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Old 10-06-2008, 03:47 PM
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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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Old 10-06-2008, 04:55 PM
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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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Old 10-06-2008, 05:07 PM
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Quote:
Originally Posted by [1ponders] View Post
It could be a flexure problem rather than guiding. Does the elongation all point the one direction?
Thanks Paul, yes it does tend to go one direction... what does that mean?
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Old 10-06-2008, 05:44 PM
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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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Old 10-06-2008, 05:49 PM
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Message deleted. I gave some advice, then realised it was wrong.
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Last edited by Geoff45; 10-06-2008 at 06:13 PM.
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Old 10-06-2008, 10:32 PM
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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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Old 11-06-2008, 07:34 AM
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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 View Post
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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Old 11-06-2008, 06:48 PM
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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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Old 11-06-2008, 07:05 PM
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Quote:
Originally Posted by batema View Post
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

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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Old 11-06-2008, 10:34 PM
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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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Old 13-06-2008, 10:10 AM
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I am not sure. I think you pay through internet.
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Old 13-06-2008, 11:07 AM
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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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Old 13-06-2008, 07:14 PM
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Quote:
Originally Posted by ozstockman View Post
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
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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