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Thanks for those nice comments everyone, much appreciated. Even though I have a very nice mount in the Tak EM200, it is not all plain sailing! I had to ditch 3 or 4 of the sub-frames due to slight star elongation – not sure of the root cause just yet.
When I later took a 3 min auto guided exposure, the stars show noticeable elongation in the RA direction, which is very visually unpleasing. I’m currently trying to chase this down, as my instinct tells me this should not happen.
In the C9.25 at F10, the ST7E FOV of the 765 x 510 pixel ccd is 10x7 arc mins (according to The Sky Pro 6). So, this converts to a 600x420 arc secs rectangular field. The star elongation in the 3 minute guided exposure is around 8 pixels. So, 8/765=0.0104575 and if we multiply this by 600 we get a trail of 6.27 arc secs.
The sales and marketing hype surrounding the Tak EM200 quotes the mount as having ± 5 arc sec PE, although I believe Takahashi actually state ± 7.5 arc secs. So, the mount appears to be in spec (6.27 vs. 7.5) although the elongation that I measured occurred in a 3 minute cycle, whereas I think the full worm cycle is between 8 and 10 minutes.
This elongation has occurred when I have auto guided using the 2nd guide chip in the ST7E and also via a separate guide scope, so I think guide scope flexure can be ruled out in this instance. However, the ST7 only push fits into the 2” JMI focuser so there could be some flexure here, which the system is trying to chase. I may have to permanently fit a 2” to SCT adapter into the JMI, so I am not relying on a couple of grub screws to hold the weight of the ST7 and Meade 647 Flip Mirror plus 32mm Plossl viewing eyepiece.
Having said all of that, the EM200 is working like a champion and if that is the only drama in my life, I can consider myself a very fortunate soul indeed, compared to those are really suffering around the world.
Cheers
Dennis
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