To PEC or not to PEC

[Screwdriverone (Chris)]

Hi Again,

The questions keep coming, sorry about this.

Do I need to run the PEC training on the HEQ5 and then use PEC when taking astrophotos? Will this provide better tracking if I am not using guiding yet?

I have noticed the drive tends to "blur" the stars every 90 seconds or so, perhaps this is the mount inaccuracies or could it be the scope is not balanced properly?

Thanks in advance

Chris

[gregbradley]

When you say the images blur after 90 seconds are you using autoguiding or is this with no autoguiding?

I take it you mean without autoguiding. Getting 90 seconds with star elongation and no autoguiding is a mark of a nicely setup mount. I am sure other super mounts could top that but that is a good standard.

Autoguiding should take it from there. PEC is something used with autoguiding not without (at least I have never heard of PEC without an autoguider).

Can't say for EQ5 but generally PEC reduces PE quite a bit.

Greg.

[Screwdriverone (Chris)]

Hi Greg,

What I mean is that in a run of say 10 x 30 second images with 10 seconds between each 30 sec sub, without autoguiding, just sidereal tracking. Every third or so image is showing coma like (teardrop) stars (even in the centre of the field) as if the mount has gone over a funny shaped tooth or something like that in the drive (worm gear?). This is where the 90 seconds comes from. The difference is only slight BUT when you flick from one good image to another using a preview in explorer, you can see which subs are duds quite easily.

This was worse when the scope was say 30 deg towards the east. When it was at zenith on Sagittarius, it didnt happen as much. Perhaps Drift alignment would highlight if I was dead on the SCP or not and fix the issue? On the run of ten images I would get 1&2 good, 3,4 bad, 5&6&7 good, then 8&9 bad, then say 10 would be good. This is an example of what I would get on average.

I had the SCP pretty much dead on in the polar scope as the octans constellation was within all the 4 star circles in the asterism shown on the polar scope so the only things I can think of that could be causing this aberration could be the alignment of the tube to the mount on the dovetail bar (which the manual cryptically tells you to adjust) or perhaps the tube is not exactly balanced?

I don't know if I am expecting too much from an unguided pic of longer than 30 seconds or not? Would autoguiding eliminate this problem?

I may have tried to shortcut the best practices in my haste to get some results so if I need to slow down and set things right, then this is probably a good place to start.

Sorry for all the questions embedded in there. I am just trying to explain what I mean.

Cheers

Chris

[ericwbenson (Eric)]

Hi Chris,

It's the mount. What is the focal length of the telescope on the mount? What camera are you using (to tell us the pixel size)? These will tell you your image scale. That information combined with the amplitude of the periodic error will tell you the expected yield in percent from a series of exposures. I have a spreadsheet for that here:
http://www.faintgalaxy.com/g11tracking.htm

There are three fixes, two cost money, 1) live with your current yield, 2) autoguide 3) fancier mount, and autoguide eventually anyways...

A good AG setup is not cheap (it either cost some real dollars or takes some time to perfect), but a mount that does not need AG is even more expensive, AG is usually the next step

I did some stuff with a EQ4 unguided. From an aligned stack of 10 sec subs you can watch a hot pixel trace out the PE:
http://www.faintgalaxy.com/ngc6720.htm
You could do the same and quantify the size of your adversary.

Good luck,
EB

[Screwdriverone (Chris)]

Originally Posted by ericwbenson

Hi Chris,

It's the mount. What is the focal length of the telescope on the mount? What camera are you using (to tell us the pixel size)? These will tell you your image scale. That information combined with the amplitude of the periodic error will tell you the expected yield in percent from a series of exposures. I have a spreadsheet for that here:
http://www.faintgalaxy.com/g11tracking.htm

There are three fixes, two cost money, 1) live with your current yield, 2) autoguide 3) fancier mount, and autoguide eventually anyways...

A good AG setup is not cheap (it either cost some real dollars or takes some time to perfect), but a mount that does not need AG is even more expensive, AG is usually the next step

I did some stuff with a EQ4 unguided. From an aligned stack of 10 sec subs you can watch a hot pixel trace out the PE:
http://www.faintgalaxy.com/ngc6720.htm
You could do the same and quantify the size of your adversary.

Good luck,
EB

Hi Eric,

Its a BD 8" reflector FL 1000mm (f5) using a Canon 1000D.

