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Magnification effect on brightness of object Vs background of sky


Ratlet
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I'm trying to understand how magnification impacts the visibility of an object compared to the brightness of the sky.

With my current scope (650mm/130mm):

my 32mm Plossl gives a 6.4mm exit pupil and x20 with 48°fov

A Meade HD-60 25mm would give a 5mm exit pupil and x26 with 60° fov

I understand that the FOV through the eyepiece would be different, but both would present a 2° area of the sky.

How would the choice of magnification affect the appearance on a dso such as m31 or m32? 

I know that the image would not be as bright as higher magnifications but would there be a difference between the brightness of the object and the sky in each case?  Which would be preferred?  Would it be perceptible with this examples or would you need to push further to something like an explore scientific with an 82°FOV and 18mm focal length?

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23 minutes ago, Zermelo said:

Cheers dude.  It's opne of those questions that throwas up a lot of similar ones which are slightly different and it made it difficult to find the correct answer.  The annoying thing is that I'm certain I've seen this question answered before but I just can't find it again!

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It’s a very complex area, with many factors at play. A chap called Mel Bartels has published a lot of work on object contrast which might be interesting for you. The second link here is to his bbastrodesigns site which is worth a trawl around.

https://www.cloudynights.com/ubbthreads/attachments/5526435-threshold forCAA.pdf

https://www.bbastrodesigns.com

https://www.bbastrodesigns.com/ObjectContrastCalculator.htm

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Contrast and stars:

The higher the magnification, the fainter the star is that can be seen.  That's because the sky in the eyepiece dims with magnification, but stars don't dim until very very high powers.

So the faintest stars are seen at high power.

 

Contrast and extended objects:

Just like the sky in the eyepiece dims with increased magnification, so does the extended object.

But, it is also larger.  So it becomes more visible and features within it become more visible with increased magnification.

But only up to the point where you start losing the ability to see those features or the object itself.

So in the two curves--decreased brightness with magnification, and increased size with magnification--there will be a magic point where

the viewing of the object is optimized for you--your eye, and your scope.

Where that is requires experimentation.

For a large faint nebula it could be at the lowest power.  For a small bright planetary nebula, it could be your highest power.

You don't know until you look, but after a few years of doing this, you'll get a feel for it and pretty much know where to go, magnification-wise, on each type, size, and brightness of object.

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For extended objects, differences in magnification generally have no effect on contrast. Sky glow and the object itself both get dimmer at the same rate.

However, the increased magnification usually aids in object visibility, so the object's perceived contrast may be higher.

For example, M31 in my SQM 20.7 - 21.0 skies seems to exhibit variable contrast depending on magnification. Using a zoom, you can find some optimal balance of magnification and view brightness where the dust lanes seem to "pop". They go from being washed out to more visible as you zoom in. But then you get to a point where they start getting harder to see again because you've zoomed in too much.

The same is true of targets like M1 and the Veil Nebula. As you increase magnification, they become easier to see and the brain is tricked into thinking the background got darker than the target did. That's not technically true, it just appears that way.

This is why I'm a big proponent of having a broad selection of eyepieces, and/or a decent quality zoom. Just like with the planets you can dial in the right magnification for the night's conditions, having lots of focal lengths to choose from lets you dial in the right balance of magnification and view brightness for different extended objects.

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Thanks guys.  With hindsight, it does kind of tie in with my observations. 

Its hard to be completely objective as I've only got x20 (32mm)and x80(8mm) in terms of power.  Will be getting an x50(12mm) or so over Christmas which should fill the gap between them for dialing stuff in, and be more useful with a Barlow.

I think after that I might have to try and resist eyepiece temptations (planetary eyepiece excluded).  My 5" Newtonian is giving me amazing views and I've got a long way to flog that horse.

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Another thing to consider with DSOs is if they are made up of individual stars.  Compact open clusters are merely low contrast smudges at lower powers, but once you reach the power point where individual star resolve, contrast suddenly jumps way up.  The same thing happens with globular clusters, but at much higher powers and larger apertures.  Technically, if you had enough aperture and steady enough skies, even nearby galaxies' stars would start to resolve at super high powers providing high contrast.  This is sort of what happens with space telescopes imaging galaxies.

Edited by Louis D
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One other consideration with the larger objects is that if they fill they fov then you end up looking through them and not detecting the contrast around the edges. If your eyepiece is filled with uniform brightness of an object then you won’t see it. If you see the edge contrast between object and background then you should.

The Rosette nebula is a good example. With a 17.5mm Morpheus in my 8” f8 I would likely only see the cluster in the centre. With a 31mm Nagler I would have more chance of detecting the nebula. Put that same eyepiece in my Genesis and it should show up nicely. Obviously a dark sky and OIII filter help a lot too.

CD637154-5CF0-4164-91FB-B18E34CD5A20.thumb.png.6bb6147ea17f04d57a80cd612ac0ccce.png

DE1E90D6-E5B1-4993-884A-21A0CDAA0B48.thumb.png.14c997a194747c324e680c5fff2c7a64.png

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What app are you using for that @Stu?  I find stellariums fov simulator to be quite clunky on android and that looks much better.

