Periodic error correction

[dawson]

I think I now understand about the permanent periodic error correction (PPEC) for my mount (AZ EQ6), but I don' understand this sentence:

 

"PEC can only compensate tracking errors on the R.A. axis. Therefore, a precise polar alignment is still required to track minor errors on the Dec. axis."

 

From:

http://ca.skywatcher.com/upfiles/en_download_caty01367276646.pdf

 

1. What does it mean "track minor errors"?

 

2. Is there a way to train the mount to compensate for periodic error in the dec motor system?

 

3. How much do the dec motors contribute to tracking a celestial object for short exposure [non-guided] astrophotography?

 

Thanks for any replies.

 

James

 

[philjay]

What it means is really what it says, Pe correction only applies to the Ra axis, therefore the dec axis will not be adjusted, thus you have to make sure the Pa is as good as you can get it. I dont know of any mount that does pe on both axes, perhaps someone else may know, if there is one im sure it would be top end

[dawson]

Thanks, I see. It's just not written in clear english. Well not in language for simpletons like me

James

[Guest Kheldar]

I'm not sure what PE correction would be needed for on DEC?

 

Once you're on target (and polar aligned) then DEC doesn't move .... lock it down tight and away you go.

If your PA is not accurate then movements in DEC (guiding) are not to compensate for PE .... it's to compensate for poor PA.

 

PE is adjusting for the glitches in RA tracking which is continually moving - defects in teeth, motors, worm, etc ...

 

Someone smarter than me can probably explain this better or explain if I'm missing something

Edited August 1, 2013 by Kheldar

[dawson]

I think i knew the dec motors were not needed for tracking stars if the mount was polar aligned, but i was just confused what that wording in 1 above meant.

If one used the mount to track say the ISS via stellarium, then the dec motors would have to be involved, and then presumably any periodic error in that system would become apparent.

I guess drift aligning does this, but it would be good if one could easily do some periodic error training of both the RA and dec once set up to fine tune ones mount (RA errors) and polar alignment.

James

[xanthic]

ISS shifts far too fast for PEC errors to be apparent. You'd see it on celestial objects. No mechanical part can be made perfectly or wear perfectly. There will be slight differences in height/width along the length (worms) or radially (gears). This will cause a very slight change in speed of the drive as they rotate. The PEC routine will detect where in the cycle of turning this happens and will compensate for it.

[Guest Kheldar]

ISS shifts far too fast for PEC errors to be apparent. You'd see it on celestial objects. No mechanical part can be made perfectly or wear perfectly. There will be slight differences in height/width along the length (worms) or radially (gears). This will cause a very slight change in speed of the drive as they rotate. The PEC routine will detect where in the cycle of turning this happens and will compensate for it.

 

What he said

 

I think you're trying to fix a problem that doesn't exist to be honest - we're not talking about huge shifts in GOTO accuracy here, we're talking about small little fluctuations in the RA axis whilst tracking a celestial object

[dawson]

What duration of time does it take for there to be one complete revolution of the RA motor system? As in if maximum unguided exposure time on a sidereal object was 5 minutes before star trailing occurred, and if the revolution period of the gears was 10 minutes, PEC wouldn't improve unguided tracking would it?

Do the non sidereal tracking modes only use the RA motors to track?

James

[dawson]

I'm not really trying to fix a problem, i'm just trying to get my head around how the mount works a bit more, sparked by the thread on the belt modification for the heq5.

I am interested, eventually, in doing a bit more unguided imaging of DSOs so keen to build up some knowledge on how best to do that.

I'm hoping my rings for my OTA will be available soon, to allow me to reduce the cone error i have (which is quite bad and makes goto pretty useless on my kit at present), and then i hope to get the kit out again now the nights are getting darker, and getting better at polar alignment. Exciting times

Thanks for the replies.

[glider]

If cone error is making your Goto inaccurate then just do 2 star aligns rather than 3.

 

Cone error is only calculated and used by the mount if you go to the extent of 3 star alignment.

 

Then us Pointing Accuracy Enhancement via the hand controller as you slew around the sky.

 

Of course, if you are imaging the same target all night a 1 star alignment is perfectly satisfactory.

[dawson]

I've tried two star and three star, and goto is crap whatever; i haven't tended to stay on one object all night or stay in one area of the sky, so PAE doesn't help either. I'm hoping the rings will help. There was a separate tread somewhere about my cone error in which i think most people don't know why i have it or why it bothers me so much; i'd just like a goto which goto's

James

[glider]

This is on the AZ-EQ6 mount?

 

Have you had it from new?

 

Have you, or could a previous owner have had it apart to tune up or modify it?

 

I remember a guy who put a mount back together and either fitted gears backwards or in the wrong place so all of his ratio was out.

