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Rick
Post Laureate
Reged: 04/12/05
Posts: 3050
Loc: Tokyo, Japan
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So I just got my first DSLR for Xmas, a Nikon D40, and was playing around with this software to see which of my current scopes would be best for imaging with this camera. I currently have a Pentax 75SDHF (fl=500mm), a Tak FC100 (fl-800mm/590mm) and a CN212 (fl=820mm).
I notice the software makes a pixel rez calculation, expressed as ArcSec/pixel. Is there an optimum number or range for this? If so, what are affects of not reaching or exceeding this range?
tia,
Rick
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www.japanastro.com
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Strgazr27
Vendor - Deep Space Mods and Composites
Reged: 10/04/04
Posts: 6984
Loc: StonyHill Observatory
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Rick,
Ideally, you'd like the AS/P number to be right around what your seeing conditions will permit as far as resolution goes. If your seeing usually limits you to around 2.5 AS/Pixel than anything lower than that will really be wasted to the seeing. I'm sure Dean or Matt can get real detailed with this but this is a good generalization.
CS's
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Bobby
StonyHill Observatory
Skywatcher EQ6 Pro (On it's way)
AT 130 F/6 Prototype
Royce 10" f/4 Astrograph (Under Construction)
Self Modded 40D
70-200 F2.8 L IS
SSAG
YAHOO TMB 130SS Group
Astronomy Technologies Yahoo Group
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lawrie
Carpal Tunnel
Reged: 01/31/06
Posts: 1744
Loc: Okanagan Valley
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How do you determine the limits of seeing to know your AS/pixel?
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Clear Skies
Lawrie
Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D
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Strgazr27
Vendor - Deep Space Mods and Composites
Reged: 10/04/04
Posts: 6984
Loc: StonyHill Observatory
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Lawrie,
If using a CCD, This is one way of doing it.
This is a very good article even though it talks of using a CCD. You can use a program such as DSLRFocus to determine FWHM values using a DSLR.
Experience also helps. Even though I have pretty decent skies a large part of the time, my seeing is rarely better than roughly 2.5 AS/P.
CS's
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Bobby
StonyHill Observatory
Skywatcher EQ6 Pro (On it's way)
AT 130 F/6 Prototype
Royce 10" f/4 Astrograph (Under Construction)
Self Modded 40D
70-200 F2.8 L IS
SSAG
YAHOO TMB 130SS Group
Astronomy Technologies Yahoo Group
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Arkalius
scholastic sledgehammer
Reged: 08/03/06
Posts: 878
Loc: Orange County, CA, USA
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Actually, you want your pixel resolution to be at least 3 times more than what seeing will allow. 0.8 to 1 arcsecond per pixel is the sweet spot. Oversampling has benefits. Just because seeing is 2.5"/pixel doesn't mean everything comes in precise 2.5" square chunks, it's a continuous image that is being digitized. It helps to oversample it some to prevent loss of detail.
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-Arkalius
11" Celestron SCT on Orion Atlas EQ-G
Celestron 100ED Refractor
8" Zhumell Dobsonian Reflector
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Rick
Post Laureate
Reged: 04/12/05
Posts: 3050
Loc: Tokyo, Japan
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Thanks for those links Bobby. Helped me alot!
clear skies,
Rick
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www.japanastro.com
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Strgazr27
Vendor - Deep Space Mods and Composites
Reged: 10/04/04
Posts: 6984
Loc: StonyHill Observatory
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Quote:
Actually, you want your pixel resolution to be at least 3 times more than what seeing will allow. 0.8 to 1 arcsecond per pixel is the sweet spot. Oversampling has benefits. Just because seeing is 2.5"/pixel doesn't mean everything comes in precise 2.5" square chunks, it's a continuous image that is being digitized. It helps to oversample it some to prevent loss of detail.
Although I agree your always better oversampling, experience has taught me that in my situation it really doesn't make that big an improvement.
CS's
--------------------
Bobby
StonyHill Observatory
Skywatcher EQ6 Pro (On it's way)
AT 130 F/6 Prototype
Royce 10" f/4 Astrograph (Under Construction)
Self Modded 40D
70-200 F2.8 L IS
SSAG
YAHOO TMB 130SS Group
Astronomy Technologies Yahoo Group
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tjensen
Carpal Tunnel
Reged: 02/16/05
Posts: 1589
Loc: Chapel Hill, NC
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Ideally, you want to meat the Nyquist sampling criteria... two resolution elements (i.e. pixels) for the resolution of your instrument. Say your scope's theoretical resolving power is 0.45 arcsec/pixel, then you would need a set up that provides .25 arcsec/pixel. Now that is the theoretically perfect system... since seeing gets in the way, we generally never get to use our scopes at their full resolving potential.
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lawrie
Carpal Tunnel
Reged: 01/31/06
Posts: 1744
Loc: Okanagan Valley
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How do you figure out your scope's resolving power?
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Clear Skies
Lawrie
Ultima 8
Atlas EQ-G
ZenithStar 80 FD
DSI Pro - Pro II
Canon 350D
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