@Louis D that interesting and makes absolute sense!

It seems like for every job there is a tool and for every object there is a scope.  You might be able to drive a nail with an adjustable spanner, but you are better off using a hammer.  

 

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For extended objects (e.g. galaxies, planets), exit pupil needs to be between say 2 and 4mm.  Lower, and the object brightness decreases more.

For point objects, brightness is a function of aperture, so they become more noticeable with smaller exit pupil (more mag) i.e. reduced sky brightness.  This is because contrast increases.

Doug.

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55 minutes ago, Ratlet said:

What app are you using for that @Stu?  I find stellariums fov simulator to be quite clunky on android and that looks much better.

It is SkySafari 7 Pro. The Plus versions have the same function though. You set you kit up then can select any combination and it will display the correct field of view circle for it, works really well. It also gives you the mag and exit pupil for the particular setup chosen.

80A698B7-AC16-444C-892B-4D3FF70AC634.png

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3 minutes ago, Stu said:

It is SkySafari 7 Pro. The Plus versions have the same function though. You set you kit up then can select any combination and it will display the correct field of view circle for it, works really well. It also gives you the mag and exit pupil for the particular setup chosen.

80A698B7-AC16-444C-892B-4D3FF70AC634.png

 

Ah, thanks Stu - I have entered all my kit to get the FOV displayed on the star charts, but I hadn't noticed that the mag and EP were also calculated.

btw, the SkySafari V7 Pro and Plus versions are on a 65% off Black Friday deal until 28 Nov.

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1 hour ago, Zermelo said:

btw, the SkySafari V7 Pro and Plus versions are on a 65% off Black Friday deal until 28 Nov.

That’s a great deal! Hopefully plenty of people manage to pick it up, well worth it 👍

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On 26/11/2022 at 11:30, Stu said:

One other consideration with the larger objects is that if they fill they fov then you end up looking through them and not detecting the contrast around the edges. If your eyepiece is filled with uniform brightness of an object then you won’t see it. If you see the edge contrast between object and background then you should.

The Rosette nebula is a good example. With a 17.5mm Morpheus in my 8” f8 I would likely only see the cluster in the centre. With a 31mm Nagler I would have more chance of detecting the nebula. Put that same eyepiece in my Genesis and it should show up nicely. Obviously a dark sky and OIII filter help a lot too.

CD637154-5CF0-4164-91FB-B18E34CD5A20.thumb.png.6bb6147ea17f04d57a80cd612ac0ccce.png

DE1E90D6-E5B1-4993-884A-21A0CDAA0B48.thumb.png.14c997a194747c324e680c5fff2c7a64.png

The Rosette Nebula has substantial H emission as well as O-III.  A narrowband UHC-type filter will help you see the Rosette a lot better than an O-III.

Something like an Astronomik UHC, or TeleVue Nebustar or DGM NPB or Lumicon UHC or Orion UltraBlock.

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4 minutes ago, Don Pensack said:

The Rosette Nebula has substantial H emission as well as O-III.  A narrowband UHC-type filter will help you see the Rosette a lot better than an O-III.

Something like an Astronomik UHC, or TeleVue Nebustar or DGM NPB or Lumicon UHC or Orion UltraBlock.

I had a look through this a while back and purchased a UHC filter (from SVBONY) to give it a shot.  Probably not the best, although I've read that is basically the same as the Optolong UHC.  I'll need to have a look at the rosette.  It's one of those one's I always seem to spot on a chart after I've packed up.

https://www.cloudynights.com/articles/cat/user-reviews/accessories/astronomical-filters/filter-performance-comparisons-r1471

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2 hours ago, Don Pensack said:

The Rosette Nebula has substantial H emission as well as O-III.  A narrowband UHC-type filter will help you see the Rosette a lot better than an O-III.

Something like an Astronomik UHC, or TeleVue Nebustar or DGM NPB or Lumicon UHC or Orion UltraBlock.

Thanks Don. I’ve seen it with an OIII before but have both UHC and NPB so will give them a go. Getting to that time of year again….

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4 hours ago, Don Pensack said:

The Rosette Nebula has substantial H emission as well as O-III.  A narrowband UHC-type filter will help you see the Rosette a lot better than an O-III.

Something like an Astronomik UHC, or TeleVue Nebustar or DGM NPB or Lumicon UHC or Orion UltraBlock.

Interestingly, Dave Knisely has the UHC and OIII as both being recommended but showing different aspects better, as is often the case with these filters.

https://www.prairieastronomyclub.org/filter-performance-comparisons-for-some-common-nebulae/

NGC 2237-9 “ROSETTE NEBULA” (diffuse nebula in Monoceros).

(100mm f/6, 22x)

DEEP-SKY: (2) Some increase in contrast, but nebula is still more of a diffuse haze around the central star cluster with hints of irregularity.

UHC: (5) Noticeable increase in contrast, with more outer nebulosity visible and some irregular light and dark structure being visible.  Nebula was visible when UHC was held up to unaided eye!

