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Astrophotography and Sketching >> DSLR & Digital Camera Astro Imaging & Processing

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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: Nils_Lars]
      #5560304 - 12/08/12 01:39 AM

Hi Erik,

Thank you,

Jerry


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Barry E.
super member


Reged: 12/27/08

Re: NGC 891 20121116 new [Re: Jerry Lodriguss]
      #5560660 - 12/08/12 10:44 AM

Quote:

But if your sub exposures are long enough to get you up out of the readout noise of the camera, using a good-quality 2-inch Barlow in this instance to get you to f/10 should be ok




Great image as usual, Jerry!

Regarding the use of a barlow for deepsky, I have done some thinking in that area:

Why are our stars bigger than the pros?

Keep up the great work.


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jimegger
Pooh-Bah
*****

Reged: 12/22/05

Loc: Palmer,Alaska
Re: NGC 891 20121116 new [Re: Barry E.]
      #5561277 - 12/08/12 05:58 PM

Jerry, you have done it once again - nice shot !!!

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petemumbower
Carpal Tunnel


Reged: 07/12/09

Loc: West Michigan
Re: NGC 891 20121116 new [Re: jimegger]
      #5561643 - 12/08/12 10:55 PM

Excellent work Jerry, you simply have the magic touch with astrophotography.

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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: petemumbower]
      #5561683 - 12/08/12 11:25 PM

Hi Barry, Jim and Pete,

Thank y'all very much!

Jerry


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LeCarl
super member


Reged: 03/16/11

Re: NGC 891 20121116 new [Re: Nils_Lars]
      #5561740 - 12/09/12 12:25 AM

Wow, I'm very impressed, tons of details, I will show this to my friend that think the c11 can't be used without focal reducer! haha!
I can't believe that!

So you mean it is better to use it without focal reducer at native focal? (a lot oversampled) for using longer exposure to reduce background noise,
but... it's not better to use lower iso at faster focal because lowering the iso keep background noise away too... For example:

f10 focal at iso800 for instance background value of X
Vs
F6.3 focal at iso400... maybe? near same exposure time background value 1/2X then the S/N will be higher

I guess that... I don't know the background value properties but I think it is proportional, so we can play with (an idea)

Edited by LeCarl (12/09/12 12:31 AM)


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Moonwalker1968
member


Reged: 10/19/09

Loc: Switzerland
Re: NGC 891 20121116 new [Re: LeCarl]
      #5561884 - 12/09/12 05:06 AM

Jerry,

I have been on your interesting website several times and have learnt a lot. Thanks! Your beautiful NGC 891 image caught my attention a few days ago. I also work with long focal lengths (2.1 m and 2.3 m) at f/5 and would have thought 18MP on DX format with 2 m of focal length is a strong oversampling. Does it still make sense to use a modified (or unmodified) Canon 600D over a Canon 20Da at 2 m focal length? Do you win anything with it?

Best wishes,
Markus


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Turf1
member


Reged: 12/02/12

Loc: SW Michigan USA
Re: NGC 891 20121116 new [Re: Nils_Lars]
      #5562659 - 12/09/12 03:36 PM

Jerry, WOW!!!! I just got a C11 on a Cgem mount. when the skies clear and I can use it for the first time, could I expect ANYTHING close to this with my Nikon D50? Any response would be much appreciated. Thank you for a wonderful picture.

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moxican
sage


Reged: 11/13/11

Loc: Toronto, ON
Re: NGC 891 20121116 new [Re: Nils_Lars]
      #5562701 - 12/09/12 04:00 PM

That is an awesome image. Maybe someday I'll get one just like it

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rainycityastro
professor emeritus
*****

Reged: 03/29/10

Loc: Seattle, WA
Re: NGC 891 20121116 new [Re: Jerry Lodriguss]
      #5563126 - 12/09/12 08:58 PM

Gorgeous image! Amazing result for only 60mins and F-10. With a DSLR! That too unmodded!

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JoseBorrero
Post Laureate
*****

Reged: 09/04/09

Loc: Puerto Rico Island
Re: NGC 891 20121116 new [Re: rainycityastro]
      #5563271 - 12/09/12 10:37 PM

Awesome picture as always, I noted the you changed from Tak mount to Losmandy. Wonderful change

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bluedandelion
Carpal Tunnel
*****

Reged: 08/17/07

Loc: Hazy Hollow, Western WA
Re: NGC 891 20121116 new [Re: JoseBorrero]
      #5563287 - 12/09/12 10:49 PM

That is a very cool image. Well done.

