The RHUL Observatory
Introduction for Users
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The RHUL Observatory
Introduction for Users |
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This document covers the basic information needed by students who will be using the RHUL observatory for project work. It should be read and understood before you begin working with the telescope and other equipment in the dome.
The telescope is a Meade LX200, a Schmidt-Cassegrain with a 10-inch primary mirror. There is a copy of the manual in the dome and it is also available online.
If you point the telescope down you can look in the tube and see the important parts: light goes through the thin lens (Schmidt corrector plate), reflects off the spherical primary mirror in back, the hyperbolic secondary mirror in front and then goes down the hole in the middle of the primary mirror.
Please do not touch the corrector plate! It looks like it has lots of dust on it but this is not really a problem. Finger prints, on the other hand, are much worse, and the lens is delicate.
Recall that a spherical mirror will not focus rays parallel to the optical axis at a single point; this is spherical aberration, and can be corrected by making the mirror parabolic. The problem with parabolic mirrors is that they suffer from a second type of aberration called coma. This is where incoming rays that are not parallel to the optical axis are not focused at a point, so the coma-free field of view of a Newtonian telescope with a parabolic mirror may be quite small, say, only a few arc minutes (Jupiter is almost an arc minute across).
Schmidt's solution was to keep the mirror spherical, hence not introducing coma in the first place, but to correct for the spherical aberration by means of a thin lens, the Schmidt corrector plate. This is the main advantage of a Schmidt-Cassegrain telescope, namely, the wide coma-free field of view. We are interested in looking at the moon and sun (each a half a degree across) and nearby galaxies (many are several tenths of a degree across) so a wide field of view is important.
The "Cassegrain" part of Schmidt-Cassegrain refers to using a secondary mirror to send the light back down a hole in the primary. The light travels through the tube (around 50 cm long) three times so naively one would think the focal length would be around 150 cm. In fact it is 250 cm. This is because the secondary mirror is not flat but rather a hyperboloid, so the rays coming from it are spread apart leading to a longer effective focal length. The diameter of the mirror is 25 cm, so the focal ratio is f/10.
The diffraction limit to the angular resolution is about 0.45 arc seconds. In fact the limit from atmospheric considerations is more like 1 to 2 arc seconds, so the large size does not really improve our angular resolution over, say, a 5 inch mirror. The main advantage of the large aperture is that it collects more light.
The telescope is equatorially mounted. The axis of the fork points north and its angle with respect to the horizontal is equal to the local latitude of 51.43 degrees (51 degrees 26 minutes), i.e., the fork is parallel to the earth's axis. This happens to point 0.8 degrees from Polaris. As the earth turns to the east, the telescope turns about the same axis at the same rate but to the west, so that the telescope stays pointing at a fixed direction in space.
Remove the telescope's plastic cover and four lens caps (main, rear cell, and the two caps on the finder scope).
Open the dome by unscrewing the nut holding the doors together and sliding open the doors. If the doors stick, make sure the blue cords are not caught and/or use the long pole to gently nudge them open. When rotating the dome, it's best to keep an eye on the blue cords to make sure they don't get caught anywhere.
Power to the computer and the telescope is connected to the mains outlets under the desk. Turn on the mains and also the power switch on the telescope. After a short initialization time you can then use the hand paddle to slew the telescope around with the N, S, E and W keys. The four keys on the left row of the paddle control the slewing speed.
Do not attempt to move the telescope manually when the motor gears are engaged; this will damage the gears. Either disengage the gears (see manual to find the declination and right-ascension locks) or use the hand paddle.
Find the 1.25-inch eyepiece holder and screw it onto the telescope's rear cell. Take care not to damage the threads or to over-tighten any telescope attachments; firmly finger tight is sufficient.
Insert an eyepiece into the eyepiece holder and tighten the set-screw. Practice pointing the telescope at some bright objects such as the moon. Start with a lower power eyepiece (e.g. 32 mm). First find the object in the finder scope and centre it in the cross hairs. You should then be able to see it through the eyepiece. The focus knob is on the rear of the telescope (see manual). This actually moves the primary mirror in and out.
