Periodic Error -> Gemini G41 Obs+

Recently, I got the mount and scope reinstalled in the dome (thanks Declan and Paul!) after some adjustments to the dome walls just to fine tune them. Now that we FINALLY had a clear night, I thought it interesting to see what the periodic error was like with my mount while carrying the 16" Meade SCT. So, before I polar aligned the mount with any real degree of accuracy I took a long exposure image with a Canon 400D. Below is a small part of the image at 100% scale factor.

While the star may not be perfectly focused, at least it seems to show a relatively smooth curve. I won't know for sure until I do a proper test of course. I must get the ST8 on the back of the scope some night and take a precise record of the actual values of the periodic error.

As a preliminary check, I tried to put some figures on the image from the 400D.
The scope focal length is approx 4400mm (I should be able to get a more precise measure with the CCD camera, but for now this will do).
The pixels are 5.7 um square on the Canon 400D.

Periodic Error = [ ((3600*180)/pi) * (Pixel Size/Focal Length) ] x Pixel Height From Top to Bottom of Curve

Periodic Error = [ ((3600*180)/pi) * (0,0057/4400) ] x 9 = 2.4 arc seconds

Now, this is approximate as there is some interpolation required in the exact measurement from the image. It will be interesting to plug the ST8XME in the back of the scope and see how close this estimate is to the real nswer.

Michael.

I'm still a learner driver when it comes to these things. I am assuming that the smaller the number the better? Is 2.4 arcseconds good or bad for your scope on that mount Mike?

Seanie.

Correct - the lower the number, the better. In this case, it would be pretty good. However, this is a fairly crude means of measuring it. By attaching a CCD camera on to the back of the scope and letting it act as an autoguider, one can record the actual corrections necessary to centre the star on the chip. These measurements can then be exported to a spreadsheet and a graph generated to see exactly how smooth the tracking really is.

Michael.

Is the CCD on its own or mounted through a smaller piggybacked scope Mike?

Mike,

I highly recommend PEMpro for dealing with all things related to PE. It is so simple to use and will give you very precise results as well as allowing you to correct down to sub arcsecond if you desire. Your image seems to show some very promising evidence for very low PE with a very smooth curve. There is a free trial of PEM pro available so you can check it out without costing you a penny.

Seanie_Morris wrote:

Is the CCD on its own or mounted through a smaller piggybacked scope Mike?

For purposes of periodic error correction, the CCD camera is attached directly in to the back fo the scope so as to get the most accurate result possible. With the camera I have, the SBIG ST8XME, it has two chips within the camera - one for imaging and the other one for autoguiding. SBIG have patented this design hence why other manufacturers can't use this method. For most applications, this makes autoguiding pretty handy as there is only one camera involved and it eliminates the need for a piggyback guide scope and separate autoguider camera. The one other instance when you might need an autoguider rig piggybacked is for imaging with narrowband filters. These narrowband filters reduce the light so much, it can be difficultl for the internal autoguider chip in the SBIG camera to see the guide stars through such dark filters.

Michael.