I can probably use my webcam and a guide scope or even this onto the 9x 50 finder, just need to research the setup on this and then guide using the autoguider port.

Thanks for the help.

Cheers

Chris

[Bassnut (Fred)]

Greg, actually its the reverse, PEC is more important not autoguideing than with.

If PE is smooth, then a suitably fast guide correction rate can correct the PE drift, PEC is not essential, its a handy bonus. But if guiding is not used, then PEC becomes very important, PE shows up immediately.

Random fast PE variations cause havoc, guiding or not, guiding wont then help a squat and it should be addressed. The only way to fight bad PE is with fast AO, and thats way more expensive than a mechanical fix.

Unless of course the mount is an unfixable nasty cheapy, in which case fast AO turns it into PME, for a lot less money ;-)

[RobF (Rob)]

Hi Chris

I've got an almost identical setup (mount/Newt) and what you describe with every third or so image smearing is due to the periodic error oscillations (as everyone here has been describing). My HEQ5 has one spot in its worm cycle in particular where it "takes off" for about 30-40secs. Any pic in progress at that point is a lost cause without autoguiding.

I've played with a QHY5 (Orion SSAG) in the back of a piggybacked refractor for quite a while, but now get far better performance with the guide cam seated in the back of a simple skywatcher 9x50 finder. Can highly recommend this route. At 1000mm FL and with careful balancing, you can get pretty tight and reliable autoguiding. Each mount is a bit different of course, and MIGHT benefit from some adjusting if you're that way inclined (I haven't been game to pull my baby down though).

I've played with PEC a bit. It does reduce the oscilations in PE a bit, but you have to religously park your mount or lose the encoder positions. Only takes a laptop crash or mount crash for some reason and you lose it. I've struggled to prove it helps with tightness of imaged stars, as balance and guide settings seem to be more important factors for me.

[Screwdriverone (Chris)]

Hmmm, Thanks Fred,

So...

Does that mean I should PEC train the mount and see if this helps?

I would love to be able to not enter into AG just yet, I know its inevitable, like the perpetual hole in the pocket, but if I can get more reliable subs longer than 60 seconds, then this would be great.

Cheers

Chris

[Screwdriverone (Chris)]

Cool, thanks Rob,

That's the way I was heading, sorry, I missed your post as I was reading up on the webcam into the 9 x 50 finder and ST4 port on the Shoestring website.

Might PEC the mount as I always park it and then sooner or later get the guiding up and running.

Cheers

Chris

[Bassnut (Fred)]

Chris, you have to get AG cranking, its a given. But if you insist on not for now, then PEC is critcal, do it.

[Screwdriverone (Chris)]

Originally Posted by Bassnut

Chris, you have to get AG cranking, its a given. But if you insist on not for now, then PEC is critcal, do it.

Yes boss, anything you say boss, I will get right on it boss,

Thanks, didnt mean to be cheeky (well, yes... I did, actually!), I will continue checking out the best way to do this with the webcam and the finder in the first instance.

Cheers

Chris

[Bassnut (Fred)]

Hey, sorry to impose, do what you want, no diff to me, if eggy stars suffice, then ignore PEC. If I was being too short and to the point, then I appologise, no offense intended.

[Screwdriverone (Chris)]

No need to apologise Fred, I was trying to be funny, sorry.

I definitely want people to tell me what to do, last thing I want to do is come off as a smart arse when it comes to doing the right thing technically, otherwise all this great advice dries up and I am on my own.

Be as short as you want, tell me to pull my head in too if I overstep by being a knob.

Thanks for your advice, I will keep the tongue out of the cheek from now on. Sorry about that.

Cheers

Chris

[ericwbenson (Eric)]

Hi Chris,
So you are at 1"/pixel native. That's getting up there in resolution. I bet you find a lot of your results are much more pleasing when binned 2x2, it gives a lot more cushion for tracking errors.

PEC sounds like a good idea on paper, but in the field, unless you have a permanent/stable setup, and a good mount, I think you will find the effect of PEC underwelming. Most cheap mounts (I include the G11 on the borderline of this group!) have tracking errors that are not an even multiple of worm rotation - they don't repeat the same for every PE cycle - hence PEC can't fix them. They come from transfer gears, worm bearings and motor shafts for example. Also, when trying to record the PE curve, they contaminate it, so you record the wrong thing and potentially make matters worse! Even once you clear things up with software (see PEMpro for example), something moves mechanically and it behaves differently or you turn off the power to the mount and the zero point of the curve is lost. Very frustrating in the middle of the night!

