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PW's web pages - Astro tips

Eyepiece tips Dew cap Polar alignment I Polar alignment II Mounting a camera I Mounting a camera II Hartmann-(Scheiner-)mask WebCam for astrophotography LED flashlight Preview





Eyepiece tips

For a long time I thought that only an eyepiece with a long focal length could have a large field of view. In fact the field of view depends only secondary on the focal length but primary on the diameter of the field aperture.
For selecting the focal length of an eyepiece there is an upper and a lower limit, depending on the size of the exit pupil. For the longest focal length of the eyepiece the exit pupil should not be larger than your own pupil diameter in darkness (normally about 6 mm), for the shortest the exit pupil should not be smaller than 0.7 mm. For the calculation you could use the following formula:
    f max = F / D * 6
    f min = F / D * 0.7
    D = diameter of the telescopes objective, F = focal length of the telescope
With the maximum focal length you get most out of your telescopes light gathering power. If the exit pupil is bigger than your own pupil, this is equivalent to a reduction of your telescopes objective diameter. If the exit pupil is smaller than 0.7 mm, the objects seems to be bigger but there are no more details visible. It is called "empty magnification". For the max. focal length of the eyepiece there could be two borderline cases:
a) The calculated value is more than 40 mm. Such a long focal length is not easy to get and mostly as 2"-version. The upper limit for 1.25"-eyepieces is normally 40 mm, but they mostly have less good viewing comfort.
b) The calculated value is less than 30 mm. With standard Ploessl-eyepieces you can not get the maximum field of view. For this case you should use widefield eyepieces.
For the eyepiece with the longest focal length you should know:
a) The focal length should correspond to the above formula.
b) Maximum field of view with max. diameter of the field aperture (about 28 mm for 1.25", about 47 mm for 2").
For the eyepiece with the minimum focal length there only should be a good viewing comfort. It is a problem for eyepieces with less than 10 mm focal length, because the distance of the eye to the lenses is very close. For this case you should use eyepieces with long eye relieve (Vixen LV, TeleVue Radian, Pentax XL, ...).
Between the eyepiece with the longest and the shortest focal length you should choose 2 or 3 more with more than half of the focal length of the previous (e.g. telescope 200/1000: 30 mm, 18 mm, 12 mm, 7 mm, 4 mm). An alternative is the use of a barlow lense, wich is mounted between diagonal and eyepiece and so increases the focal length of the telescope (mostly by factor 2).
The (mostly overrated) magnification of the telescope is calculated as follows:
    V = F / f
    F = focal length of the telescope, f = focal length of the eyepiece
I myself love minimalistic eyepieces: No widefields, no barlows, construction as simple as possible. Therefore a good Ploessl or the excellent Baader Eudiascopic. If possible I even avoid the diagonal.
For further calculations of eyepiece and other telescopic parameters you could use "Telescope calculator" at Software.

Cheep dew cap from corrugated board     photo

Everyone who has a closed telescope like a Schmidt-Cassegrain or even a refractor knows the problem with dew. At temperatures below 10°C and high humidity the front lense or the corrector plate respectively steams up in short time. The most effective solution is the (complicate) heating of the front lense. A more simple solution is a dew cap. For nearly every telescope you can get a dew cap optional. Mostly it's from metal, heavy, bulky and expensive. And metal attracts dew - especially at the outside. I found the most simple, less expensive and very effective dew cap in a roll of black corrugated board. On the tubes side I fix it with some rubber bands, on the open side with a paper clip. The ideal length is the single to double diameter of the telescopes tube. After use you can roll it up saving space. If it is destroyed you can buy a new one for few money. Until now I had no problems with dew even under bad conditiones and additional it protects from stray light.

Polar aligning the mount - I (by PW)     photo

For visual observations normally a rough alignment of the mount to the celestial northpole will do. Just align the rectascension axis to Polaris, as described in most user manuals. For high magnification and especially for photography you should do a more precise alignment. Of course you could correct a insufficient polar alignment with the telescopes hand controller but not the resulting field rotation. For the following method you need a polar finder, which you can get at least optional for nearly every mount. Mounts from Vixen and compatibles have even a circle for the position of Polaris and a cross for the true position of the celestial northpole. The problem is, in which direction from Polaris lies the true pole? Vixens user manual describes a method by using the rectascension circles, but which needs some calculations and the knowledge of your geographical position. But the celestial northpole lies nearly exact on the line from Polaris to Beta UMi (see red line on the photo). My method works as follows: Align Polaris to the middle of the polar finder (you must not level the tripod exactly horizontal) and rotate the rectascension axis until the counterweight axis is parallel to the line from Polaris to Beta UMi (see photo). Now use the screws on the mount to position Polaris exactly in the middle of the circle in the polar finder (or more precisely to the position 11 o'clock relating to the cross directly on the circle). Ready. For this method you must have a exactly aligned polar finder (see manual). This method of polar alignment is good enough for photos with lenses up to 1000 mm focal length. A more precise method is the Scheiner-method that can be found below. I make polar alignment after completely seting up the mount, telescope and accessories, because otherwise it is risky to deajust the equipment by pushing on it.

Polar aligning the mount - II (by Scheiner)

coming soon ...

