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Contrast Efficiency-what is it and how do you know IT?

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#1 PKDfan

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Posted 26 May 2019 - 07:57 AM

This is from a new member here so howdy all!!

I've been an observer for forty years and had two scopes a 60mm play scope and a black c8.

One had contrast one had light gathering power.

Guess which ones?😉 sure sure the c8 had super optics but could never show me so powerfully like that super little scope, the supposedly so difficult Horsehead nebula.

In a good refractor the Head is ridiculously easy even in LP skies in fact when I looked through a Televue 70mm in S.E. Vancouver the whole region Runningman etc.etc. was so bright I thought it was clouds  real clouds! AMAZING!!

Why I love refractors could have been my thread instead and  Contrast efficiency is my mantra, that is why I bought a SW-100ed pro and a decent mount, to get back into the wonders above. I think we have sime jargon-strehl  ratio being the most important I believe.  So when does it become hard to see and can  only  a lense get PERFECT images?


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#2 CHASLX200

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Posted 26 May 2019 - 08:04 AM

You must have better eyes than i do.  43 years and 240+ scopes from 2" to 18" and i never saw the horse head in any shape or form in city or dark skies.   I must be blind or doing something wrong.


Edited by CHASLX200, 26 May 2019 - 08:05 AM.

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#3 PKDfan

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Posted 26 May 2019 - 08:10 AM

Edit here sorry was confusing flame nebula for the runner  still same thing all reflection nebulae

are targets. Or all Low contrast objects really. 

Thanks for reading my first real post here at CN.

Long live the glass in All thy wondrous ways

Amen

Clear skies 

And may the dreaded new equiptment curse be

BROKEN!



#4 PKDfan

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Posted 26 May 2019 - 08:15 AM

Yes that was certainly youthful eyes but not 20's

more like 30ish but my point is that you should be able to see those Hard to See things with really eaziest is with a refractor



#5 PKDfan

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Posted 26 May 2019 - 08:16 AM

Wow what did I do??



#6 PKDfan

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Posted 26 May 2019 - 08:27 AM

Hi there chaslx200

I see that you are very knowledgeable and you have never seen it!! Wow I'm shocked everyone who has a frac has'nt  caught it!



#7 25585

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Posted 26 May 2019 - 08:30 AM

With Hydrogen Beta filter?



#8 CHASLX200

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Posted 26 May 2019 - 08:43 AM

I have seen the X mas tree neb.  But never the horse. Not in my skies.



#9 TOMDEY

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Posted 26 May 2019 - 08:47 AM

This is from a new member here so howdy all!!

I've been an observer for forty years and had two scopes a 60mm play scope and a black c8.

One had contrast one had light gathering power.

Guess which ones? sure sure the c8 had super optics but could never show me so powerfully like that super little scope, the supposedly so difficult Horsehead nebula.

In a good refractor the Head is ridiculously easy even in LP skies in fact when I looked through a Televue 70mm in S.E. Vancouver the whole region Runningman etc.etc. was so bright I thought it was clouds  real clouds! AMAZING!!

Why I love refractors could have been my thread instead and  Contrast efficiency is my mantra, that is why I bought a SW-100ed pro and a decent mount, to get back into the wonders above. I think we have sime jargon-strehl  ratio being the most important I believe.  So when does it become hard to see and can  only  a lense get PERFECT images?

 

Edit here sorry was confusing flame nebula for the runner  still same thing all reflection nebulae

are targets. Or all Low contrast objects really. 

Thanks for reading my first real post here at CN.

Long live the glass in All thy wondrous ways

Amen

Clear skies 

And may the dreaded new equiptment curse be

BROKEN!

Hi, PKD! That's pretty amazing... Is it possible that you are confusing the Flame for the Horsehead? NGC2024 is much much easier to see/savour than is Barnard33 to even detect. I've seen the Horsehead only under unusually good conditions... then pan over the Flame... just to reward myself for having detected the difficult Horsehead!

 

On the other hand... I find the Horsehead easy if using a Night Vision Eyepiece with 6nm H-alpha filter. But that's a different approach!    Tom


Edited by TOMDEY, 26 May 2019 - 08:49 AM.


