8″ LX200 with QSI583 WSG

My observation site is in a small town, called Falset, in Tarragona, Catalonia (Lat: 41.1333 degrees North, Lon: 0.8167 degrees East).

My scope is a 8″ Meade LX200 GPS. This is the model which was marketed by Meade with the Advanced Coma Free (ACF) technology. The GPS feature was a nice add-on to the classic LX model, and it really eases the setup of the telescope.

My first observations were made by moving the heavy Meade Tripod with the scope every night. Not having a permanent pier, the whole thing was stored inside the house, and moved to the outside when needed. A part from this inconvenient, a boring alignment routine was needed every session. I developed an improved alignment process, which I (more or less) mastered, so I was able to observe after about 30-40 minutes.

The really big improvement to this was the installation of a permanent pier in the terrace. The pier is sturdy, and was carefully oriented toward North. It includes the capability of moving about 2 degrees in azimuth, so a fine orientation is possible.

With this setup, I am able to mount the OTC, and set all the accessories (cameras, cables, …) in about 15 minutes. And, very important, the aligment can be used for some time (a couple of years) after a new refinement is needed.

Here we can see the telecope mounted on the pier and the Meade equatorial wedge.

telescopi 2

In the next picture, observe a plastic strip which is holding the CDD in place: this was added after the QSI fall down one night (!!! I was very fortunate, and the CCD wasn’t damaged!). The weight of the QSI is remarkable, and the screws at the back of the telescope optical train are somehow insecure to hold the camera tight in place.

QSI and DSI

In this picture you can also see the Meade DSI Pro camera mounted at the top of the QSI. This small CCD functions as my guide camera, taking advantage of the QSI WSG off-axis design. For clarity, no cables have been mounted.

In this configuration (which is the one I use the most), I attach a focus reducer/field flattener to the optical train, so as to achieve a F.7.7 system (down from the native F.10 configuration, which is too demanding). This value of F7.7 has been computed from real images taken through my configuration.

The QSI is an exceptional piece of tech. At the time of buying, I was considering other options, mainly from SBIG. The main advantage of SBIG CCDs was that they come with a second, smaller CCD inside, devoted to guiding. But, some of the features of the QSI CCDs captured my attention.

One of these is that the models WSG from QSI have an off-axis capable body. This means that they have a prism which send ssome light off to a side opening where you can attach a second, guider CCD. Yes, you must have another CCD to guide (SBIG CCDs come already with the CCD guider inside), but this configuration allows that light for the guider be used BEFORE going through filters (as the prism is located in front of the internal filter wheel). This way, locating and using a guide star is much easier, and it’s not dependant on which filter is currently used. In my case, I already had a DSI Pro at hand, so I didn’t need to buy a guider. In other words, I thought that QSI performance would be higher as it comes without a second CCD for roughly the same price.

Another nice feature of this beauty is that it uses an internal filter wheel which accomodates 5 1.25″ filters (so cheaper than the big filters other CCDs use).

The QSI583 is based on the monochrome, anti-blooming Kodak KAF-8300 sensor (8.3 megapixels, 5.4 microns). This sensor is used by many manufacturers in other cameras. The QSI583 can use binning in different modes, from unbinned and all the way up to binned 4X4 (unbinned, this beast delivers a 3,326 X 2,504 pixels frame!). It lso has a cooler which is able to keep the sensor at about -20 C.

583wsg-2

You can read the technical specifications of the QSI583 WSG from here: QSI583

My preferred use of the QSI is binned 2X2, as it fits well with my 8″ LX200 together with the Optec focal reducer (to a F7.7 system). Working this way, the key parameters of my configuration are as following:

Configuration description: 8″ LX200 F10 + Optec focal reducer + QSI-583WSG binned 2×2

Effective F ratio: F7.722
Effective focal length: 1544.4 mm.
Resolution: 1.442 arcsec/pixels
Effective FOV: 40.1′ X 30.1′

As I said before, guiding is done with a Meade DSI Pro CCD. I purchased this camera back in 2007, as it was cheap and allowed me to play and have fun with digital astrophotography.

This is a very easy-to-use CCD, based on the Sony® ExView HAD chip, with 510 X 492 pixel frame (note that it’s not square), each one of 9.6 X 7.5 micron, monochrome with a manual filter wheel, and without any cooling mechanism.

As expected, I really had lot of fun with this little CCD. Of course, after some years using it, I got to a point in which I wanted more, so the QSI came in. But my old DSI Pro is still serving, now as a guide CCD coupled with the QSI 583 WSG off-axis system.

With my normal configuration, explained before, the key parameters for the guiding filed obtained with the DSI Pro are:

Resolution: 1.11 arcsec/pixels
Effective FOV: 8.2′ X 10.9′

The prism position into the QSI makes that the imaging field of the guider is shifted about 36 arcmin from the center of the imaging field of the QSI (as measured by myself from some tests).

qsi i dsi

When mounted, the optical chain in my setup shows a clear shift in the iluminated field. Being uneven is somehow expected from the optical elements involved (includind the focal reducer), but the shift in the Y axis indicates some kind of flexure o missaligment in the chain (collimation?). This image shows the uneven field and the shift (note how the center of the iluminated field is off-center):

without flat analysis

From my experience, flexure seems not to be the main culprit, as images like this show little (if any) differences regardless of the altitude to which the scope is pointing at. Therefore, I assume that there’s a tinny missalignment in the optical chain, most probably due to misscollimation (although collimation improvements don’t seem to work so far).

The system shows also a little of curvature, as you can see in this simulation.

This curvature can have different expalinations and contributions. One of these is the native optical configuration of the LX200 ACF. It uses a Ritchey-Chretien system, which is a modification of the classic Cassegrain telescope, and that improves some flaws of the classic configuration. Although the Ritchey-Chretien configuration delivers a more plannar field, they are also much more expensive to manufacture, and, as with everything, there are grades in the perfection such systems are built.

This degree of curvature doesn’t represent a big problem for me. In part, because other factors, as guiding errors while imaging, contribute much more in the final outcome, so the tinny efects of the non-plannar field are just hidden.