Stopping the aperture of my scope

[Michael131313]

Do you think that stopping the aperture of my AR102 by making a mask would be beneficial in viewing bright objects or any object? Thanks very much for your opinions.

[Bigzmey]

For nights of average seeing it might produce better views of Moon and planets.

80mm is a good mask to try, it will turn your scope to F8. Easy to make from cardboard.

[The Wave Catcher]

Yes! I have Bresser 102 mm, f/4.5 achromatic refractor and I built an aperture mask from an old plastic yogurt lid. I cut a 60 mm hole in it. I really thought my telescope had very little chromatic aberration but the mask does wonders. My fast refractor is designed and works best for low power views of comets and large DSOs. The mask turns it into a pretty good planetary telescope too. My views of the Moon, planets, and double stars are exceptional with the mask. I call it my “poor man’s apochromatic” telescope.

Here is a photo of my mask after cutting and painting (still has masking tape):


Here is the mask in use:

[The Wave Catcher]

I used the following “CA Cart for Telescope” to help me select an aperture size that met the Sidgwick standard for CA.

https://www.cloudynights.com/uploads/g ... _27450.jpg

[Michael131313]

Thank you very much Bigzmey.
Thanks Steve for your help and instructions. I am going to make one and try it out.

[gregl]

I made one from the bottom of a black plastic oil drain pan from a dollar store. Check there if your yogurt container isn't big enough.

[j.gardavsky]

The aperture mask on an achromatic refractor will reduce the spherochromatism (Gaussian error in optics), which is to a large extent due to the intrafocal marginal rays of light.
Ninety years ago, the designers at CZJ have found out a rule of thumb, like 1 devided by the square root of 2, as an optimum aperture mask downsizing to reduce the spherochromatism, but still balancing the gain with the loss of resolution.

An application of the Sidgwick matrix to masking the aperture may be misleading, because masking the aperture has no effect on the longitudinal chromatic aberration, masking changes the Zeiss RC-value of an refractor, making the longitudinal CA less visible. The math backgrounds are in the wave properties of light, and specifically thanks to the Fourier optics, the Fourier integrals somehow kicked down the road by Sidgwick, and lots of others.
Otherwise, our telescopes would be looking like German Makkaroni, I mean not the Italian Maccheroni,
https://www.gustini.ch/blog/wp-content/ ... gleich.jpg

Hoping, this helps,
JG

[Michael131313]

Thanks so much JG. I am going to use the CZJ formula to make my mask.

[pakarinen]

Does your AR objective dust cover have a smaller cap on it? Both my ST80 and ST120 have an extra removable cap in the middle of the dust cover that I can pull off to stop down the aperture.

[Bigzmey]

Does your AR objective dust cover have a smaller cap on it? Both my ST80 and ST120 have an extra removable cap in the middle of the dust cover that I can pull off to stop down the aperture.

Synta does it for their achro fracs, but not ES.

[Michael131313]

Thanks for answering for me Bigzmey. I am running late today. It would be a nice addition if they did.

[Baurice]

I'm dubious about this because it reduces the resolution, as well as well as the brightness. For photography, you cam remove chromatic aberration by photo processing. You can use filters to reduce it for visual use. At one time, I used sunglasses to view bright objects through telescopes (doesn't work with the SUN!!!). Instead I bought a moon filter and variable polarising filter, which also work on Jupiter and Venus.

BTW, I like your avatar with your dog. We have two.

[Bigzmey]

I'm dubious about this because it reduces the resolution, as well as well as the brightness. For photography, you cam remove chromatic aberration by photo processing. You can use filters to reduce it for visual use. At one time, I used sunglasses to view bright objects through telescopes (doesn't work with the SUN!!!). Instead I bought a moon filter and variable polarising filter, which also work on Jupiter and Venus.

BTW, I like your avatar with your dog. We have two.

As JG indicated stopping down the aperture is a balance between controlling the glare and optics aberrations, and loss of resolution. While it may appear counterintuitive, you may improve the views of bright targets (Moon and planets) and resolve more fine details. Typically, it is most effective on larger fast scopes (DOBs, achro fracs). I would not stop down a well configured APO or Mak.

There is danger in using filters to. If they are not of good quality you loose fine details due to light scatter on filters. I have stopped using polarizing filters because with samples I had from Orion and GSO I clearly saw loss of resolution and contrast at high powers.

[Baurice]

I'm dubious about this because it reduces the resolution, as well as well as the brightness. For photography, you cam remove chromatic aberration by photo processing. You can use filters to reduce it for visual use. At one time, I used sunglasses to view bright objects through telescopes (doesn't work with the SUN!!!). Instead I bought a moon filter and variable polarising filter, which also work on Jupiter and Venus.

BTW, I like your avatar with your dog. We have two.

As JG indicated stopping down the aperture is a balance between controlling the glare and optics aberrations, and loss of resolution. While it may appear counterintuitive, you may improve the views of bright targets (Moon and planets) and resolve more fine details. Typically, it is most effective on larger fast scopes (DOBs, achro fracs). I would not stop down a well configured APO or Mak.

There is danger in using filters to. If they are not of good quality you loose fine details due to light scatter on filters. I have stopped using polarizing filters because with samples I had from Orion and GSO I clearly saw loss of resolution and contrast at high powers.

I have a Mak.

[WilliamPaolini]

I only mask for two reasons: to cut down on CA when using an achromat, or to make larger more aesthetic spurious disks on stars when observing double stars. The latter is also good for getting a more stable star when the seeing is less than desirable. Sometimes I will mask all the way down to 55mm, although 65-80mm is more likely. At 55mm even Sirius is almost always a nice pure white star with a completely stable airy disk pattern!

[chris_g]

Does your AR objective dust cover have a smaller cap on it? Both my ST80 and ST120 have an extra removable cap in the middle of the dust cover that I can pull off to stop down the aperture.

My Sky Watcher ED80 has one on the dust cover. Now I know what it's for.