#10 PKDfan

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Posted 26 May 2019 - 09:12 AM

Hi tomdey, no  really was the horsehead there was the tiny little dark indent on the reflection nebula. The TV 70 had a 12mm nag in her it was a quick view though



#11 PKDfan

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Posted 26 May 2019 - 09:21 AM

And by the way the flame was as large as the the most impressive CCD pbitos Maybe I had unbelieve seeing and  transparancy tbat  night?!?



#12 PKDfan

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Posted 26 May 2019 - 09:27 AM

 No filters



#13 PKDfan

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Posted 26 May 2019 - 09:29 AM

Sorry for typos darn superphone



#14 John Huntley

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Posted 26 May 2019 - 02:49 PM

It's take me 4 years, a 12 inch scope, a H-Beta filter and the darkest skies that I have had to see even the slightest glimpse of Barnard 33 from here in the UK.

 

I'm a great fan of refractors and have a number of really good ones but none has got near to showing me the Horsehead Nebula,yet.

 

Your eyes must be really something to be able to see this target "ridiculously easy" under LP skies with a 70mm aperture and no filter.


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#15 ris242

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Posted 26 May 2019 - 03:37 PM

 i never saw the horse head

 

Were you looking at the other end of the horse?


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#16 Magnetic Field

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Posted 26 May 2019 - 03:59 PM

This is from a new member here so howdy all!!

I've been an observer for forty years and had two scopes a 60mm play scope and a black c8.

One had contrast one had light gathering power.

Guess which ones? sure sure the c8 had super optics but could never show me so powerfully like that super little scope, the supposedly so difficult Horsehead nebula.

In a good refractor the Head is ridiculously easy even in LP skies in fact when I looked through a Televue 70mm in S.E. Vancouver the whole region Runningman etc.etc. was so bright I thought it was clouds  real clouds! AMAZING!!

Why I love refractors could have been my thread instead and  Contrast efficiency is my mantra, that is why I bought a SW-100ed pro and a decent mount, to get back into the wonders above. I think we have sime jargon-strehl  ratio being the most important I believe.  So when does it become hard to see and can  only  a lense get PERFECT images?

Contrast is not an absolute thing (although it is normalised on the y-axis and can take on values from 0 to 1).

 

A lot of people make the same mistake: you need to scale the x-axis on the MTF (Modulation Transfer Function) graph accordingly otherwise you are comparing apples with oranges. Your 8"SCT will still go strong where your 4" apochromat has already left the scene no matter what you think how good the contrast in your apochromat is.

 

Try to understand post #14 and especially the graph and its meaning in the following:

 

https://www.cloudyni...or-obstruction/

 

For example you cannot use the graph in post #3 in the link above to compare say a 5" refractor to a 8" SCT. But most people do exactly this. When someone tells me a refractor has so much contrast. I always think this is hogwash and reminds me: how long is a piece of string. You must use something similar  to the graph (adapted to your instrument size and secondary obstruction) in post #14 in the above link.


Edited by Magnetic Field, 26 May 2019 - 04:28 PM.

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#17 CHASLX200

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Posted 26 May 2019 - 04:07 PM

Were you looking at the other end of the horse?

Maybe.



#18 PKDfan

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Posted 26 May 2019 - 09:12 PM

I think alot of us kinda overlook the fact that an obstructed scope kills the sharpest part of the eye and disregard what that does to our observations
Clear akies

#19 Magnetic Field

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Posted 27 May 2019 - 03:26 AM

I think alot of us kinda overlook the fact that an obstructed scope kills the sharpest part of the eye and disregard what that does to our observations
Clear akies

 

Read my post #16  and follow the link therein and educate yourself. The information is out there.

 

 

There are also people out there who think smoking is good for your health. And then there are the flat earthers and moon landing conspiracists.


Edited by Magnetic Field, 27 May 2019 - 03:34 AM.


#20 Vla

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Posted 27 May 2019 - 08:56 AM

Try to understand post #14 and especially the graph and its meaning in the following:

 

https://www.cloudyni...or-obstruction/

 

For example you cannot use the graph in post #3 in the link above to compare say a 5" refractor to a 8" SCT. But most people do exactly this. When someone tells me a refractor has so much contrast. I always think this is hogwash and reminds me: how long is a piece of string. You must use something similar  to the graph (adapted to your instrument size and secondary obstruction) in post #14 in the above link.