Ajay


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: LeCarl]
      #5563399 - 12/10/12 12:21 AM

Hi LeCarl,

This is a complicated question. There has been a lot of contentious material written on this subject.

I am definitely oversampling at f/10 with 4.3 micron pixels at 2794mm of focal length, the sampling is about 0.3 arcseconds per pixel.

But at f/6 the sampling is 0.5 arcseconds per pixel. Not that much difference. It's still oversampled by a lot.

Signal-to-noise ratio goes down, but image scale goes up. If you reduce the image to the same image scale, I think you get the s/n back.

A lot depends on whether you want to consider it from the perspective of pixel s/n or object s/n.

See:

Steve Cannistra: The Relationship between f ratio, subexposure time, and pixe...

Stan Moore: The F-Ratio Myth

Craig Stark S/N Part 1

Craig Stark S/N Part 2

Craig Stark S/N Part 3

Craig Stark S/N Part 4

Craig Stark S/N Part 5

Emil Martinec: Noise, Dynamic Range and Bit Depth in Digital SLRs

Martinec seems to be saying that the same number of photons fall on the chip, regardless of pixel size, so the s/n should be about the same as long as you are shot noise limited.

I have found that a much bigger consideration is the seeing with long focal lengths. You really need good to excellent seeing to shoot at 3,000mm of focal length. If the seeing is not good, you are wasting your time.

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: Moonwalker1968]
      #5563403 - 12/10/12 12:25 AM

Quote:

I have been on your interesting website several times and have learnt a lot. Thanks! Your beautiful NGC 891 image caught my attention a few days ago. I also work with long focal lengths (2.1 m and 2.3 m) at f/5 and would have thought 18MP on DX format with 2 m of focal length is a strong oversampling. Does it still make sense to use a modified (or unmodified) Canon 600D over a Canon 20Da at 2 m focal length? Do you win anything with it?




Hi Marcus,

You are welcome!

18mp is on APS-C not DX, but it is still way oversampled.

What you gain by using a 600D vs a 20Da is the improved technology in the camera that produces less noise.

As far as the sampling, you're not going to resolve anything more spatially, but I think the s/n is about the same because the same number of photons fall on the chip, regardless of what size the pixels are (as long as you are shot-noise limited and not read noise limited).

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: Turf1]
      #5563408 - 12/10/12 12:29 AM

Quote:

Jerry, WOW!!!! I just got a C11 on a Cgem mount. when the skies clear and I can use it for the first time, could I expect ANYTHING close to this with my Nikon D50? Any response would be much appreciated. Thank you for a wonderful picture.




Hi Turf1,

Thank you!

If you have the same equipment, and the scope is thermally stabilized, and correctly collimated, and the seeing and guiding are the same quality as the night I shot, and the sky darkness is the same, and the exposure is the same, and the image calibration and processing are the same, then the results should be somewhere in the same ballpark.

The Nikon D50 is much older technology than the Canon T3i so the camera is going to probably have more noise, so you might need more total exposure to improve the signal-to-noise ratio.

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: moxican]
      #5563409 - 12/10/12 12:31 AM

Quote:

That is an awesome image. Maybe someday I'll get one just like it




Hi Moxican,

Thank you.

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: rainycityastro]
      #5563410 - 12/10/12 12:32 AM

Quote:

Gorgeous image! Amazing result for only 60mins and F-10. With a DSLR! That too unmodded!




Thank you!

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: JoseBorrero]
      #5563411 - 12/10/12 12:34 AM

Quote:

Awesome picture as always, I noted the you changed from Tak mount to Losmandy. Wonderful change




Hi Jose,

Thank you!

I'm only shooting with the Losmandy because the Tak mount is in for repair.

The GM100EQ is a great mount, but I can't live without goto anymore with 3,000mm of focal length!

Jerry


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Jerry Lodriguss
Vendor


Reged: 07/19/08

Loc: Voorhees, NJ
Re: NGC 891 20121116 new [Re: bluedandelion]
      #5563413 - 12/10/12 12:34 AM

Quote:

That is a very cool image. Well done.




Hi Ajay,

Thank you,

Jerry


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