Checking PA with PHD - This is madness

[swag72]

That's exactly what I do Daz, but I don't use the programme as the polar scope reticule tells me where I need to put Polaris - I don't understand why I'd need to use the programme after I've set the date and time on the polar scope. As it has been calibrated one time, I know that setting the date and time will put the little circle in the right place.

So, looks like I'm not doing anything too wrong then.

[blinky]

No, it sounds like you are fine. So, you are getting trailing when shooting 5 min subs, hmmmm..... What length are your guide subs - 1s or 2s, I usually use 1second, you may just find that the focal length of the guide scope is not long enough or... there is possibly something like differential flexture between the guide scope and main scope. How are you attaching the guide scope?

[Tim]

Thanks for all this folks - I may pluck up the courage to go out again some time. Just to confirm though, I need to recalibrate after each adjustment to the scope while I am using PHD, yes? So I find the star, get PHD guiding, then once I make my first adjustment of the screw to stop the graph from drifting, I have to recalibrate again, I can't just click on the star again and tell it to carry on tracking - Even if the move is minute?

That sounds right Sara when you first start out. But when you get close to good polar alignment, you probably wont have to recalibrate every time.

To be honest, if you dont slacken the ALT/AZI adjusters, and you can put the mount back exactly where it was the last time you had good alignment, and, most importantly, the mount is level, then you should have plenty good enough alignment for astro photography.

Have you calibrated your polar scope on a distant object? There are instructions to do this in the manual for the HEQ5 usually. Once that is done you know that the polar scope is centred properly. Then when you have achieved very good alignment using the PHD method, take a careful note of where Polaris appears in relation to the inscribed markings on the polar scope. If you rotate the mount in RA you should see Polaris more or less following the ring of the inscribed circle.

next time you set up, just use the polar scope view to manouvre the legs of the tripod (you didnt alter alt/azi remember) so that the view in the polar scope is the same as it was when you had good alignment. Using this method you should be up and running in 10 mins or less.

Do check that the top of the mount is level though. You need to do this before attachaching the mount head, put a proper spirit level across the top plate in two directions, and flip the level over each time to make sure. You usually need to press down hard on the mount to allow for the extra weight when you do attach the rest of the kit.

At any rate, as Ant says, that graph isn't THAT bad Some nights, poor seeing would give you a graph like that even with perfect alignment.

I look forward to the pics

Tim

[kookoo_gr]

I also tried the PHD drift method with my 9x50 finderscope - guidescope with little success.

but i would definitely suggest this drift align method http://www.observatory.digital-sf.com/Polar_Alignment_CCDv1-1.pdf that way you eliminate any possible errors form flexure between your guide scope and your imaging scope and within 5-10 minutes you have a spot on polar alignment. the only thing you need to pay attention for this method is that according to the FOV of your imaging scope you may want to vary the exposure time, in my imaging system with a small FOV i use 30 seconds of exposure.

also when you get the spot on polar alignment tighten the bolts of the mount AZ-Height in order to keep from loosing the PA. also if you have a long tube with long focal length it may cause issues at imaging since it is prone to vibrations. and that will cause trail. try to remove any flexure form your imaging system. this flexure may come from vibrations from the cooling fan of the imaging camera, the focuser itself, between the imaging scope and the guidescope, the tube rings, the tube rings and the dovetail, any loose wires, the secondary or the primary mirror (for newts) so check the alignment of your optics. also i would suggest the use of an OAG sine it solves the problem of flexure between imaging and guidescope along with any weight issues

i use an heq5 pro and a 8'' - f/6 newton and although i have managed to get 15 mins subs each time i image it's a real headache and i am going to upgrade to a GSO RC 8'' in order to reduce length and weight along with an OAG

[dlp]

The polar alignment routine built in to EQMOD/EQASCOM is excellent. Takes me about 5mins to polar align now I've got used to the routine, and once you've done it you know you're bang on. There's a video on YouTube with the exact process.