That graph doesn't tell what contrast transfer looks like in the field. For that, we need to take into account the seeing error, as well as inherent aberrations. If we do that in this case, 8" vs. 60mm, this is what it looks like. The seeing is assumed 1 arcsec (those best moments most people can hope for), with the approximate seeing error calculated to about 0.16 wave RMS for 8", and 0.036 wave RMS for 60mm aperture. The poly Strehl for 60mm f/15 achro is 0.92, assumed no significant SA, i.e. not more significant than the SCT's chromatism.

 

Turns out that the two instruments come to very similar contrast transfer level within most of the resolving range of the smaller aperture (since we are talking relatively large objects, many times the resolving limit of 8" aperture of 0.57 arcsec, relevant part of frequencies is well below 0.2, i.e. five times the resolving limit, or 2.8 arc seconds). If seeing worsens, it will take the 8" below the 60mm level in this frequency range.

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#21 HydrogenAlpha

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Posted 27 May 2019 - 09:08 AM

I think alot of us kinda overlook the fact that an obstructed scope kills the sharpest part of the eye and disregard what that does to our observations
Clear akies

How does it kill the sharpest part of the eye? If you're referring physically to the human eye, the light from every part of the objective, as long as the incident angle is correct (i.e. parallel to the principal axis), contributes to the emergent light at the centre of the optical axis of the telescope.

 

Also, the fovea has poor sensitivity to dim light. 



#22 HydrogenAlpha

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Posted 27 May 2019 - 09:11 AM

Hi tomdey, no  really was the horsehead there was the tiny little dark indent on the reflection nebula. The TV 70 had a 12mm nag in her it was a quick view though

The flame also has a dark lane running across it that is very prominent. But I don't do visual so I'm not too sure


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#23 CHASLX200

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Posted 27 May 2019 - 09:28 AM

I don't know about the rest of ya, but i will never see the horse head with my scopes and skies.



#24 Magnetic Field

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Posted 27 May 2019 - 12:50 PM

That graph doesn't tell what contrast transfer looks like in the field. For that, we need to take into account the seeing error, as well as inherent aberrations. If we do that in this case, 8" vs. 60mm, this is what it looks like. The seeing is assumed 1 arcsec (those best moments most people can hope for), with the approximate seeing error calculated to about 0.16 wave RMS for 8", and 0.036 wave RMS for 60mm aperture. The poly Strehl for 60mm f/15 achro is 0.92, assumed no significant SA, i.e. not more significant than the SCT's chromatism.

 

Turns out that the two instruments come to very similar contrast transfer level within most of the resolving range of the smaller aperture (since we are talking relatively large objects, many times the resolving limit of 8" aperture of 0.57 arcsec, relevant part of frequencies is well below 0.2, i.e. five times the resolving limit, or 2.8 arc seconds). If seeing worsens, it will take the 8" below the 60mm level in this frequency range.

You should at least give the source and not just posting Figures from other people's work (if you are not the author; if you are the author of the IDL drawn Figure I apologise in advance).

 

There is more to the story.

 

Looks like a Figure from here (I haven't checked it against your Figure though):

 

https://www.telescop...nd_aperture.htm

 

I don't like the telescope optics site. I am not saying it is wrong what one can read there but it is  not peer reviewed and poorly written and no references given (as you would in a technical document).

 

 

 

Btw: Seeing: yes this is the reason why Damian Peach uses a 6cm achromat instead of his  C14  for his astrophotography masterclass. And planetary sketchers prefer a 6cm achromat over a 40cm or 50cm Dall-Kirkham (https://www.cloudyni...n-jupiters-grs/ or https://www.cloudyni.../#entry9380595) in the UK were seeing was never that great.


Edited by Magnetic Field, 27 May 2019 - 12:55 PM.


#25 Gabby76

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Posted 27 May 2019 - 01:24 PM

I have managed the HH with a 80mm refractor under good skies, 5mm exit pupil and Hb filter before but not often. 




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