Stellar Journey

24-Jul-2007      00:00:00      (00:00:00 UT)    +Howard

Survey - Messier Objects in 4-1/4" F4 Newtonian Telescope on Meade LXD75 Mount

OBSERVATORY SHAKE DOWN/MESSIER SURVEY

This is the first program for the observatory and was started in October of 2011. It's primary goal is to test the autonomous operation of the observatory and it's associated equipment to identify and correct any shortcomings. A secondary goal is to create an image catalog of all Messier objects visible from the observatory with it's 4-1/4" F4 Newtonian telescope and modified Meade DSI II Pro camera. I want operations with the smaller telescope to be routine before switching to the 10" F4 Newtonian.

Collimation

The first problem I encountered was with collimation of the optics. A cheshire collimator was used to get things close by the usual methods (as in this document) and then the camera was inserted and set to be slightly out of focus. At this point, the primary mirror's collimation screws are used to bring the out of focus star images into symmetry about the center of the field of view. You can see an improvement due to collimation in the images below:




Polar alignment

I spent about half a night getting the mount polar aligned. I used the drift alignment method and the drift explorer in k3ccdtools. After several hours of iteration I got to within about 10 arc-minutes of the pole. I also shimed the OTA so that the optical axis is more parallel with the RA axis of the mount.


Focus

Another problem was with maintaining focus. My telescopes have very little temperature related shift in focus and I was hoping that an motorized focuser wouldn't be needed (at least not right away). But due to a combination of factors (filters in the light-path, mechanical movement, residual temperature related movement) I do end up needing to tweek the focus every now and then. Hence the addition of computer controlled motors to my two JMI focusers, more information is here.


Goto Accuracy

After fighting to find objects (in the smallish CCD field of view [FOV]) I realized that something would need to be done about the limitations of my Meade LXD75 mounts goto accuracy. So, I setup a virtual machine (running Linux/CentOS) and installed Astrometry.net software. This software accepts images of star fields and returns the location (RA, and Dec.) Then, I integrated Astrometry.net with my Sky Planetarium software and this allows me to quickly locate objects.


Autoguiding

This involves attaching an small refractor with it's own CCD camera to the side of the main telescope. This smaller telescope/camera provides images every few seconds to allow corrections to the motion of the (LXD75) mount so it tracks more accurately during long exposures with the main telescope. Without autoguiding I was limited (by star trailing) to 40 second exposures or so, with it, exposures can be of any practically any length. This opens up a range of targets that are simply unavailable at shorter exposures. The following raw unprocessed images illustrate the difference (40 seconds vs. 8 minutes).




Software - Sky Planetarium

I use my own custom software to control the telescope and cameras. I needed to add some features to aid in getting more productivity out of my setup.

Camera sharing: Sky Planetarium supports the Meade DSI family of cameras (and most web-cams too). I added camera sharing support as the plate solving and imaging console must both use the same camera.

Telescope control: Now can use refracted coordinates for ASCOM supported telescope mount gotos to help with the initial pointing accuracy.

Plate solving: Sky Planetarium aquires images and passes them off to be plate-solved (by the Astromentry.net software). It then picks up the resulting coordinates and shows where the location is in relation to the stars/objects on it's map. In automatic mode, after a user requested goto completes it takes an image and waits for a successful plate solve, then moves the mount to the corrected location. The mount can be moved via another goto, a sync-and-goto, or a slew (Autostar only). This is all implemented but more debugging and testing is necessary. Still, this works well, getting the object easily within the CCD's field of view (+-1/4° from center) every time. More work needs to be done with the slew support and mount tuning to improve upon this. I want about 4-8X this level of accuracy.

Imaging, automation: I've added the ability to pre-configure the filter/exposure-time/number-of-images and have it shoot all of the frames for color with a single button press. The way it's setup now, there can be only have one exposure length, I'll eventually add the ability to specify the exposure length/number of frames for each filter.

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SOME NOTES FROM THE SURVEY ARE BELOW:

3-14-12, Messier Survey:
1-26-12, Image Gallery:
1-21-12, Messier 33, again:
1-15-12, Messier 1, 42, 43, and 78:

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