David

[Whippy]

Thing is Sara, even with a reducer your ED120 has a fairly long focal length so if you're aiming for long exposures accurate polar alignment is essential. TBH, I don't think aligning just by the polar scope is going to cut it for what you want to do.

Your subs in your other thread looked like field rotation to me, not flexture and the best way to cure that is getting that PA nailed. Like Craig, I use WCS to help out and while all this now seems a terrible faff, it does become easier once you do it a few times.

The other alternative is to buy a shorter scope like an ED80 or Megrez 72 and try that. Shorter focal lenghts aren't so demanding of PA accuracy and hence are a bit easier to use. Not a cheap option though!

Tony..

[kookoo_gr]

The EQMOD polar align routine although it is very easy it is only accurate as long as the polar scope is aligned. also i have found out in my mount that the circle where you place the polaris is not very accurately drawn and each time i drfit align my mount is always overshooting the true north a bit so a drift alignment for imaging should be made if you don't have a permanent observatory

[swag72]

Thanks for all that folks - I hope that the sky is clear tonight and I'll have another go. I think I have more of an idea now of what I need to do and what I need to achieve.

Thanks for reading about my PA process, I didn't think I was doing anything too wrong, so that's good to know.

When I set the date scales, I did focus the reticule on a distant object then, but I think I'll probably have another bash at that as I know realise how important that is and it may have been "Oh, that's alright" at the time.

Don't think I could change scopes, without ending up with a divorce!! So that's out of the question really as I quite like my husband!!

Thanks for the continued support.

[Psychobilly]

Focus of the reticule isnt an issue whats important is that its propelry aligned (centered) with the mounts RA axis... you know the flip through 180 degree check...

Thats why I chose to use my EQ3-Pro over the HEQ5 lastnight as I know the polar scope is spot on...

Billy...

[Glider]

I use Polarfinder as above ^^^^ to polar align.

I use PHD exactly as described above for guiding.

I don't think you are doing anything wrong.

The graph looks exactly like the ones I get.

Are you chasing a solution for the miss shaped stars you saw in the images you posted earlier this week?

Have you taken anymore images since?

[swag72]

Well, have just been out and checked out the reticle alignment, and it was incredibly a little way off out. Knowing whey it needs to be spot on - It now is!!

Yes Glider, I am trying to sort out the field rotation that was identified in the M31 images I took guided for the first time. And I've not taken any images since.

Tonight - Weather permitting, I will set up and mark where the tripod legs go. I'll then make indentations in the tiles so that the three legs always go back in the same place. I'll check the level as well, just to make sure it's spot on, PA and align as per usual and then try the PHD thing again.

In order to pick the stars, I looked on Stellarium and picked as low as star in the south as I could get and the same for the East - That should be OK?

I feel that each time I go over this with you guys it is cementing a little more in my head.

[Glider]

Make sure you do an imaging run. What you have seen is a graph that shows a jagged line but not necessarily any correlation of how bad that looks to how bad your star shape might look.

Could be that what you have learnt has solved the image problem and you just need to get that feeling of good/bad tracking/PA.

[RobH]

Sarah.

Am I correct in thinking that you are looking at the graph while polar aligning?

If you are, it's no surprise that you've been having a hard time.

That isn't the way to polar align using PHD.

What you do is standard drift aligning, but use PHD as your reference.

Here's what I do....I'm assuming that you're familair with the main drift aligning techniques.

Roughly align your mount.

Pick your star on the PHD screen, and run the calibration routine.

Make sure you know the directions the star moves on the screen....I do this first by getting a star and moving the mount north for example....whichever way the star moves on the screen is South.

Once the calibration routine is run, then go to the 'brain' icon, and disable guide commands.

Then tell PHD to start guiding. You will soon see the star start to drift away from where it was originally.

Adjust your mount until the star has not drifted north or south for about 5 minutes.

In both RA, and DEC adjustments, you are just watching for any N/S drift....don't worry about E/W as this will generally be periodic error.

Once you've done all of that, then you can look at the graph, and adjust balance etc to smooth it out.

On the graph you posted, there is a lot of overcompensdation showing.....reduce the RA aggressiveness by quite a lot....try it at about 65%.