You can also log into the PC and start up e.g. Internet Explorer, Starry Night, and programs for controlling the CCD cameras. For reference, the PC in the dome is PC62.PH.
For details you should refer to the LX200's manual. Here we will summarize the important features.
The LX200's computer has five main "modes", which can be cycled through by pressing the "Mode" key. Almost all of the work you will do is from Mode 1 (Telescope / object library). For now, stay in this mode. Don't confuse these "main modes" with the various sublevels of menus that you will get into from Mode 1.
To select a menu item, move the arrow using the arrow keys in the lower right of the paddle until it points at what you want, and then press the "Enter" key. This may cause you to enter into a submenu. To go back from a submenu, press the "mode" key.
The tracking motors may or may not be on depending on the state that the computer was left in when it was last turned off. To check this, select "telescope" from the main menu and press enter. You will then see the options "site" and "align". Select "align". You will then see the options Alt/Az and polar. There are more options below that you can see by scrolling with the arrow keys. To turn off the tracking motors, select "land". To turn them on, select "polar". Then press "mode" twice to return to the main menu.
The LX200's computer has a library of astronomical objects and it can be told to point the telescope at them automatically. When the telescope is first turned on, however, it does not know in what direction it is pointing. So you should first point it at a bright star, say, Vega, which you can identify from Starry Night. Then you need to tell the LX200 that you are pointing at Vega. To do this, press "star" (number 6 on the keypad). You can then enter the number of the star; there is a list in the back of the LX200's manual, and some common stars are noted on the whiteboard in the dome. (Vega is star number 214.) Alternatively you can press "enter", and you will be prompted for a name. Press "enter" again and you will see an alphabetical list of names. Scroll through the list until you find the star you want and press "enter". The readout should indicate that it recognizes the star, but it does not yet know whether you are actually pointing at it.
Now make sure that the star is well centred in the telescope's eyepiece. Then press "enter" and hold until the hand paddle beeps. It should display the message "coordinates matched". From here in principle the telescope knows which direction it is pointing. If at any point you disengage the RA or dec drives this will be lost. So if you want to slew the scope to a different direction you should only use the hand paddle.
You can now use the goto feature to go to any other star or astronomical object known to the LX200's database. First, select the object to which you want to go. For the Messier objects, press M (key 9) followed by the object's number; for stars, press "star" and enter the number or select the name as above. For planets, use "star" and enter the numbers 901 through 909 for Mercury, Venus, etc.
WARNING. If you have a CCD camera or other large device connected to the rear cell of the telescope, there may not be enough clearance to move the telescope to declinations near 90 degrees (i.e., north). The computer will not, of course, know what is mounted on the rear cell, and it may happily crash the device into the fork. You should keep an eye on the scope as it moves. You can stop the scope by pressing goto again. If all else fails, be prepared to turn off the power switch.
We currently have two CCD cameras that allow you to take photographs that can be analyzed. When you're done taking a series of pictures you should move them to a folder elsewhere on the computer. It's probably best to first create a folder with your name on the C drive. Then inside this, create a folder with the date of your observations, e.g. 8Oct02. From there you can either transfer them to your Y or W drives (if you have enough room), or you can write them to a CD.
Our main CCD camera is an ST-7E made by the Santa Barbara Instruments Group (SBIG). It has two CCD chips, which you can see by looking in the front end (other than for quick looks please keep the cap on the camera to keep dust out). The larger chip (6.9 mm x 4.6 mm) is to record the actual image and has 765 x 510 pixels (390,000 total), which are 9 x 9 micron squares. The smaller one is a guide chip which sites on a star in the field of view and sends feedback to the telescope motors to keep it pointing in the same direction over the course of a long exposure (in principle up to an hour).
