Takahashi EM-400 Periodic Error

[marc4darkskies]

I was all set to do some imaging last night. Beautiful, clear, cool day. Got focused and calibrated for a new target, took a dark and started imaging. Got 15 minutes into a 20 minute sub and it clouded over!! Aaarrggh! :mad2:

Anyway, I thought I'd have a look at the PE for my mount in between the clouds.

Here's the result - not bad really, inside what Tak specifies for the EM-400 (+-3.5 arcsec)

http://www.pbase.com/gailmarc/image/93621871

(Note: Data gathered by autoguiding in CCDSoft with corrections disabled)

[g__day]

Well I call that pretty impressive!

[gbeal]

Cracker. Is this method reasonably self-explanatory in CCDSoft? I would be keen to try mine and see, of course if it is garbage I won't tell you I did it, LOL.

[marc4darkskies]

Hi Gary,

Dead easy. Turn on logging (creates autoguide.log file) and turn off the guiding for X & Y (see the Autoguide tab : Settings button). Then select a guide star as normal and start autoguiding for a few worm revolutions (15-20 mins). Import the log file into Excel. You'll need to know which axis is RA - depends on the camera rotation.

You're also supposed to do the test on a bright star near Dec 0 deg. I was at about -30 deg Dec for my test.

I'll be watching out for your report - within the week - good news or bad! :lol:

Cheers, Marcus

[RB]

Excellent Marcus and thanks for sharing the method.

[gbeal]

Hi Marcus,
OK, will do, I'll give it a go. I may have to refine the polar alignment a little though, as it isn't perfect. Good or bad, I'll report back.
Gary

[marc4darkskies]

Look forward to it! :)

As long as your polar alignment is good enough to keep the guidestar in the autoguide window for 15 minutes or so you'll be right. You're measuring the accuracy of the RA drive not polar alignment. By the way, you may also see a linear trend in the graph (polar alignment plus maybe the sideral tracking rate isn't perfect for the elevation of the target). I did, just subtract it out.

It's a shame more people don't do this kind of simple analysis on their equipment - for the sake of those looking to buy. It's says a lot about a mount's quality. Of course, PE can be corrected by autoguiding (depending on the magnitude and frequency of the PE), but that's not really the point. I like to know how well engineered the mount is before I buy.

Cheers, Marcus

[g__day]

I'll see what I can do once my alignment is all set up and perfect. Adjusted I expect the Atlux to do +/- 3.5 arc seconds, raw 7/8 arc seconds. Its been a massive effort to get aligned and calibrated correctly - the manual just isn't detailed enough in that it had me cursing for months at a time until I stumbled onto the correct set-up settings by chance!

Can PEMPro show an equivalent type graph Marcus? I don't have CCDSoft5, but can run Atlux -> SkySensor200-PC -> MaxPoint -> PEMPro -> MaximDL -> Canon 400D might just show an equivalent graph from either its polar align wizard or it PEC wizard?

Matthew

[gbeal]

To that end, can Maxim DL?

[marc4darkskies]

Sorry, don't know guys - haven't used either PEMPro or Maxim. Naturally though, you'll need to use Excel to plot the data that your camera / autoguide software logs.

Cheers, Marcus

[gbeal]

Tried a couple last night, and actually thought I had cracked it with CCDSoft, but when I came back about 20 minutes later, the star had drifted out of the window, so it was a waste of time. I normally have the polar alignment pretty good, but obviously not enough. Auto-guiding makes me lazy huh? Perhaps I will throw the log file into Excel anyway.

[jase]

...and just as well Marcus... If your EM400 was over the manufacturer specs you'd want a quick sale and get another (or alternative mount). Tak mounts don't support PEC, so forget about PEMPro. The performance you get, you're stuck with. IMHO this is a disadvantage. I know someone will quickly respond and say, yeah they're built so good they don't need it or who cares I autoguide anyway. That's fine if you want to think along those lines.

The build quality statement just doesn't fly. The PME would have a similar or slightly better build quality...and it supports PEC. If you take the PEMPro stats - sampled out of 11 PME's the average reduction of peak-to-peak PE using PEC went from 3.8" down to 0.8". Enough said. (http://www.ccdware.com/products/pempro/examples.cfm). Don't understand Tak's theory here. PEC isn't too hard to support.

Secondly, indeed autoguiding will counteract PE problems...but that's not the point. The more you can reduce your mounts PE before autoguiding is a major advantage. The less autoguiding corrections made, the less chance of error i.e over/under corrections etc. As you increase your focal length, you'll quickly realise this is very important. Your TOA-150 is a wide field instrument so the tolerance is quite high. Pushing the 2500mm+ boundary starts getting serious.

Will try upload some stats from the Titan I have. I recorded PEC over six months ago so may still have the figures. If I recall correctly, after PEC I experienced a peak-to-peak of ~1.8". Out of the box it operated at -/+ ~4.2" (still under manufacture specs of 5.0"). Thus a considerable improvement.

Moral of the story...don't underestimate the power PEC will deliver when it comes to imaging. If I could afford a mount that didn't need to be autoguided while operating at 3000mm+ focal length (such as the NTM-500 (http://www.astelco.com/products/ntm/ntm.htm) that has a tracking accuracy of < 1" over a 120min period, not to mention the 0.3" when guiding), I'd have a huge smile from ear to ear.:)

[g__day]

I am kinda stunned that no one has written a PEC utility for the Taks. If you can simply model and inverse a PE curve in PEMPro, surely you can easily send a repetitive series of pulse guides being the inverse curve back to the mount as an auto-guide add on.

It should be an easy add on for any guiding software to add this inversion curve to the guide commands and sync it to the worm position.

Just a thought for anyone with programming skills!

[jase]

I should be careful what I say here... Tak's can support PEC, but not out of the box. i.e. using the supplied Tak Temma I or II software. You can of course replace the goto engine with something more sophisticated such as the FS2 - http://www.astro-electronic.de/. FS2 supports a few Tak mounts. I've also heard of people adapting their Tak mounts to use SkyWalker - http://www.astrometric.com/index.html, but its a considerably larger migration. (Don't believe SkyWalker has PEC - never researched it).