Re: "scaling" exposure time to account for varying focal ratio and ap


Jan 7, 2010

 


----------------------------

#48459 Jan 7, 2010

In a message dated 1/6/2010 11:51:56 PM Central Standard Time,

lmbuck2000@... writes:



> first "his" f/4 vs. my f/4.8

>

> (4.8^2/4^2) = 1.44 means the f/4 is collecting 1.44 times as much data in

> the same interval as me due to "speed" of the system.

>

> then aperture: the 200mm vs. my 130mm

>

> (200^2)/(130^2) = 2.37 times as much "stuff" coming onto the sensor in his

> system vs. mine.

>

> so 75min x 1.44 x 2.37 and i need 256 minutes!

>



The only thing that governs data collection is aperture, not focal ratio.

But also remember this. A 200mm mirror system with secondary obstruction is

not as efficient as an unobstructed refractor. A 130 oiled lens with high

tech multi-coatings loses on the order of 1% of the light coming in. A 200mm

system with 2 mirrors and a 45% obstruction can lose between 30% to 40% of the

incoming light depending on coating type.



A simple aluminum coating will have 90% efficiency coming out of the

coating chamber, but will drop below that very quickly as it is exposed to the

oxygen in the air. The older the coating, the lower the overall efficiency. It

takes a high tech coating to maintain 90% for any length of time (high tech

meaning expensive).



Probably the greatest difference in image "richness" is camera type, how

well it's cooled, how well you have done the dark frames, light frames etc.,

and how well you have processed the image. Tony Hallas has released a set of

videos (available thru AP website) that explains the best way to get low

noise images from your data. How long the exposure should be for optimum

results, etc. By the way, he has used his 206EDF refractor to produce far deeper

images than he was able to do with his original 14.5" Cassegrain, both having

F8 focal ratios, so aperture is not always everything.





Rolando





[Non-text portions of this message have been removed]



----------------------------

#48460 Jan 7, 2010

Assuming exact same camera for both, the real difference is approximately

this between a 130 lens and a 200mm with 45% obstruction (65% light

transmission efficiency): 75x 2.37 x .65 = 115 minutes.



Rolando



In a message dated 1/7/2010 10:22:20 AM Central Standard Time,

chris1011@... writes:



> > so 75min x 1.44 x 2.37 and i need 256 minutes!

> >

>

> The only thing that governs data collection is aperture, not focal ratio.

> But also remember this. A 200mm mirror system with secondary obstruction

> is

> not as efficient as an unobstructed refractor. A 130 oiled lens with high

> tech multi-coatings loses on the order of 1% of the light coming in. A

> 200mm

> system with 2 mirrors and a 45% obstruction can lose between 30% to 40% of

> the

> incoming light depending on coating type.

>

> A simple aluminum coating will have 90% efficiency coming out of the

> coating chamber, but will drop below that very quickly as it is exposed to

> the

> oxygen in the air. The older the coating, the lower the overall

> efficiency. It

> takes a high tech coating to maintain 90% for any length of time (high

> tech

> meaning expensive).

>

> Probably the greatest difference in image "richness" is camera type, how

> well it's cooled, how well you have done the dark frames, light frames

> etc.,

> and how well you have processed the image. Tony Hallas has released a set

> of

> videos (available thru AP website) that explains the best way to get low

> noise images from your data. How long the exposure should be for optimum

> results, etc. By the way, he has used his 206EDF refractor to produce far

> deeper

> images than he was able to do with his original 14.5" Cassegrain, both

> having

> F8 focal ratios, so aperture is not always everything.

>

>





[Non-text portions of this message have been removed]



----------------------------

#48471 Jan 7, 2010

thanks, very much! i knew i get "the facts" from you.



lee

--- In ap-ug@yahoogroups.com, chris1011@... wrote:

>

>

> Assuming exact same camera for both, the real difference is approximately

> this between a 130 lens and a 200mm with 45% obstruction (65% light

> transmission efficiency): 75x 2.37 x .65 = 115 minutes.

>

> Rolando



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