Periodic Error measurements average around 45 arc seconds peak-peak, so either very short exposures or auto guiding is needed for imaging. This chart is a 19 minute plot which shows 2 full worm cycles, each about 9 minutes in duration. Notice the PE is smooth with no sudden jumps. This makes guiding corrections more effective and usually larger PE can be guided down.

Periodic errors with auto guiding using GuideDog through the mounts auto guider port using the ShoeString Guide Port Interface adapter. Minimum guide errors was set to 2.0 arc seconds which seemed the best this mount could do. The LPI/C8-N combination image scale was 1.65 arc seconds/pixel. This plot was taken 8 minutes after turning the guiding software on and letting the tracking settle down. The mount is carrying the weight it would normally carry during photography sessions.

In an effort to lower the PE even more and also to get better tracking using a longer focal ratio OTA, I polished the RA worm and ring gears and the results were surprising. For more info click here.

Given the 1000mm focal length of the C8-N OTA and the Canon Digital Rebel (EOS) pixel size of 7.4X7.4 microns, the image scale is 1.52 arc seconds per pixel. The formula is: image scale=(pixel size * 206)/1000. Based on the recommendation in Ron Wodaski's book The New CCD Astronomy that states for high quality images, 1-1/2 times the arc seconds per pixel is a safe limit for guide errors. This would put that value at 2.28 arc seconds for the C8-N OTA and Canon camera. The LPI pixel size of 8X8 microns and the Tasco scope used for these tests has a 700mm focal length would give an image resolution of only 2.35 arc seconds. Adding a 2X barlow lens would increase the focal length to 1400mm and give an image resolution of 1.18 arc seconds. This will allow the guide camera and software to detect image shift sooner thereby sending corrections to the mount before affecting the image from the Canon camera. In actuality, atmospheric conditions will cause stars to be spread over more pixels, so you'll have more leeway.