telescop

Meade 10" LX-200 Telescope - Helpful Hints for Astrophotography

I am using a 10 inch Meade LX-200. It is a superb, affordable telescope (in the vicinity of $3000. - Some people spend more on stereo systems). The telescope system is electronically controlled so that there is no guesswork involved in pointing the telescope at a particular object. I have successfully photographed objects that I could not see visually through the telescope (The Horsehead Nebula and the Cone Nebula are examples). The database built into the scope contains 64,000 objects. In addition, you can enter the coordinates of any object desired. This is handy for comets and other things not already in the database. MegaStar is a software system I use to locate current comet coordinates or other objects of interest that are not already in the LX200's database. I also use MegaStar to plan my photographic expeditions.

The "wedge"

For astrophotography, an equatorial mount is essential. I have the Meade "SuperWedge". The SuperWedge bolts to the top of the standard tripod. The telescope is then mounted on the wedge. The wedge has mechanical controls so that the elevation and azimuth of the telescope forks can be adjusted. The equatorial mount allows the telescope forks to be adjusted so they are parallel to the earth's axis. This is essential for astrophotographs whose exposure time is greater than about five minutes. If the standard tripod is used with the telescope in "AltAzimuthal" mode, "field rotation" will be seen on photographs whose exposure time exceeds five minutes or so. This means that the stars appear to circle about the center of the photographic plate and leave circular trails. These "star trails" are generally undesirable.

I have added a "thrust-bearing" to the declination adjustment rods to improve the smoothness of the adjustment mechanism. The thrust bearing can be obtained from ACE hardware. (Well I didn't know that initially and thought such an oddly named item would be hard to find...) I have also recently (May 97) added thrust bearings to the Azimuth adjustment rod. The smoothness and "turnability" of the knobs has been greatly improved.

The Declination Drive

I found it useful to make a few tweaks to ensure the telescope's ability to handle astrophotography. The telescope comes with an excellent declination drive. The declination drive consists of a motor that drives a series of circular gears that drive a worm gear that drives a very large gear that directly drives the telescope declination axis. The worm gear is pushed against the large gear by a spring that is part of the mounting of the Declination drive assembly. Mine came with two springs presumably because someone thought more pressure was needed. I was having difficulty with a phenomenon called "retrograde" motion in the Declination axis. This means that during guiding, after having gone South for a while, a command to go North results initially in movement to the South THEN movement to the North. The initial Southerly movement is "retrograde", i.e., it is going in the wrong direction. This makes it difficult to guide the telescope during photography. It can confuse an autoguider also under some circumstances. I used a Radio Shack laser pointer and a small mirror to investigate the behavior. The mirror was stuck to the end of the telescope with double sided tape. The laser beam was reflected off the mirror to the wall some distance away. By carefully observing the dot during guide commands, I was able to try various things to eliminate the retrograde motion. I noticed that during reversal, the declination worm gear reverses and puts torque on the declination assembly which rotates a bit compressing or decompressing the aforementioned spring. This rotation is directly related to the retrograde motion although it seems to defy rational analysis. (I believe it is because the worm gear is parallel to the large gear and the large gear's teeth are angled to fit it. If the worm gear were mounted at an angle and the large gear's teeth were parallel to the axis of the large gear, the problem might not arise?)

By eliminating one spring, the rotational motion was reduced. A field test in my back yard confirmed the wisdom of reducing the spring constant in the case of my particular telescope. All retrograde motion has been eliminated. Reversal backlash delay (the time it takes time for the system to react to reversal of the worm gear rotation with no correction , i.e., "backlash" setting of 0) went from 6 seconds to 4 seconds. This is GOOD. Meade makes allowances for backlash. There is a backlash correction setting that causes the motor to run quickly when a reversal is commanded. The length of time that the motor runs quickly is user-controlled by the "backlash" setting. This essentially takes up the slack and results in zero backlash if set properly. I have noticed that different values may be required depending upon the telescope's orientation. I usually check it and set it for minimum backlash delay with no image jump right after I identify a guide star and right before I start setting up the autoguider for my next photograph. Image jump will occur if the backlash correction value is set too high because the motor runs too long and overshoots the required backlash amount. I can usually get a setting that reduces the backlash to half a second or less without image jump.