PEC is useful for making the AG work less, but by itself it is less valuable IMHO.

EB

[Screwdriverone (Chris)]

Hi Eric,

ummm, how do you bin 2x2 on a DSLR?

I have no idea what you mean?

Chris

[ericwbenson (Eric)]

Hi Chris,
Oh, I meant resize the image down by a factor of 2 after calibration and raw->color conversion. DSLRs still have lots of pixels left over!

EB

[mch62 (Mark)]

Originally Posted by Screwdriverone

Yes boss, anything you say boss, I will get right on it boss,

Thanks, didnt mean to be cheeky (well, yes... I did, actually!), I will continue checking out the best way to do this with the webcam and the finder in the first instance.

Cheers

Chris

Chris I use a finder converted to a guide scope to keep weights down and it works at F10 on an 8"SCT on both an EQ6 and EQ5 Pros.I made mine from a 8x50mm finder ,removed the eyepiece and then got an old variable eyepiece projection adapter. I used the inner tube from the eyepiece projection adapter that has the threads for a guide camera such as a QHY5. The inner tube fits with a little gentle persuasion. The trick is to insert it just far enough so the camera comes to focus. What I found was you need to cut off and use only about 20mm of the inner tube and insert it so the camera mount threads are about 5 mm out of the end of the finder, but this depends on the brand of finder. This allows an QHY5 to focus with the front objective in the middle of its focus adjustment. When you have done this you never have to worry about focusing your guide scope again. It picks up more than enough guide stars as the 50mm lens is F4. I used a Stellavue twin ring finder bracket. looks just like a little guide scope bracket set up with good quality s/s adjusting bolts nylon tipped and is quick release. The whole lot weighs well under a kg –about 700-800grams from memory.
Just make sure to keep your set up balance slightly biased so the drive gears and worm stay loaded. I found this can cause intermittent elongated stars even with guiding or PEC.

Mark

[Screwdriverone (Chris)]

Originally Posted by ericwbenson

Hi Chris,
Oh, I meant resize the image down by a factor of 2 after calibration and raw->color conversion. DSLRs still have lots of pixels left over!

EB

Oh, I see, thanks Eric for the tip, I will definitely add it to the list of things to do.

Cheers

Chris

[Screwdriverone (Chris)]

Originally Posted by mch62

Chris I use a finder converted to a guide scope to keep weights down and it works at F10 on an 8"SCT on both an EQ6 and EQ5 Pros.I made mine from a 8x50mm finder ,removed the eyepiece and then got an old variable eyepiece projection adapter. I used the inner tube from the eyepiece projection adapter that has the threads for a guide camera such as a QHY5. The inner tube fits with a little gentle persuasion. The trick is to insert it just far enough so the camera comes to focus. What I found was you need to cut off and use only about 20mm of the inner tube and insert it so the camera mount threads are about 5 mm out of the end of the finder, but this depends on the brand of finder. This allows an QHY5 to focus with the front objective in the middle of its focus adjustment. When you have done this you never have to worry about focusing your guide scope again. It picks up more than enough guide stars as the 50mm lens is F4. I used a Stellavue twin ring finder bracket. looks just like a little guide scope bracket set up with good quality s/s adjusting bolts nylon tipped and is quick release. The whole lot weighs well under a kg –about 700-800grams from memory.
Just make sure to keep your set up balance slightly biased so the drive gears and worm stay loaded. I found this can cause intermittent elongated stars even with guiding or PEC.

Mark

Hey thanks Mark, I have a camera projection tube I can use for this and I think the webcam should just slide into this. I will give it a bash and see if I can rig it up without substantially modifying the finder in case I want to use it as a finder again.

Thanks

Chris

[gregbradley]

[QUOTE=Bassnut;615555]Greg, actually its the reverse, PEC is more important not autoguideing than with.


Thanks Fred. I wasn't sure as I have never done PEC as the NJP doesn't provide for it. But I'll have to learn it soon.

By the way there is a service that coats the worms of Chinese mounts with a super slippery coating that is supposed to reduce the PE dramatically.

If you look at the startarg site you'll see the link for it. Its not that expensive so it may be worth doing.

There is also a machining service promoted on Astromart to improve the machining on mounts so that would be another approach.

Greg.