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f29pc
professor emeritus
Reged: 05/23/05
Posts: 651
Loc: Maine
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I'm getting some diagonal star elongations in my 20 min subs.
My polar alignment is within 1 arc min according to PemPro, and PAM. PE is 1.7 arc sec peak to peak, and my guiding errors are all less than .4 in maxim throughout the entire 20 min. I'm guiding with maxim using an 80 mm piggybacked on my 12" LX200 (@ 1780mm FL)ota, on an MI 250. I have done 20 min subs with no problem in the past, but I think I was in a side by side setup, not piggybacked.
Here is a side by side of a 5 minute unguided exposure on the left, and a 20 min (Ha)guided on the right. The max guiding error durring the 20 min exposure was .32 in RA and .40 in Dec. Are we looking at flexture here? Guided 10 min subs are ok. What causes the diagonal elongation, I don't think it's my Polar Alignment if I can get 5 min unguided stars shown on the left of the image.
Need some suggestions, thanks.
Paul
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12" LX200 UHTC OTA
10" LX200 EMC
WO 105mm APO, 80mm APO, 66mm APO
MI250
Next day air makes every night X-mas eve!
www.burkesobservatory.com
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Most likely flexure Paul. Piggyback guiding in my experience will always show some flexure, at sometime or another. Rings, different metal thickness contracting at different rates etc. Unless your focal length is very short then IMHO piggybacking will be troublesome. Could your primary mirror be shifting in your SCT a bit as it tracks? There is a bit of play around the baffle for the mirror to move.
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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Stathis
super member
Reged: 10/05/07
Posts: 171
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So, if you are shooting at longest focal lengths, side by side on a plate is better?
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http://stathisastro.blogspot.com/
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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I'm no expert but no. Off axis or internal guiding are the best methods. Your image train moves as one. With piggyback or side by side you still have the same forces at play. If your guidescope focuser moves a bit or your the tube swells or contracts at a different rate than your imaging scope things go downhill. I'm not saying guidescopes are impossible to use as there are 1000's of images out there done in this manner. Off axis/internal guiding are the best methods but the each have their own pitfalls as well.
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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f29pc
professor emeritus
Reged: 05/23/05
Posts: 651
Loc: Maine
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Thanks Chris, I don't think its the mirror, the scope has very little image shift, and a mirror lock. I agree that OAG is the best way to go, but using a FR, spacing becomes an issue, and the internal guide chip is useless with narrow band. Looking back at my logs, I was in a side by side setup (a Casidy triad bar) when I did narrow band at this focal length. I changed to a piggy back to reduce the weight.
One other thing to note, I did a 20 min unguided image, and the drift was in the same direction as the elongation of the 20 min guided image.
Paul
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12" LX200 UHTC OTA
10" LX200 EMC
WO 105mm APO, 80mm APO, 66mm APO
MI250
Next day air makes every night X-mas eve!
www.burkesobservatory.com
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Fabio Papa
Pooh-Bah
Reged: 03/01/05
Posts: 1460
Loc: Piacenza, Italy
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Is the movement constant? I mean, across say 3 images, do the star field keep moving away in one direction? If so, and if the star doesn't move in the guide program, then it can only be flex. Maybe not about mirror movement, but flex nonetheless.
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Fabio Papa
Celestron C8 F/10
Vixen ED81SWT F/7.7
Orion Atlas EQ-G
Gruppo Astrofili di Piacenza
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f29pc
professor emeritus
Reged: 05/23/05
Posts: 651
Loc: Maine
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Fabio, Yes, I took 3 20 min guided exposures, they all look the same. Same direction, and same amount of movement.
I was hoping it wasn't flex, but back in my mind, it was the first thing that I thought of.
Thanks, that confirms it, I'm tearing my system apart as we speak. Don't know what I'm going to do, maybe a leaner side by side, or an OAG setup.
Paul
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12" LX200 UHTC OTA
10" LX200 EMC
WO 105mm APO, 80mm APO, 66mm APO
MI250
Next day air makes every night X-mas eve!
www.burkesobservatory.com
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bill w
Postmaster
Reged: 03/26/05
Posts: 6597
Loc: southern california
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i feel your pain
this is a toughy
might re-check polar alignment (i've been burned by mysterious shifts in PA)
?sag in image train
might also consider guider image scale
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-bill w
nexstar 8 GPS
canon 300D, Toucam Pro II
SXV-H9C, H9, SX Exview autoguider, SX-AO
FS 102 (OLV), FS60 CSV, 8" LX200R, G 11
http://astro.whwiii.net/
image processing monitor calibrated to just differentiate darkest boxes:
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f29pc
professor emeritus
Reged: 05/23/05
Posts: 651
Loc: Maine
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Bill, thanks, the first thing I checked was the PA, with both PAM, and PemPro, it was less than 1 arc min from the pole. Then I re-calibrated the guider, and checked the image train.
What I ended up doing is setting up a side by side using my ADM setup. The 12" on one side and the WO 105 on the other. The 105 is mounted in a good set of rings on a Losmandy plate, with a Moonlite focuser. Both imaging trains are all "screwed" together, no compression rings or thumb screws. Everything balanced out great, so we will see. The 12" and the 105 in a sbs setup used less counter weight than the 12" with a piggybacked 80mm!
Paul
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12" LX200 UHTC OTA
10" LX200 EMC
WO 105mm APO, 80mm APO, 66mm APO
MI250
Next day air makes every night X-mas eve!
www.burkesobservatory.com
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Nocturnal
Pooh-Bah
Reged: 09/14/05
Posts: 1033
Loc: CT, USA
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Read this to get some ideas how to measure DF:
http://deepskystacker.wikispaces.com/Measuring+differential+flexure
Even if you don't (want to) use DSS you should be able to use the same technique with your stacking software as long as you can pick the reference frame and output the Dx and Dy values.
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Best,
Sander
C11-HyperStar on Atlas EQ-Q driven by EQMOD
William Optics M110 With FR-III/TRF-2008
DS2090 guide scope
QHY-8, DSI-Pro and DSI cameras
watec 802h video camera with KIWI OSD
Astro stuff: http://www.tungstentech.com
My Astro Photos
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lineman_16735
Tak-o-holic
Reged: 12/04/04
Posts: 2604
Loc: Central PA
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Quote:
Thanks Chris, I don't think its the mirror, the scope has very little image shift, and a mirror lock. I agree that OAG is the best way to go, but using a FR, spacing becomes an issue, and the internal guide chip is useless with narrow band.
I never considered the fact you were using a focal reducer...that does complicate the use of an OAG. I have never had an issue using the internal guide chip while shooting through a 6nm HA filter. Although I'm using a rotator and it is pretty easy to find a bright enough guidestar at my focal length. I'm sure at your focal length it is much more difficult. Best of luck with your tear down.
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Chris
A mount from Illinois
A scope from Japan
A camera from Cal-I-Fornia
A dog from Kentucky
A wife and kids from the "Twilight Zone"
The Geek Shed
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