I have taken 3 hour photos (The Cone Nebula photo shown earlier is one) using the Meade Pictor 201XT autoguider that are guided perfectly. I have been extremely pleased with the results!

I also took a photo during windy conditions where the wind was literally whistling through the telescope setup! (The wind came up after I had started the photo. Interrupted my nap when it shook the motor home.) The guide lock was maintained over a two-hour period. The gusts of course did affect the photo. The stars were slightly enlarged and fuzzed a little. Not suitable for anything but I was surprised that the autoguider did maintain its lock.

Iterative Alignment of the Meade LX-200 Telescope

There has been much discussion on the Meade Advanced Products Users Group (MAPUG) regarding the best way to align the telescope for photography. Some of the most respected amateur astrophotographers still use the "drift method" which is described in the Meade Manual. My personal preference is to do what is called iterative alignment which I first suggested on the list server that is now known as MAPUG. (See Iterative Alignment E-Mail Messages if you are interested in the history of the development of this method.) Refinements of the procedure have been made by several other active MAPUGers since that time and I believe it is now in use by many people. I always do iterative alignment. I have never used the drift alignment method. I believe the three-hour exposures I've taken demonstrate the accuracy of the iterative alignment procedure.

Telescope Fork Adjustment

This is something that I found to be rather important if pointing accuracy were to be ensured. Note however that this is a difficult adjustment and should not be undertaken unless you are absolutely sure you know what you are are doing.

The telescope "Optical Tube Assembly" (OTA) is attached between two fork arms and is free to swing between them. The axis of that swing is essentially parallel to the base of the fork arms. Now for precision pointing accuracy, you want that axis to be precisely perpendicular to the Right Ascension rotation axis. There are little adjustment screws that allow one to improve the precision. On my telescope, I noticed a minor pointing accuracy problem which I traced to this adjustment. With the scope electronics turned OFF and by using Polaris and iteratively adjusting the OTA axis between the fork arms, rotating the telescope about the RA axis (RA is unlocked), adjusting the wedge, and adjusting the declination angle, I was able to reduce the error significantly. I am afraid that I cannot accurately describe the procedure here without writing volumes. The end result though is that with the scope power off and when the declination is accurately set to 90 degrees and the wedge is adjusted so that Polaris is in the center of the view field, Polaris remains in the center of the view field when you rotate the telescope about the RA. It is this adjustment that I believe is the reason that I was able to enter the coordinates for the Cone Nebula with absolute confidence that the Cone Nebula would be right in the center of the film frame even though I have no hope of ever seeing the Cone Nebula visually. (There is a "High Precision Pointing" mode for the Meade LX-200 series scopes that I have never used because I do not need to. Again I believe this is due to the telescope fork adjustments I have made.

I was told that there is a procedure for aligning a monopulse radar called "plunge and rotate" that accomplishes this same task for a radar dish. You accurately point the dish to a transponder on the horizon. You then rotate the dish 180 degrees in Azimuth. Then rotate the dish elevation 180 degrees and check the transponder signal. You repeat and adjust the antenna angle between the forks until the "plunge and rotate" operation centers the transponder every time. Don't think the LX-200 would allow us to do something similar to this though.

Off-Axis Guider Prism Adjustment

Photography requires use of an "off-axis guider". I use the Meade Off-Axis Guider. I discovered that the prism used to divert off-axis light to the guiding eye-piece or autoguider should be adjusted to suit the particular telescope you are using. This adjustment can vastly improve the quality of light seen through the Off-Axis Guider. The prism angle depends on the physical characteristics of your telescope so this is not something that can be set at the factory. This is probably the only place you will hear about how to make the Off-Axis Guider Prism Adjustment.

Taurus Tracker III Off-Axis Guider

Since 19 May, 98, I have been using the Taurus Tracker III Off-Axis Guider. It has two eyepiece ports instead of one. One is a "flip-mirror" the other is for the autoguider. The "flip-mirror" port is actually a slide-in-and-out item but it allows one to see what the camera would see and you don't have to disconnect the camera. It also allows you to do all focussing using an eyepiece in the "flip-mirror" port. Once it is set up, you never have to try to look through the camera again to focus. In the past, I spent a *lot* of time focussing. The tracker III totally eliminates the need for bright camera screens. The only drawback is that you have to remember to "flip" the "flip-mirror". This little problem has cost me a couple of photos. Its just hard to think straight at 2:00 AM. Otherwise though, I am sold totally. I was a little surprised when I got it because it is not made of metal. This has not been a problem. It is a in fact a lot lighter than it would have been had it been made of metal. I also did tests regarding vignetting when using an f/6.3 focal reducer. It is actually seems to be a little better than the Meade off-axis guider. Certainly no worse. Anyway, I am quite happy with it.