With the DEC commands, you will find that on one side of the meridian, the drift will be either North, or South, and it'll be the4 opposite once you've done a meridian flip.

Find out which direction your DEC drift is in, and then set the DEC guiding to that direction only.....e.g, if the star drifts south, then set your commands to North.

If you have DEC guiding on auto, the mount will be having to take up backlash every time it changes from N to S commands, and this will give you the sort of trace you're getting.

Remember too that it'll be 2 or 3 minutes from when you start to guide before the mount settles down...don't expect it to guide perfectly straight away.

HTH

Rob

[daz]

The reason that I (and many others) don't use the date-scale thingy is that it's just not accurate enough, and you can find that the silver disc can move

[swag72]

Sarah.

Am I correct in thinking that you are looking at the graph while polar aligning?

If you are, it's no surprise that you've been having a hard time.

That isn't the way to polar align using PHD.

Cheers for that Rob - Wonder why on the guide john posted above it says that it's pretty immediate and that you don't need to wait the 5 mins or so. I guess that it's best to wait a little to let the mount settle as well as starting to see a drifting pattern whether it be up or down.

If you don't use the date scales Daz, do you move the RA axis so that the reticule matches the Polar app and then get Polaris into it, or make no use of the reticule circle at all and just get Polaris in the right place along the outer circle?

[jgs001]

Sorry Sara, I don't know, that was a cut and paste...from another thread. I'll be adding rob's comments to my notes though

[swag72]

Sorry Sara, I don't know, that was a cut and paste...from another thread. I'll be adding rob's comments to my notes though

Hey no probs John, That is a cut and paste from a site that has been previously recommended. I can see when looking at my graph the need to let it settle and not make / expect immediate changes as it wasn't until the 3rd part of the graph that it started to drift - Only noticed as Martin pointed it out!!

[jgs001]

Hope it's clear and you're getting to try it out. It's clouded over again ...

[swag72]

Not a chance - Cloudy for me tonight!

[adamsp123]

Thanks for all this folks - I may pluck up the courage to go out again some time. Just to confirm though, I need to recalibrate after each adjustment to the scope while I am using PHD, yes? So I find the star, get PHD guiding, then once I make my first adjustment of the screw to stop the graph from drifting, I have to recalibrate again, I can't just click on the star again and tell it to carry on tracking - Even if the move is minute?

NO!

Point scope at near meridian and near zero DEC, run the full calibration, turn off DEC guiding and look at dx/dy DEC graph, note the which way the graph is heading, stop PHD, make a suitable mount adjustment, start PHD again (no cal) see what the DEC dx/dy is doing if the trace is flatter good... if steeper you have adjusted the wrong way, keep repeating till trace is virtually level, now move scope to East or West redo calibration and carry on with the procedure as before.

HTH

[martin_h]

NO!

Point scope at near meridian and near zero DEC, run the full calibration, turn off DEC guiding and look at dx/dy DEC graph, note the which way the graph is heading, stop PHD, make a suitable mount adjustment, start PHD again (no cal) see what the DEC dx/dy is doing if the trace is flatter good... if steeper you have adjusted the wrong way, keep repeating till trace is virtually level, now move scope to East or West redo calibration and carry on with the procedure as before.

HTH

Like Pete says - you only need to re-calibrate PHD when you move from South to East/West - or the other way. NOT every time you tweek the mount!

[daz]

If you don't use the date scales Daz, do you move the RA axis so that the reticule matches the Polar app and then get Polaris into it, or make no use of the reticule circle at all and just get Polaris in the right place along the outer circle?

The only thing the date-scale calibration does is to get the asterisms etched on the reticule in approximately the correct orientation, so the little polar circle is in approximately the correct place to put Polaris - providing of course, you have aligned the polarscope to the RA axis (which I see you have!)

Yes, I do use the little polar circle, but only because its easier to see. You would find that the outer circle actually covers Polaris so getting it in the right spot is a little tricker - although not impossible.

EQMOD users can make use of the Polar Alignment routine, which again, does the same thing.

HTH