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The Tak's back!


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Our Takahashi EM200 mount reappeared two days ago after going back to the supplier to see why it was so hoplessly inaccurate. It seems that two bolts were not tight enough. They retain an upper part of the mount just below the saddle plate. All that trouble for two bolts. Grrrr.

So how did it do on first test again? Pretty well. With the polar illuminator now working the legendary Tak alignment routine was a real treat. (You don't level the mount on a Tak. Ours is observatory based so the mount is level, but it doesn't have to be. The bubble level is on the RA shaft and is just there to calibrate the polar offset.) It takes no more than three minutes and is incredibly accurate for a non-drift routine.

Unguided tracking was promising, with perfect stars at one minute with 980mm focal length. Three mins produced slight trailing.

Autoguided in AstroArt initial results were bearable but no better than an on-form EQ6. I was using half second corrections as I often do with the EQ sixes. (0.5 to 1 sec, typically.) Aha, but then I switched to 2.5 second intervals and the accuracy leapt up no end.

Using a similar guide setup my friend Tom's EM200 gives an average guided tracking error of around 0.04 pixel. The EQ sixes vary for me between 0.05 and 0.15, with 0.09 being typical.) The repaired Tak settled at 0.06 and pretty well sat there at that.

By day I had another look and found that I had no weight bias on the west, as is usually a good idea, so I hope the EM200 will be even better once properly offset in this way. Also I will have to play with the guiding parameters to optimize its performance but so far it is looking good.

I have had all sorts of premium mounts visit and most have the odd glitch from time to time. I hope mine are now over!

Olly

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Glad to hear that the mounts back Olly.

It must have been a worry, typical that it was just two bolts that were causing the issues.

When I first started guiding I thought that a low guide exposure would be better, but very quickly found that 2sec was much better than 0.5sec.

Also found out recently (at SGL5) that a slightly out of focus guide star is also very helpful!!!!

Ant

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Glad to hear that the mounts back Olly.

It must have been a worry, typical that it was just two bolts that were causing the issues.

When I first started guiding I thought that a low guide exposure would be better, but very quickly found that 2sec was much better than 0.5sec.

Also found out recently (at SGL5) that a slightly out of focus guide star is also very helpful!!!!

Ant

That's intersting, Ant. I have tried longer intervals on our EQ sixes and they don't work for me. Too much PE. However, the slightly soft guide star is a new one and worth looking into. I wonder why that is. Less variation from the seeing? A more averagely round star for which to calculate a centroid?

Thanks for that snippet, I'll try it.

Olly

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I think the out of focus guide star is for a more reliable centroid calculation.

I've spent a silly amount of time killing flex from my system and I think the very last bit is being caused by a long guide exposure (2+ secs). At Salisbury I forgot to change the default 1 sec exposure and hey presto the last of the trailing had gone....

James

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I have never worried too much about guide scope focus, so long as it isn't miles out it's ok. You definitely do not want a guide star that occupies 2 or 3 pixels because it is so blazingly sharp - you cannot centroid it.

Dennis

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