Amateur telescope making

Re: Aperture masks

chipdatajeffB — Sat, 04 Jul 2009 18:05:27 GMT

Also, if double star measurements are something you're keenly interested in doing, then you'll want to research the diffraction-grating micrometer. This is like a mask in that it goes over the objective, but it looks a bit like an adjustable shutter.

The construction and use of such an instrument is described in Chapter 14 of Bob Argyle's excellent book, Observing and Measuring Visual Double Stars. It's also described in Muirden's, Amateur Astronomers' Handbook.

It is essentially a set of adjustable slats in a frame, looking something like a heating system vent, where the spacing of the parallel slits is precise and the angle of the whole frame of them can be varied and measured accurately over the objective.

This device produces several orders of spectra of the observed stars, aligned at a right angle to the slit. For a given grating size (slat size) the distance between the first and second order spectrum is proportional to the spacing of the slats and can be used to measure the separation and position angle of the stars. This works for brighter pairs and can be quite accurate.

Here's a description of how to make and use a simple one.

Re: Aperture masks

chipdatajeffB — Sat, 04 Jul 2009 17:09:40 GMT

You can certainly trust Dave's answers and experience.

I like the drift method (you need to figure the declination adjustment required), but I normally use a different method.

I mainly observe with two specific telescopes and one set of eyepieces. For each telescope, I prepared a chart of the measured fields of view with each given eyepiece in the set (it's a set of Televue Radians).

I then acquired a set of circle templates (architects and mechanical draftsmen use these) which I use to draw star-hopping circles on star charts. This gives me something quite close to customized TelRad finder charts. If you use this method, you need to observe the sky with your charts at hand, and plot the actual field of view using the circle template that most nearly matches it.

You can use charting software such as Cartes du Ciel to do the same thing. Many other planetarium programs allow you to create fields of view, or overlays, that match your eyepieces. I kind of waffle back and forth between using a laptop at the telescope, and not. I prefer not, but for observing double stars it's quite nice to be able to get online while observing and look things up. Ditto for asteroid searching (at which I'm not yet very good!).

If you use the software method, you can prepare a very precise overlay using circle-drawing tools in a graphics program, then print that on a transparency. This allows you to exactly match in the template what you'll see with a given eyepiece, and you can also label each of the overlays with the telescope/eyepiece combination in fits.

Re: Aperture masks

DaveMitsky — Sat, 04 Jul 2009 16:47:10 GMT

The AFOV divided by magnification formula is a fairly close approximation of TFOV. A more accurate approximation can be obtained by dividing the field stop diameter of the eyepiece in question (which is rarely supplied by the manufacturer) by the telescopic focal length and multiplying that result by 57.3 (i.e.,180/pi). However, the best way to determine TFOV is to use the drift method.

Off-axis aperture masks may result in a "prettier" image but one loses resolution (and light grasp) by stopping down a telescope. Using full aperture and patience results in the best views.

http://www.skyandtelescope.com/howto/visualobserving/3305656.html?page=5&c=y

My experience with using apodizing masks with Newtonian reflectors has been very postive. Some of my best planetary views have been through large Dobsonians that were equipped with apodizing masks.

http://www.csastro.org/gallery/article4.htm

Dave Mitsky

Re: Aperture masks

savyone — Sat, 04 Jul 2009 16:12:09 GMT

Very thorough answer. It would take me a day surfing the net to find that much good info. I started reseaching off-axis masks, and think I have found a new hobby. Apparently, having a SCT limits the types of masks I can employ such as the apodizing filter. If I could find something that worked like a Fstop camera iris, I could have a good old time experimenting around. Not to change the subject but, in looking for info on the types of masks you desribed, I came across this little Golden nugget. Apparent field of view (72deg) divided by the magnification (69x) equals the actual field of view. (1.043 arc degrees). Can you verify this to be correct? I enjoy using my setting circles in finding DSO's and this formula could be very usual. Thanks again for such a good clear description of aperture masks.

