Last update 99-12-23

Experiences with the Meade LX10, part 3


LX10


LX10 and CCD,

99-12-23

Experiences with LX10, dec motor, CCD and autoguiding:

Here first some basic things to know of using CCD'S  instead of the normal photographic film:

1) CCD are very sensitive. This is top and one of the major advantage. The quantum effeciency (how much of the incoming photon's is used) is somewhere between 15% and 60 %. This is equivalent to a photographic film with a sensitivity of 10.000 ASA or so.

2) They are linear and are not suffering from the Schwarzschild effect. There is no minimal amount of light required. That means they are ideal for magnitude measurements.

3) They are in general not only sensitive for normal light but also in the near infra-red and less sensitive in the blue part. (That's why the combination refractors and CCD does not work very well.)

4) The chips for amateur use are in general small. Typical 3 x 4 mm till 7x 5 mm. A large expensive amateur CCD chip has the dimensions of 14 x 9 mm. That's small compared with the standard film 24 x36 mm. The field the camera sees is accordingly also small !!

5) They need to be cooled to reduce there thermo-electric noise. A power supply and a computer is required for there operation.

6) Some chips need a shutter.

So mayor thing for selecting a CCD camera:

A) Chip size.

B) Mechanical shutter required ? (Depends on type, planet or deep sky use.)

C) Guiding. How to guide the camera. Some have an extra guiding chip or can accumulate the result automatically.

D) Software, how is transfer to  PC working. Transfer time.

E) Price.

Experiences with auto guiding the LX10:

I bought the Alpha Mini from OES. This camera uses the Philips chip FT800 of 6.4 x 4.8 mm2 (pretty large). This chip is intended for professional camera's and has a quantum efficiency of around 20%. Being cooled the camera chip reaches in my room in about 20 minutes saturation due to the noise.(pretty quick but workable) The maximum exposure time is the limited around that value. The chip seems to be out of production since November 99. The big advantage of this chip is the electronic shutter build inside the chip,  so planet exposures till 0.001 seconds are possible. Almost all professional CCD's need for this a mechanical shutter. The provided software was for DOS but very good. (The company makes CCD's, for 8 years or so)

First I did several experiments with a standard camera lens mounted on the Alpha Mini CCD camera. After pointing it to the dark streets around my house, is was nice to see how they changed in lighted streets using a exposures of seconds. Also interesting is the use of binning (combining chip pixels) and transferring only a part of the chip to the computer. This influenced to exposure time and transfer time dramatically.

The camera was then mounted behind the telescope. With or without the off-axis guider. A problem occurred getting the off-axis and the camera both in focus. With an extension ring to the camera this was solved.

I also bought from OES an guiding camera, the LC-CCD07. This one has a Texas TC221 chip of  2.6 x 2.6 mm2. Cost new around 300  Euro. This camera is very light and has a short picture transfer time.

With this guiding camera, I made some pictures using my standard Minolta camera. Finding a guide star is with a off-axis not so easy and with a small chip even more difficult. In some cases, the easiest thing was to first find a guide star using an eye-pc and then after centering switching back the guiding camera. With some rings, I was able to get both the main and off axis camera focus good. With the eye-pc half pulled out, this one was also in focus but it is something to notice !.

The pictures will follow but one of the main experiences where:

1) If focus of main camera is good, the  off-axis should be fixed with rings. If a good combination is found this problem is solved for ever.

2) Due to the possible orientations of th camera, it took a long time to find the correct corrective direction in RA/DEC. After document the few possiblities, this problem was for my finally fixed.

3) The LX10 needs guiding every 15 or 20 seconds or so. This limited the exposure time of the off-axis camera (and transfer time)

The setup, installing the computer and telescope, focus, finding the object and finding guide star, setup guiding camera and software could take hours. But as soon the setup was running, it was a imense pleasure to see in the garden the telescope humming and following every 10 or 30 seconds or so and I was enjoying a nice warm drink in my house. I only left the the nice warm living room every 15 minutes or so the inspect the guiding and then returned to the room. A few pictures of an hour exposure where made, but  clouds spoiled the fun again.

The declination motor of the LX10:

I was already warned, but the declination motor is not a top design. I does wat it has to do but hand control directly becomes very difficult. The problem is the the motor can not be quick disconnected. The is still one hand button left. but you have to turn through the clutch. Adjusting the clutch so, that is slips nicely is very difficult. In my case it slipped too much or it was bight-ing itself into fixed connection and no hand control directly was possible. There is in my opinion a spring in the design missing.  I finally improved it by introducing a small rubber ring and a washer under the nut of the clutch. This introduced enough spring so that the two clutch wheels could adapt (flexible) to each other.

Experiences making CCD picture using the LX10:

In december there was finally a clear night again after waiting for weeks. Based on the hassle of setting up auto guiding , I decided to try hand guiding. Till my surprise it worked out very well. Secondly due to the high sensitivity, hand or manual guiding of a 120 seconds or so, is not big job. Guiding was required every 15 or 30 seconds or so. Here are my first results:


m42-9912.jpg

Unfortunally this JPG picture of M42 does not give all the fine details of the orion cloud.

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The famous horse head in 180 seconds.

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Just a galaxy I could find in the clear night.

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This demonstrates the electronic shutter of the Alpha Mini, but also that focal length of telescope should be optimized to the CCD chip.


98-7-10 I have ordered an CCD camera, the "Alpha Mini" from OES germany. The camera will arrive within a few days. Before that I already prepared for automatic guiding. First I ordered the LX10 declination motor and secondly I made an interface box for connecting the LX10 to the  PC parallel port. In this box  I used some small reed relays as follows (see also the schematic of the hand controller in LX10 experiences part 2):

Additional to this schematic, I added 4 LED's with a 2.2 Kohm resistor parallel to the 500 Ohm relays coils and 4 buttons parallel to the 500 Ohm relay contacts. Using this 500 Ohm relays there is not additional power supply required. The parallel port has for each pin a typical shortcut current of about 50 mA and is just be able to deliver enough power for the 500 Ohm relays.