ICEINSPACE
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Waning Gibbous 92.8%
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10-06-2008, 03:47 PM
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aiming for 2nd Halley's
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Join Date: Oct 2005
Location: Australia
Posts: 2,959
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Will a Barlow improve my PhD guiding?
Hi All, looking at my images from Sat night I notice considerable star elongation (ovalling) despite the fact PhD was locked on guding happily throughout. Now the prime focus FL was about 450mm and I'm guding with a ZS66SD that is about 380mm... would a barlow improve my precision?
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10-06-2008, 04:55 PM
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Retired, damn no pension
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Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
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You wont need it Rob. With the ZS66 at that FL you should be able to easily guide 1- 1.5 meter FL assuming your mount has the accuracy). Can you post a pic of the areas of your images you are concerned about. It could be a flexure problem rather than guiding. Does the elongation all point the one direction?
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10-06-2008, 05:07 PM
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aiming for 2nd Halley's
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Join Date: Oct 2005
Location: Australia
Posts: 2,959
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Quote:
Originally Posted by [1ponders]
It could be a flexure problem rather than guiding. Does the elongation all point the one direction?
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Thanks Paul, yes it does tend to go one direction... what does that mean?
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10-06-2008, 05:44 PM
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Retired, damn no pension
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Join Date: Nov 2004
Location: Obi Obi, Qld
Posts: 18,778
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could mean a number of things. Can you tell if it's in RA, DEC or diagonal to both?
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10-06-2008, 05:49 PM
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PI rules
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Join Date: Jul 2006
Location: Sydney
Posts: 2,631
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Message deleted. I gave some advice, then realised it was wrong.
Geoff
Last edited by Geoff45; 10-06-2008 at 06:13 PM.
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10-06-2008, 10:32 PM
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Registered User
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Join Date: Mar 2007
Location: Kenmore Hills, Brisbane, QLD
Posts: 392
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Hi Rob,
I gave you a link in your previous post about WO flattener and vignetting which exactly describes your problem. Moreover it's about the same scope your are using. It does not happen because your guiding is bad it happens because this scope has astigmatism. I have the same with my ED80 which is a clone of WO ZS80.
If you add a barlow it wont fix problem but you'll make your guidescope much slower(f/11.5 if you add 2x barlow). It means that you'll have to increase exposure time to pick up the same star. It also means that PHD will send commands to correct your mount less frequently.
cheers,
Mike
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11-06-2008, 07:34 AM
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aiming for 2nd Halley's
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Join Date: Oct 2005
Location: Australia
Posts: 2,959
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Thanks Mike, if it's astigmatism (must confess I haven't star tested these optics yet) do you think it would have a differential impact depending on where you put the guide star?. I'm curious as to how this could impact guiding though. The star resolves to a point?
cheers,
Rob
Quote:
Originally Posted by ozstockman
Hi Rob,
I gave you a link in your previous post about WO flattener and vignetting which exactly describes your problem. Moreover it's about the same scope your are using. It does not happen because your guiding is bad it happens because this scope has astigmatism. I have the same with my ED80 which is a clone of WO ZS80.
If you add a barlow it wont fix problem but you'll make your guidescope much slower(f/11.5 if you add 2x barlow). It means that you'll have to increase exposure time to pick up the same star. It also means that PHD will send commands to correct your mount less frequently.
cheers,
Mike
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11-06-2008, 06:48 PM
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Registered User
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Join Date: Jul 2006
Location: Sunshine Coast
Posts: 1,829
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Rob,I am Guiding through a zenithstar66 however I find that I have to use a barlow to get the star in focus with the phillips toucam. I have heard of something like an extension tube for focusing. Do you use one of these or does your webcam go dirctly into the star diagonal and achieve focus. There is also the third possibility that I am doing something incredibly stupid and have been doing it ever since we got the zs66. Mark B
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11-06-2008, 07:05 PM
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aiming for 2nd Halley's
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Join Date: Oct 2005
Location: Australia
Posts: 2,959
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Quote:
Originally Posted by batema
Rob,I am Guiding through a zenithstar66 however I find that I have to use a barlow to get the star in focus with the phillips toucam. I have heard of something like an extension tube for focusing. Do you use one of these or does your webcam go dirctly into the star diagonal and achieve focus. There is also the third possibility that I am doing something incredibly stupid and have been doing it ever since we got the zs66. Mark B
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Hi Mark, I just put the webcam (Phillips 900nc) into a 1.25in star diagonal and it finds focus no problem. Is yours perhaps one of the Petzval ZS66's which might be different to the ZS66SD?
Do you use PhD guiding on the 66?
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11-06-2008, 10:34 PM
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Registered User
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Join Date: Jul 2006
Location: Sunshine Coast
Posts: 1,829
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Rob, Yes we do have the Petzval. I use PhD guiding using this scope mounted on a skywatcher ED80. We have this set up on a Heq5 pro mount and when all is well it goess well. We have just found out that our order of guide rings and dovetail bars will arrive this Friday which will allow us ( school) to move our celestron C8 from the wedge mount to the HEQ5 with the Ed 80 as a guide scope. Hopefully not too heavy. I'll have to get back to where I bought the 66 from as the barlow does make star selection for guyiding more difficult. Mark.
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13-06-2008, 10:10 AM
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Registered User
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Join Date: May 2008
Location: CA, USA
Posts: 10
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I am not sure. I think you pay through internet.
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13-06-2008, 11:07 AM
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Registered User
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Join Date: Mar 2007
Location: Kenmore Hills, Brisbane, QLD
Posts: 392
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Hi Rob,
I don't think so as it isn't related to guiding at all or your guiding scope. The problem comes from your imaging scope.
A WO 0.8x reducer should fix it. Not 100% but stars will look much better if you use it.
Check this link http://www.ewellobservatory.com/best...es/player.html
and select #15 "Field Curvature" from the "Outline" list on the left. There is a good explanation there why it happens.
cheers,
Mike
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13-06-2008, 07:14 PM
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aiming for 2nd Halley's
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Join Date: Oct 2005
Location: Australia
Posts: 2,959
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Quote:
Originally Posted by ozstockman
Hi Rob,
I don't think so as it isn't related to guiding at all or your guiding scope. The problem comes from your imaging scope.
A WO 0.8x reducer should fix it. Not 100% but stars will look much better if you use it.
Check this link http://www.ewellobservatory.com/best...es/player.html
and select #15 "Field Curvature" from the "Outline" list on the left. There is a good explanation there why it happens.
cheers,
Mike
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Thanks Mike, but I'm already using one (a WO 0.8x reducer) . I'm pretty sure my problem is guiding.
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