Periodic Error Correction (PEC) Adjustment

This is well covered in the Meade manual. No gear is perfect. The teeth have tiny imperfections that cause the telescope to wander slightly as it tracks Right Ascension. This motion error is "periodic" in that it behaves the same way during each revolution of the worm gear which takes 8 minutes. By using a guide star and an eyepiece with cross-hairs (I use the Meade 9 mm eyepiece) the LX-200 will "learn" the corrections that you make as the worm gear turns. Those corrections will then always be applied automatically and electronically in the future so that the error is nearly perfectly counteracted. (Assuming that you did a nearly perfect job of teaching it. You can reteach it anytime you want of course. Count on 20 minutes of tedium...)

More recently, I have set up Maxim DL to autoguide on a Magnitude 6 or so star and train the PEC using the corrections generated by the autoguide process.

Training the PEC of course is somewhat dependent on seeing so it should be done during good seeing conditions. Otherwise the guide star will be jumping all around due to atmospheric activity in addition to its apparent movement caused by periodic errors of the worm gear.

Meade Pictor 201XT Autoguider

I am totally happy with the Meade Pictor 201XT autoguider. Without the autoguider, you have to look through an off-axis guide scope during the entire exposure and keep making adjustments to keep the guide star centered in the cross-hairs. I was able to do this for exposure times up to 30 minutes but no longer! With the autoguider, I plug it in, make some initial adjustments until it is tracking, open the camera shutter, and go take a nap. The autoguider "looks" at the guide star and gives commands to the telescope to keep the guide star centered. Works great!

Meade 208-XT Autoguider

Well, I was totally happy but I started trying to photograph a few more galaxies such as Stephan's Quintet. These galaxies are located out of the plane of the Milky Way so there are very few surrounding stars from our galaxy. That's why you can see them well in the first place, i.e., nothing is in the way. Unfortunately, that means that there are few if any bright stars that can be used for auto-guiding. In fact Magnitude 11 and beyond seems to be typical. The 201XT is right at its limit at Magnitude 11 in my opinion. What is happening is that the "dark-current", the current that flows in a CCD device even when it is totally dark, is enough to swamp the few electrons generated by Magnitude 11 stars. One has to reduce the dark current. One does that by cooling the CCD device with a thermoelectric cooler. The 208-XT and the SBIG ST-4 both have thermoelectric coolers so both are candidates. The overwhelming and prevailing choice among the amateur astronomer community is the ST-4. When I asked on the APML list if anyone had done any real comparisons though, no one replied very definitively. So about that time July 1999 or thereabouts, I was offered a good deal on a 208-XT. But I hedged my bet by also acquiring a SBIG ST-4. My plan was/is to actually do some comparison tests and get some idea whether one really is a hands-down winner. If you've seen any Arizona weather maps lately, you will understand why that has been on hold. I have not yet updated the little box I made containing the voltage regulators that provide power to the camera and to the autoguider. I need to be sure that the regulator for 12 volts to the 208-XT can handle an amp without melting down.

SBIG ST-4 Autoguider

Took a long time to collect the connectors to interface the ST-4 to the LX-200. Bought wires and connectors originally from Atronomics but they did not have an 8-pin phone connector connected to the end to plug into the LX-200. Then had to sort out the wires. Took additional time to realize that the ST-4 left-right-up-down buttons don't operate at times when I expected them to. (Software has to be in certain modes but the effect on the buttons is undocumented - If I recall properly, when the ST-4 is not "tracking", the relays do not operate the telescope so I kept thinking I had it wired wrong when I command movement and nothing happened...) Anyway ST-4 is ready to go at this point. Tradeoffs are that the ST-4 has a control box and the 208-XT doesn't. The ST-4 camera is lighter than the 208-XT. So each has its good points and bad points. The thing that really matters though is how easy it is to get a lock on a dim star and how likely is it to maintain that lock. We will see...