: Assuming that the math is correct, a potential negative to this concept is the reduction in the field of view. And now i await people smarter than me on this topic to offer up their experiences.

Engaging Supportive

Lomingen

Jun 15, 2019

I am not sure how it works with a mirrored telescope but I can add some information from photography.

Increasing the opening of a lens reduces the sharpness because light in different wavelengths reacts differently to the optical system. When you take a picture of a single dot the blue light tends to be more towards the outside of the picture and the red light more to the inside. The effect is stronger in the corners of the picture. In photography you can react to this by taking just the center part of the picture. Simply cutting the edges with the problems.
Decreasing the opening of a lens reduces the sharpness because the light is scattered on the edges of the opening and a small opening has a lot more edges than a big opening (twice the rim, four times the opening). In photography there is a sweet spot in the opening of every lens where a lens is sharpest. Usually around f/8.

Out of my head two thoughts about teleskopes that should go into this discussion.
Firstly in telescopes there are more obstructions in the way compared to a lens. The holding of the secondary mirror or screws on the rim for example. Resulting in more edges that decrease sharpness. Reducing the opening and avoiding obstructions on the edges could increase sharpness. Or reducing the opening a lot and move it off center to avoid the holding structure of the secondary mirror. (I have a telescope cover for watching the sun with an off center opening.)
Secondly the resolution of a telescope is dependend on the diameter of the opening. Reducing the opening reduces the resolution. I have no idea if the effect is measurable considering the resolution of a camera or the seeing but here is the thought: Two near stars that are just not separated are seen as a unsharp line. Reducing the opening keeps the two stars from separation and the result is just on dot making a shaper. At least appear sharper. (Just like binning.)

Disclaimer: All of this are just thoughts. I have not made any calculations or measurements.

Roman Pierce

Jun 15, 2019

Really helpful to me, many thanks!

Siegfried

Jun 15, 2019

I am not sure how it works with a mirrored telescope but I can add some information from photography.Increasing the opening of a lens reduces the sharpness because light in different wavelengths reacts differently to the optical system.

We taking about a mirror system ....... and on mirror the wavelengths will not really effect.

Siegfried

Siegfried

Jun 15, 2019

Ali Alhawas:
Hello everyone ..I heard from a friend that if you decrease the aperture of the primary mirror of the telescope with a circular shield ,
for example.. a circular cardboard to make 6 inch telescope a 5.5 inch telescope ..and you will get a sharper image especially in the corners..
did any one try that ..

Thanks
Ali

If you use a correct coma corrector and the right distance on an collimated system, it should be sharp ! I like my fast F/4 Newton. F/5 is sure easier, but especially on deep skyphotography I would not reduce lower.
I would more think that your optical path is good.

Siegfried

Lomingen

Jun 15, 2019

Siegfried:
I am not sure how it works with a mirrored telescope but I can add some information from photography.Increasing the opening of a lens reduces the sharpness because light in different wavelengths reacts differently to the optical system.
We taking about a mirror system ....... and on mirror the wavelengths will not really effect.

Siegfried

True. The wavelenght are not effected with mirrors. Thanks for the reminder.