Mounting a camera on the mount - I     photo

For photographing star fields you normally use a camera with wide-angle, normal or telephoto lense, mount it on the telescope or on the mount and take guided photos with exposure times of several minutes. For focal length up to 135 mm and exposure times up to about 15 minutes it is not necessary to control the guiding, as long as the pole positioning is done accurately (see chapter polar alignment). The most simple way to mount the camera is a piece of wood with square cross-section, that fits in the dovetail slot of the mount. The square cross-section makes it easy to change between landscape and portrait format (see photo). Additional you need also a photo-tripod-screw. Of course you should control the guiding, at least to find out if the batteries for the mount are giving enough power. But above method is just simple, cheap and effective. The rubber lens hood you can see on the photo is a good (and necessary) dew protection.

Mounting a camera on the mount - II (V2.0)     photo

Above method of mounting the camera onto the mount has the disadvantage of no control of the guiding but this is absolutly necessary for lenses with a focal length of 135 mm and more and you also should do that for shorter focal length. And with above method you could turn the field of view only by 90°. Therefore I build a device that carries the camera and the telescope at the same time. You need the following materials:
- A piece of aluminium to file the dovetail plate or a ready made part.
- (Guide scope) tube rings (e.g. Baader).
- A guiding scope with (illuminated) crosshair eyepiece (e.g. Baader/Celestron Microguide 12.5 mm - this has additional concentric circles with defined diameters and with a formula to calculate the necessary accuracy for guiding (e.g. with 135 mm focal length for photography and 400 mm focal length for guiding the guiding star must stay within the second circle).
- Photo ball head for better alignment of the camera (you also could mount the camera directly to the tube rings) as well as a photo tripod screw with fitting length for the ball head or the camera respectively.
The whole equipment should be well balanced relating to the dovetail plate.

Hartmann-(Scheiner-)mask for focusing astrophotos     picture

At astrophotography with long focal length or through the telescope - especially with the popular, older, mechanical cameras - it is very difficult to focus, because the focus screens are rough and dark. With changeable focus screens you have alternatives, also you could make use of the very exact "knife-edge-method" or other equipments for focusing, but they all have the disadvantage that you must control the sharpness at the filmplane and therefore you must open the camera before you take the photo. Not quite as exact but much more simple is the Hartmann- or Scheiner-mask. Cut out a disc from a piece of (black) cardboard as shown on the drawing with the diameter of the telescopes tube and two (or three) holes. The diameter of the holes should be 1/3 to 1/4 of the telescopes lense diameter for refractors and 1/3 to 1/4 of the telescopes lense diameter minus the diameter of the secondary for reflectors (the holes should not "touch" the secondary). You must find a compromise between holes as big as possible and as far away from each other as possible. Now fix the mask in front of the telescopes lense and aim at a not too bright and not too dark star. Now you should see two (or three) stars in the cameras viewfinder or the eyepiece respectively. If you focus, the stars move together. If you could see only a single star, you reached the optimum focus. Attention: Don't forget to remove the mask befor you take the photo!!!

WebCam for astrophotography     photo

In the internet I read something about modifying webcams for astrophotography. As my results on photographing the planets and the moon with standard SLR-camera and eyepiece projection were not satisfying (problems with focusing, exact exposure time, some shots are needed for eleminating turbulences, ...) I bought a Logitech QuickCam VC parallel. The parallel version was not easy to get but I needed it because my notebook-computer does not have an USB-interface. I did the modification as follows: You could open the body with a paper clip inserting in the little holes of the body. Then you are able to separate the two halfs. Then you must remove the lens and the IR-filter (green film), the lens-holder can be left on the plate because it is a good distance device. At e.g. Conrad-Electronic you can find cases of plastic of different sizes. Chose one, where the plate fits. Then drill a hole in the case, big enough for the case of the lense and one for the wire. Then paste the case of the lense with the circuit board to the inner side of the case and screw up the lid. Finally paste the 1.25" tube of an old eyepiece to the case (you also could use a 1.25" water pipe, also available with flange). Ready. Attention: Be very careful not to damage the electronic components on the circuit board, don't even touch it (because of electrostatics).
Some tips for taking photos with the webcam:
- Deep sky photos of nebulas, galaxies etc. are not possible because of the short exposure times. Bright clusters (Pleiades, etc.) can be tried. Never take photos of the sun without a special solar filter.
- For focussing you should use an eyepiece which is parfocal with the camera or produce a distance ring for the eyepiece, i.e. focus the webcam at daylight on a far distanced object and measure the length of the focuser. Then remove the webcam and replace it by a 5 mm Ploessl eypiece (this has a field of view corresponding to the QuickCam VC), focus and measure the length of the focuser once again. The difference of the two meassurements is the lenght of the ring. With ring and eyepiece you could search and position the object in the telescope, then remove ring and eyepiece by the webcam, make a precise focusing and take your photos or movie.
- Always use the webcam directly on the telescope, never with diagonal. Barlows and eyepieceprojection only with very good optics.
- Experiment with brightness, contrast and exposure times to get the best possible results.
- Always take many photos. For one good photo you have to take a min. of ten shots. Later you choose the best images and add them to get a even better result (for adding I use the software Giotto). You could increase the image quality by subtracting a dark frame (take a photo in an area with no stars or with the cover on your scope and the same parameters as the photos).
My photos with the webcam can be found at Astrophotogallery (mainly planets and moon).

LED flashlight     photo

Translation is coming soon. Look at the german page.



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More tips for the hobby astronomy can be found here soon.
  • Enhancement of CCD-photos
  • General tips for astrophotography
  • Diffuser for astrophotography

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