October 1999 Newsletter

Stockton Astronomical Society

Valley Skies

Valley Skies - October 1999 Issue

The Telescope Nut
by Jeff Baldwin

Aperture Stops

Have you ever seen an observer looking through his/her telescope with a card in front of the telescope that only allows a small amount of light to enter the scope? This is an aperture stop, and it can come in handy once in a while. Let's talk about why this is true.

Imagine two telescopes side-by-side observing the same object in the sky under the same turbulent atmospheric conditions. One telescope is observing with 12" clear aperture (diameter of the mirror), and the other is blocking all of the light except for a 3" off-center aperture. They are both looking at Saturn, which is so bright that light grasp and light gathering is not an important issue. The scope with 12" of aperture is looking through a cylinder of turbulent air that is 12" in diameter, while the telescope with an aperture mask is looking through only a 3" column of turbulent air. The amount of air that the larger optic is viewing will be 12/3 squared, or 16 times the amount of air volume. It should make some sense that the image degradation due to turbulence has the potential to be 16 times greater. The smaller scope comes out ahead this time for clarity.

Imagine another set of telescopes, also 12", one with a mask and the other without, only this time they are looking through clear, smooth air. However, this time the two telescopes have optical errors, each having a 1/2 wave peak to valley error. The 12" unmasked scope has a 1/2 wave error messing up the image. However, the 12" masked scope has 1/4 of its diameter passing light to the eyepiece, so the error of 1/2 wave is reduced by 4 times, implying that the image is formed by a 1/8 wave peak to valley error, a much sharper image than the unmasked scope.

Another interesting issue about using a mask with an eccentric aperture (see illustration) is that you no longer have spider vanes causing parallel and perpendicular diffraction patterns. You also no longer have the secondary mirror obstruction. You also can eliminate the effects of a turned down edge on the primary mirror. You end up with a slower telescope. These things all add up to a refractor type of image quality with one advantage: Refractors have chromatic aberrations, and this masked reflector has none. Theoretically, it should outperform a refractor of the same aperture, f ratio and wave-front error (I intentionally did not say surface error since refractors can get away with twice the error on the surface for the same image degradation).

Bottom line, masking a telescope can help on planetary, solar and lunar observing. If you have perfect optics and perfect seeing, masking may be unnecessary for lunar or planetary observing, but these are not the usual conditions.

Solar observing is a special case, because of the heat concentration. For Newtonians, solar viewing requires masking to reduce heat entering the telescope. Warning: always use caution and proper full aperture filtration when viewing the Sun. Without filtration, it will burn a hole in your head where your eye used to be.

Then there is the apodizer mask. This is neat. You make a mask that avoids the spider vanes and the secondary mirror obscuration and the tde near the edge of the mirror. Then you place screen door screen onto the mask with a hole cut out so the circle is 52% the diameter of the mask's hole. Then you place another screen over it with a hole that is 76% the hole's diameter. Then you place a third screen over with an 88% hole. The screens should have their rectangular patterns fairly well registered. When you look through this mask at Saturn, you see hundreds of messy Saturns with one extremely perfect Saturn in the center.

I tested this on Black, an imperfect 1/12 wave f/3.7 mirror of 24" aperture. I tried a 4" mask, a 6" mask, an 8" mask, and a 4" apodized mask. The apodizer beat all others based on the amount of black sky seen between the individual double stars of the double-double, epsilon Lyra. Juliano's 17.5" Sky Design scope is another experiment, Saturn being the target. Seeing multiple images of Saturn is unpleasant for some observers, but if you can concentrate on the center image, it's a killer.

I like to talk people into building fat, fast mirrors. You can always make a thin, slow scope out of a fat, fast scope by masking. You get both types of scopes this way. You can't make a fat, fast scope out of one that is thin and slow, you're stuck. Also, masks are very cheap to make, usually the cost of a piece of cardboard.

Clear Glass...Jeff Baldwin
For more information on Telescope Making jump to the ATM page.