[Daz Type-R]

James, confused as to why you can not get your GOTO to GOTO, I can get the mother-in-laws 130P GOTO to track for ages, and that mount is nowhere near as good as yours, even in ALT/AZ mode I would of thought yours should be better.

 

Now forgive me for saying the next sentence but here goes.....

 

Are you sure you are doing it right?

 

I have never seen a AZ-EQ6 mount not have I tried to make it track, but surely in ALT/AZ mode it is just the same as any other ALT/AZ Sky Watcher mount (by that I mean the principle of it)

[Guest Kheldar]

How good is your PA?

Are you using it manually (setting circles) or the inbuilt hand controller PA routine?

If you're using the hand controller are you recalibrating your GOTO after PA?

[dawson]

Lots of questions to answer, here goes:

 

I've had it from new (< 1 year).

I've not tampered with the insides at all.

I have played with it in alt-az mode briefly but most of my experience is in EQ mode.

I think I am doing it all right.

Tracking isn't a problem given it is unguided.

Polar alignment seems OK, but could always be better, but again this shouldn't (as far as I'm aware) affect the GOTO accuracy.

I have been polar aligning by looking through the polar scope and getting Polaris in the little circle once I've worked out where the little circle should be using the hand controller to tell me where the circle should be, and adjusting the ALT and Az screws accordingly; but again this shouldn't (as far as I'm aware) affect GOTO accuracy.

I was under the impression any GOTO data was cleared once the mount is turned off/on and when one does a new star alignment.

I do the star alignment after PA.

 

I checked for cone error by rotating the scope around the RA from one side to the other after centering a far object; the error was massive and uncorrectable, hence my search for OTA rings so I can at least make an attempt to correct it.

 

GOTO may be reasonable if staying in one area of the sky, but to flip from east to west GOTO is out.

 

My alt-az synscan mount had much more accurate GOTO.

 

As I say, tracking hasn't been a particular problem.

 

This is all un-related to this thread though, as I started this thread to learn more about periodic error and the motors, not to find a solution to my GOTO issues.

 

GOTO has got up and gone

 

Thanks all

 

James

 

P.S. I'll find the cone error thread.

[dawson]

Cone error thread:

 

http://www.eastmidlandsstargazers.org.uk/topic/5021-mak-cone-error/?hl=%2Bcone+%2Berror

 

James

[glider]

Are you using PC software called Polarfinder, and are you remembering to reverse the image on the computer screen so it looks like what you'll see through the polar finder?

 

Have you aligned you polar finder?

[dawson]

I think i'm doing the polar alignment ok, and i have aligned the polar scope to the mount, but again is there any relationship between poor polar alignment and crap GOTO accuracy? My tracking has been ok.

James

[+RonC]

Reverse view of Polar Finder?

I use as it is on screen without a problem?

At least my Goto goto's OK!

 

Ron

 

Sorry, just noted I use Polar Align on the Ipad!

Edited August 1, 2013 by Ron Clarke

[Brantuk]

Check the alignment and collimation of the polar scope first. When you polar align by getting the pole star in the little circle - get someone to turn the RA through 360 degrees while you keep looking through the polar scope - the pole star should stay on the circumference of the larger circle. Then you can be reasonably confident of your PA'ment.

[philjay]

James, what mag are you using. If you are using the mak it has a high fl so you really need to use a nice mag ep fl say 25mm plus. If you are using your mak at say f12 and an ep of say 6mm thats gonna give you 300 mag, not many gotos on non permanent set up will cope with that narrow fov. This will give the impression that your goto is crap. A nice 25 to 30 mm or even longer 40mm will increase the fov and give you and the mount a fighting chance

[glider]

Good point Phil. I remember in my webcam planetary days Goto Jupiter with 20mm EP, centre image, go to 10mm EP centre image and finally webcam (effective 6mm EP) and centre again.

 

Trying to get an image on the webcam was impossible otherwise.

[dawson]

I'm f/15. I would need to check, bit generally i start with a 24 or 36mm EP. Even so, it would be good if the object was in the field of view!

The accuracy of my old alt-az synscan goto mount was better, and the goto i've done a few times with my 127mm mak was ok, it is just rubbish with the 180mm mak, which makes me think the cone error is to blame.

James

[catman161]

Did you align using dions tv aerial method James? I do it like that and it works fine for me well actually I know it is a minuscule amount off perfect bit by that point I had been staring at this tv aerial and messing around with the thumb screws on the polar scope for about an hour and a half and figured it was close enough! Even with that tiny error in aligning the reticule to the mount I still get good go to accuracy with the NEQ6. I mean its always a little off centre of the FOV in the 13mm Ethos but not bad by any standards.

[dawson]

I aligned the polar scope to the mount on an aerial yes. It is pretty well centred; astro-leigh looked too and thought it was good.

My goto accuracy is no where as good as yours felix

Again, i didn't think polar alignment (which i think i do ok) had ANYTHING to do with goto accuracy.

Jd