OIII: (5) Higher contrast than with UHC, with more dark irregular detail throughout the region (especially in the 10 inch), but not quite as much nebulosity visible as in UHC.

H-BETA: (1) Very faint glow around the star cluster, not much better than without a filter (but much dimmer).

RECOMMENDATION FOR ROSETTE NEBULA: UHC/OIII.

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6 hours ago, Ratlet said:

I had a look through this a while back and purchased a UHC filter (from SVBONY) to give it a shot.  Probably not the best, although I've read that is basically the same as the Optolong UHC.  I'll need to have a look at the rosette.  It's one of those one's I always seem to spot on a chart after I've packed up.

https://www.cloudynights.com/articles/cat/user-reviews/accessories/astronomical-filters/filter-performance-comparisons-r1471

The Optolong, like many "UHC" filters, has a bandwidth almost twice as wide as the better narrowband "UHC-Type" filters.

As such, it also yields much less contrast on nebulae.

Yes, it will work some, but the better narrowband filters have 22-28nm bandwidths, whereas the Optolong is 47-49nm.

Compare with David Knisely's comments about the Deep Sky broadband filter.

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Following Star Hunter's lead, I bought one of the cheap spectrometers off of ebay and have been modifying it to image filter spectra.  I'm still getting the hang of it, but I composited several of my line filters together with an image from Star Hunter to show the relative passbands of several line filters together.  I may not have got the scaling or alignment quite right, but it's close.

1634938310_LineFilterswithStarHunter.thumb.jpg.84ea201cdaed5f2c1c7e31688e457418.jpg

My 1990s vintage Lumicon UHC is considerably narrower than the Svbony UHC.

My 1990s vintage Lumicon OIII leaks a lot of red light, more than the Lumicon UHC!  At least it hasn't rusted.

The Zhumell OIII doesn't leak any red light at all!

The Lumicon OIII in my observing experience and the test sticker on the filter case indicate it is well centered on the OIII lines.

The Zhumell OIII is well to the right, more in the Swan bands (Carbon) than the OIII bands.  I'll have to try this filter on comets in the future.

The two OIII's stacked indicate a bit of overlap, so maybe the Zhumell passes at least one of the OIII lines?

The Zhumell Moon & Skyglow (M&SG) is basically the same as Baader's Neodymium filter at a far lower price (between $10 and $15 about a decade ago).

The M&SG probably increases contrast by separating the overlap of the red and green and to a lesser extent green and blue parts of the spectrum.

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Current Lumicon UHC and O-III filters pass no red.

 

Here are some other filter tests using prisms and spectrophotometers:

NOTE: many tests are older and do not represent the current filters.

Astronomik narrowed their filter bandwidths in the 2016-2017 range

TeleVue did so in 2018

Lumicon changed their bandwidths and transmission spectra in 2018.

So reports from earlier may not represent current filters.

This site is quite current:

https://searchlight.semrock.com/?sid=a08a1af9-84ee-49d2-959d-153d7e7c0eb8#

As is this:

https://www.cloudynights.com/topic/817105-2022-nebula-filters-buyers-guide/#entry12318142

 

Here is the list of test sites:

http://www.loptics.com/articles/spectra/spectra.html
http://astrosurf.com/viladrich/astro/instrument/filters/Filters.html
http://karmalimbo.com/aro/reports/Astronomy Filter Research Paper_by Jim Thompson.pdf
http://www.reinervogel.net/index.html?/Filter/Filter_e.html
http://www.cloudynights.com/item.php?item_id=1520
http://karmalimbo.com/aro/reports/Band Pass Filters For Visual & Video Astronomy_Part1.pdf
http://www.reinervogel.net/
http://www.astronomy.ru/forum/index.php/topic,68257.0.html
https://www.cloudynights.com/topic/734419-radian-triad-triad-ultra-performance-vs-f-ratio/#entry10581011
http://www.reinervogel.net/index.html?/Filter/Filter_Hb_e.html
http://www.astronomy.com/~/media/import/files/pdf/8/c/7/0805_nebula_filters.pdf
http://karmalimbo.com/aro/reports/Test Report - LP Filters Under Dark Skies.pdf
http://www.astrosurf.com/buil/filters/curves.htm#TeleVue NebuStar
http://www.karmalimbo.com/aro/pics/filters/narrow.jpg
http://www.karmalimbo.com/aro/pics/filters/o3b.jpg
http://www.karmalimbo.com/aro/pics/filters/o3a.jpg
http://home.freeuk.com/m.gavin/grism2.htm
http://sweiller.free.fr/spectro/filters/Wratten/Filter 26-29-30.jpg
http://www.astrosurf.org/buil/filters/curves.htm
http://www.astrosurf.org/buil/filters/curves.htm#Lumicon UHC
http://www.carlostapia.es/curvas_filtros/revisiones.html
http://www.astroamateur.de/filter/
http://www.carlostapia.es/curvas_filtros/Comparison_OIII_Visual.html
http://www.astrograph.de/artikel/Spectral Transmission.pdf

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