What is all this arc-second accuracy stuff?

[collangm]

Okay,

Basically I'm looking to purchase a tripod/mount with GoTo function for a scope (as yet undecided on model/type) and intend it to be used for both visual and photgraphic observations. I understand that the cheaper models suffer from 'slip', meaning they do not keep accurate time with the earths rotation. Many mounts also quote 'arc-second' accuracy figures. What exactly do these refer to and is there a comparative table of mounts and their relevant accuracy in real-world terms?

I currently looking at an HEQ5 Pro with SynScan but want to ensure as long exposure accuracy as possible?

Thanks,

Gary

[vulcan]

An arc second 1/60th of an arc minute which is 1/60th of 1°. So, an arc second is 1/360°.

I wouldn't get too hung up on accuracy of gotos. I have an NEQ6 Pro and it still only gets me in the 'ball park' and I have to fine tune manually with a slow slew.

Goto accuracy has little to do with tracking for long exposures. In the end, you have to use a guiding setup for accurate long exposures.

[collangm]

Forgive my ignorance but what exactly is a guiding setup then and is this and automatice process or does it require manual intervention to maintain accuracy?

[vulcan]

Guiding requires two scopes. One has your camera or webcam - the imaging scope - and the other (cheap refractor) has a webcam which locks onto a nearby star. This can either be done with your computer or with a separate guiding device. Basically, it is correcting the goto controls to lock onto a fixed star. Your EQ5 should have a socket for a guiding device/computer. It takes a bit of setting up but is automatic thereafter.

[ronin]

An arc second is 1/3600°

60x60 = 3600, factor of 10 out.

[vulcan]

An arc second is 1/3600°

60x60 = 3600, factor of 10 out.

Sorry, typo. I always do that

[Ad Astra]

Mounts usually have two quantitative 'accuracy' values you need to be aware of.

1. Tracking accuracy, or periodic error. Drive systems have worm and crown gears to get the proper reduction and drive the scope at a rate of 15 degrees per hour. But the worm is a machined piece of metal, and it cycles every so few minutes, so the microscopic errors in machining will show up as repeating - ie: periodic - errors. You can literally see the star trails wobbling in time as they drift across a piece of film or a CCD image. Computer systems now will accept a digital 'recording' of your manual (at the eyepiece) corrections and either play them back, or average them out to help take out the periodic tracking error. Some mounts can get quite precise!

2. Pointing accuracy. This is not just the accuracy of the software model, but the mount's alignment, mechanical perfection & accuracy, and optical encoder accuracy all combined together. It refers to how accurately a desired object will be placed in the center of the field of view when we select and slew to it when the telescope is in use. As you can imagine, the "goto" function isn't worth much if the scope doesn't point to what the hand control says is on the menu. Scopes usually have the ability to 'sync' to stars as they slew around the sky to improve pointing performance. The software model also assumes that you are correctly aligned and the tripod is level and rigid. In the field at night, you don't always have all of these things down pat every single time. A permanent mount is different - you just dial it in once, and leave it set up after that. No fiddling! Much better pointing and tracking accuracy.

Hope that helps,

Dan