When light hits one of the pixels of the CCD, electrons are liberated by the photo-electric effect. The probability for a single photon to produce a photo-electron (the quantum efficiency) is around 65% for wavelengths around 600 nm, falling off to around 30% at 400 and 800 nm. This is an order of magnitude better than photographic film. Note that a factor of 10 in quantum efficiency is equivalent to a factor of sqrt(10) in telescope aperture, i.e., a 25 cm mirror with a CCD collects as much light as an 80 cm mirror with photographic film.
The photo-electrons can be stored in the pixels over long periods of time with very little loss of charge. At the end of an exposure, which can last from 0.1 s to 1 hour, the contents of each pixel are read out row by row into the computer. The number of photo-electrons in each pixel gives a direct measure of the number of photons.
Even in the absence of incoming photons, the pixels will start accumulating electrons (dark current) and since these are indistinguishable from photo-electrons, the dark current is a limiting factor in the camera's ability to detect faint objects. The dark current can be suppressed by about a factor of two for every 5 degree C reduction in temperature, being about 36 electrons per pixel per minute at 0 degrees C. The camera has a thermoelectric cooler and in normal operation we can maintain its temperature at about -20 degrees C.
In addition to the SBIG CCD, there is a webcam CCD that has been remounted in an aluminium box with a 1.25 inch tube that can be inserted into the eyepiece holder.
The webcam is a Logitech QuickCam Pro 3000 with a 3 x 4 mm CCD chip containing 640 x 480 pixels. If you remove the lens cover you can peer in and see the chip. As with the expensive CCD, please don't do this more than for quick looks as we would like to keep the CCD as dust free as possible. The purpose of the fan in the box is to cool the CCD to reduce the dark current. This is only important while looking at very faint objects; for things like the sun or moon the fan is not needed.
To use the QuickCam CCD, insert it into the eyepiece holder and connect its USB cable to the USB cable lying by the base of the telescope, which is connected to the PC. From the PC's program menu, start "Logitech QuickCam". To adjust the camera's exposure and brightness, click on the `Settings' button and go to `Advanced camera settings'. From there, you can turn on or off the automatic exposure controls and adjust the exposure, brightness, contrast, etc. To take a picture, click on "take picture". And so forth.
When you're done observing, be careful to go through the shutdown checklist carefully. Several of the steps, if missed, could lead to serious damage to the equipment.
How much did it cost? The telescope was around 3000 pounds, the CCD camera a bit more. The nu-view spectrometer was 2500. In the US, replace pounds by dollars but keep the number the same.
If you have a hole in the telescope, why don't you get a hole in the picture? Answer please in 25 words or less.
Do we have problems with light pollution? Yes, a lot from the M25. These are high pressure sodium. Incandescent lights are the worst (continuous spectrum cannot be filtered out); mercury is better but it still has many lines all over the spectrum. High pressure sodium is better and low pressure sodium, which gives a very pure yellow, is the best, since its light is concentrated almost entirely in a closely spaced doublet of spectral lines. Still, with the CCD camera we should be able to measure the sky brightness due to local lights and subtract this from the astronomical images.
Why is there a red light in the dome? This is for reasons of dark adaptation. There are two things that the eye does to adapt to the dark: the pupil opens up to a diameter of 5 or 6 mm, and the chemical rhodopsin builds up in your retina, which greatly increases the sensitivity to light. Your eye has two types of receptors called rods and cones. Under normal (bright) light levels, most of your vision is with the cones, and the three types of cones provide colour vision. The rods are only active if the rhodopsin molecule is present, and under normal light levels this is broken apart. In the dark, its concentration builds up over a period of a half an hour or so. This gives the rods about a 100 times greater sensitivity than the cones, but no colour information. It turns out that rhodopsin is not destroyed as much by red light, so if you have to see something while you're dark adapted it's best to use red. There's also a night vision mode of the Starry Night software where everything is in red.
Does the telescope vibrate? Hopefully less now that we have filled the steel pier with sand. If you have the QuickCam CCD hooked up, set it in 320 x 240 mode and try tapping on the pier. You can see fairly large movements that take several seconds to damp out.