Good PA needed? Humbug!

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goldstar
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Good PA needed? Humbug!

#1

Post by goldstar » Mon Sep 23, 2019 3:27 pm

2019 09 18.
As I commenced PA with my AVX, an old friend swung by for a chit-chat.
Now I find 4 star PA needs some concentration and chit-chat is very distracting.
As I entered the final Polar Align by adjusting the ALT/AZ knobs I found the pole was WAYYYYY off by about 7 degrees.
WTH? Normally my initial eyeball gets me within a degree at worst.
I was eager to show my skills at this and to get going on exposing.
I repeated the ASPA and got the same results.
OK, lets whitewash over this and get imaging!
I began with 120 secs of M 31 just to show off. I checked for focus and trailing and saw just the usual trailing due to the autoguider not being engaged yet. Then I invoked guiding and demonstrated that the trailing was now gone and the guide star locked solidly.
Old friend was duly impressed. Soon he had to leave.
I continued to image for 50 X 180 sec frames and checked the result. Focus good and no trailing or field rotation. I wasn't happy with alignment but everything was telling me it was OK. Huh?

Later after an excellent series of images and processing I happened to refer to a star atlas to see what I had caught DSOwise around the star Caph in Cassiopeia. To my shame I instantly realized that I had mistaken Caph for Gamma Cass ! I tell ya, I don't know my Caph from my Gamma.
This explained my PA error. Caph was one of the alignment stars and I had been aligning on Gamma. Doh!!
No wonder the pole was off by about the same distance between Gamma and Caph.

So you can guess what my next comment will be- what's the big deal with accurate PA?
Result as follows on the Perseus Double Cluster. 180 sec X 25 . SW 80 ED 600 mm.
Next time out I might just forget the final mount align and do just goto (and double check I know my Yed Prior from my Yed Posterior).
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2019 dub clus ab.jpg
Scopes: 200mm f6 DOB:CR 6 150 mm f 7.5 refractor, SW ED 80 mm f 7.5. 2 X 80 mm f7.5 guides.
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#2

Post by SkyHiker » Mon Sep 23, 2019 4:05 pm

Great that it worked for you. You should get better images with good PA though because for one thing the DEC is usually sticky on the AVX because of the bushing, and you generally want to keep corrections at a minimum.

Personally I first level the mount roughly with a bubble level on the spreader, then I do the PA before goto alignment. The reason being, the goto alignment will be much less erratic if you start out about right. In fact if you have no cone error a one star alignment should suffice. I too am one of those APers who don't know one star from another because we rely on goto. So I just find the brightest star near where my goto alignment takes me, and presume it's the one.

I tried ASPA too but since it's an iterative procedure you end up doing much more work than when getting the PA right up front.
... Henk. :D Telescopes: 6" Mak-Newt (Comet Hunter), ES ED127CF, ES ED80, Zhumell Z12, Coulter Odyssey 10, AT6RC, Venture RX-7, Celestron Skymaster 20x80, Mounts and tripod: Losmandy G11S, AVX, LXD55, Tiltall, Cameras: Fuji X-a1, Canon SX40, Xt, XSi, T6, ELPH 100HS, DIY: Dob and camera barndoor trackers, afocal adapter, Dob with foldable base and Az/Alt setting circles, Accessories: SSAG, Plossls, Barlows, Telrad, laser collimators (Seben LK1, Z12, Howie Glatter), Cheshire, 2 Orion RACIs 8x50, Software: DSS, ImageMagick, PHD, Nebulosity, Photo Gallery, Gimp, CHDK
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#3

Post by bobharmony » Mon Sep 23, 2019 4:58 pm

I agree with Henk on this one. Polar alignment first, then some sort of alignment.

For me that takes the form of selecting quick align from the hand controller, Polar drift alignment with PHD2, slewing to a bright star close to my target, doing a plate-solve on the star and syncing the mount, focusing while I am there, slewing to the target, doing another plate-solve and sync to center it,doing a final framing check, and starting the sequence. No more 2+4 or ASPA for me, and I don't miss it a bit! Within 20 minutes of powering the mount I am collecting data.

Bob
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#4

Post by JayTee » Tue Sep 24, 2019 7:21 am

And don't forget that the further away you are from a good PA the more evident field rotation becomes on the periphery of your images. So it is always a good practice to have a good PA, hopefully, inside of 2'.

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Main: C-stron CPC1100 #2 Scope: 8" f/7.5 Dob mounted Newt AP Scopes: TPO 6" f/9 RC, ES 80mm f/6 APO G&G Scope: Meade 102mm f/7.8, Bresser 102mm f/4.5 Guide Scopes: 70 & 80mm fracs Mounts: C-stron AVX CGEM & GT Alt-Az, Meade DS2000 Cameras: Canon T3i (x2), ASI120MC Binos: 10X50,10.5X70,15X70 (x2), 25X100 EPs: ES: 21 100°, 30 82° X-Cels: 9, 12, 18, 25 Clubs: RCA & HAS
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#5

Post by XCalRocketMan » Tue Sep 24, 2019 10:20 pm

Hmmm. Wide field image (600mm FL). Won't fair as well with longer FL. But yes, I've been able to do well on my 562mm (WO GT102) with just so-so PA.
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#6

Post by Astroglide65 » Tue Sep 24, 2019 10:30 pm

Hello everybody,
I'm new here and came across this post. I wonder, since Jupiter is easily seen in the sky I often double check is everything centered up. Can I PA my AVX mount ASPA to Jupiter or does it only work with stars?
Thanks,
Dano
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#7

Post by Graeme1858 » Tue Sep 24, 2019 11:26 pm

SkyHiker wrote:
Mon Sep 23, 2019 4:05 pm
........So I just find the brightest star near where my goto alignment takes me, and presume it's the one.

Same as that!!!

Regards

Graeme
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