Randy

Re: Aperture masks

chipdatajeffB — Sat, 04 Jul 2009 14:16:01 GMT

Good question. There are several different types of masks, and different reasons for using them.

Whether and how much they help is a subject of considerable debate, but since they're easy to make and apply then the best way to determine whether a mask will help you in a particular way is to make and apply one.

The Hartmann Mask (and the newer variants called Bahtinov masks) are simple covers for the objective of a telescope that allow you to easily achieve focus. These are used mainly by imagers, who need such assistance to get "close" enough to focus to avoid wasting long exposure times going back and forth between image capture and image analysis. The technique is simple: an objective cover with symmetrically arranged holes (usually two or three, often round or triangular) diffracts the incoming light and presents diffraction spikes around bright light sources. You focus the telescope while observing (or imaging with short exposures) and continue until the diffraction spikes merge into a minimum number of sharply focused spikes. That gets you close enough to begin your imaging or observing run. The usual routine is to focus in this way on a bright star, then lock focus and move the telescope to your target area with the mask removed and begin imaging.

A simple aperture mask can also be used to "stop down" the aperture of an achromat by masking off the outer few percent of the aperture's diameter. This is the thinnest region of the lens and contributes most to chromatic aberration. By masking it off, you reduce the "false color fringing" in the view or image. For a 150mm aperture, a mask with an aperture of 130mm typically presents a much more pleasing view, from the standpoint of color fringing.

An apodizing mask can be used to enhance the contrast of an image. These normally are used with larger apertures, where there is sufficient image brightness to offset the light loss due to the filtering effect. An apodizing mask is a series of screens of different density, presenting a gradient that is densest at the outer edge and thinnest in the center. For a 200mm aperture, you'd typically apply a 50% screen to the outer 10mm or 20mm, a 25% screen to the next (inward) few millimeters, and then a 10% screen for the next (inward) few millimeters, leaving the inner 130mm or so unfiltered. Depending on who you talk to, and the quality of the original view, you may find a visible improvement in contrast in the image. Planetary observers and imagers often find these masks very useful.

An off-axis mask is sometimes used to greatly restrict the amount of light reaching the eyepiece or camera for solar or lunar observing. These masks are generally separate apertures cut into a full-aperture-blocking mask, thus reducing the aperture. For a 200mm aperture, a lunar off-axis mask would commonly provide a 102mm opening. For solar viewing, a 60mm or 90mm opening is more common.

While I have used such masks in the past, I typically today rely on the quality of the optical system instead, using an aperture appropriate for the task at hand. If you have a single telescope, however, the masks provide additional flexibility. They are very popular, for example, for users of the 20-cm SCTs.

I still frequently use masks that are square or hexagonal when splitting close double stars where the primary and seconday are of very different magnitudes. A good example is Sirius and its companion. By rotating a full-aperture mask with a square hole cut in it, you can discern the companion at lower manifications, or employ higher magnifications to allow for measuring its separation. This works because as you rotate the mask, you also rotate the diffraction spikes it produces. Since most of the energy in the diffraction spikes comes from Sirius, and less from the companion, you can rotate the mask until the companion appears between the spikes and thus see it more easily.

The mask I use for splitting close doubles with a 6-inch apochromat is a simple carboard box fitted over the dewshield. The box has a round hole on its objective side, sized to be a close fit to the dewshield. On the side opposite the objective (the side facing the star), there is a square hole about 5 inches on a side. I fitted "straps" made of large rubber bands to the box, one on each side of the square hole. I then use opaque material (cut from a sheet of stiff, flat vinyl) with holes of different sizes and shapes as the actual mask. By slipping the mask beneath the rubber bands, I can apply different hole shapes and sizes easily. The shapes that seem to work best are squares and hexagons, about 2/3rds the diameter of the objective.

Does this help?

Aperture masks

savyone — Fri, 03 Jul 2009 23:42:57 GMT

Can someone give me a run down on the different types of aperture masks and they're use? I've heard about them being used to focus during astrophotgraphy, but can they be used in splitting double stars? Do they help in planetary observations????

Randy