201XT Trials

May 30, 2003 On a partly cloudy night I decided to run some guiding tests to try to come to grips with the excentricities on the 201xt. Here's how it went:

Test 1 Description: Collimated the guide scope with the main scope, put Arcturus on the guide chip and started taking some guided 5 minute unbinned exposures.

Test 1 Results: Of the 5 images I took, 4 had round stars, but all 4 showed signs of star elongation in the lower right corner. It was also apparent that focus was soft with faint stars showing as fuzz instead of points. In shorter exposures this was not noticeable, so it looks like focus is even more critical with longer exposures.

Test 2 Description: In the second series of tests I tried guiding on fainter stars. I found that I could not establish guiding on any star that required more than a 3 second exposure. Once I had found a suitable star requiring only 3 seconds exposure, I ran a series of 10 minute exposures.

Test 2 Results: Of the 3 10 minute exposures I took, all had round stars, and all showed the focussing and star elongation issues already noted under test 1. These issues are demonstrated in this 10 minute exposure from test 2:

Test 3 Description: I finally decided to try guided images of a DSO. M4 presented itself as a decent target. I was unable to find a suitable guide star and had to point the guidescope at a star outside the FOV of the imaging area. Using the 3 point rings to find and line up a star on the guiding chip was NOT easy. There has to be a better way to point the guide scope. Once guiding was established, I got 9 5 minute shots of M4.

Test 3 Results: The first image had star trails, I don't think the guider had settled down. The next 5 images were good, then the guider lost the guidestar because of clouds so the final 3 images were unguided. Here are sample guided and unguided results from this session:

May 30, 2003 Testing Summary: As demonstrated by the images above, guiding makes a big difference and clearly the 201xt is capable of delivering reasonable results once guiding has been established. Establishing guiding is the difficult part though. Here is a summary of lessons and issues resulting from these tests:

1) The 201xt works best with exposures of 3 seconds or less.

2) The guidescope must be pointed to a guide star bright enough to use 3 second or less guide exposures. This sometimes means pointing the guidscope at stars that are out of the image field of view. To do this it is necessary to use the 3 point rings mounting the guidescope to the main scope. Finding and centering the selected star with the 3 point rings is difficult. A better method for pointing the guidescope is needed.

3) Focus at long exposure lengths is more critical; fainter stars can come out fuzzy instead of as points. It is necessary to refine focus using longer exposure times before beginning the actual imaging session.

4) Many images showed star elongation, especially in the lower right corner of the images. Need to determine source, polar alignment off? Guide scope flexure?

 

Circa October 2002:

We've had some serious cloud cover this Fall greatly limiting my time at the observatory. That said, I've made some progress with the Pictor 201XT.

Focussing the 201xt:

I read somewhere that a parfocal eyepiece can be made using a standard Meade 26mm eyepiece and a parfocalizing ring, leaving 1/8" of space at the top of the eyepiece. By using a bright star, I was getting a brightness reading from the 201xt of between 0 and 5 when focused with the 26mm (range is 0-99). I also read that the Meade 9mm reticle eyepiece is parfocal with the 201xt. I don't have one, but I do have a Meade 9mm MA eyepiece that came with the ETX70. When I focussed for the 9mm I instantly got a reading of 99!

So it seems that the 9mm MA eyepiece is parfocal with the 201xt.

Centering the target star in the 201XT:

After the focussing breakthrough I've been playing around with centering the guide star. I've been toying with the slow motion controls to get a centered reading (55). I don't have an adjustable reticle eyepiece, so I used a parfocalizing ring to make my 12mm reticle eyepiece parfocal with the 201xt. I've been drawing the view in the 12mm eyepiece when I get a centered reading. It appears that the 201xt chip needs the target to be slightly left and slightly above center as seen through the parfocal 12mm.

Next steps:

Now that I think I can focus and center a bright star, I need to try it with a dimmer star. A brightness reading of between 20 and 40 is suggested as the right range. Assuming I have success with a dimmer star, I'll need to get the 201xt talking to the LX200, callibrate, then guide. If I can get it guiding without errors and keep a subject centered at high magnification, I'll be ready to try extended exposures with the SAC7, can't wait!

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