Image scale vs exit pupil

[parallaxerr]

I have just added an ST120 refractor to my collection with the intention of using it for DSO only, in the hope it will gather a good bit more light than the ED80.

It was my understanding that gathering more light was the aim of more aperture and transmitting that light to the eye. However, I  just read a statement that said increasing aperture is more about allowing a larger image scale by being able to use higher power EP's.

This has confused me slightly as reducing the EP FL serves to reduce exit pupil, which I thought was all important. I can get a similar FOV with a 32mm 52° EP  and a 20mm 70°  EP, with mags x19 and x30 respectively. Great I thought, go with the 20mm for a larger image...BUT the exit pupil reduces from 6.4mm to 4mm.

So what's more important here with respect to seeing "more"?, image scale or exit pupil?

[parallaxerr]

Actually think I may have just found the answer from Mr. Nagler himself.

Extracted from the following article, he says

"A low-power eyepiece produces a bright image. Longer

focal length produces a larger exit pupil. While extended ob-

jects like the Moon, planets, and galaxies will be brightest,

stars do not change their brightness as exit pupils are reduced.

The higher power and resulting smaller exit pupil dims the sky

background, enhancing contrast of a star field, and allowing

fainter stars to be seen"

http://televue.com/Pdf/Eyepiece_AASNY_2014_May_June_ChoosingEPs.pdf

[Stu]

It's a complex subject, which I am only just starting to fully understand.

Stars behave differently to extended objects such as nebulae.

With increased magnification/reduced exit pupil, stars act as point sources and do not dim, but the sky background does, so dimmer stars become visible at higher mags.

For extended objects, they dim at the same rate as the sky background, but as you increase the mag, you increase the image scale and your eye is better at detecting the contrast with larger objects, so small faint galaxies, or small features within nebulae become visible as you enlarge them with higher mag.

A scope can never make an object have higher surface brightness than with the naked eye, but a larger scope can maintain the surface brighness whilst it produces a larger image scale, thus helping object visibility.

The easier comparison is to keep everything the same, but just change the aperture.

So, take an 8" f8 scope and a 16" f4 scope. Both have a focal length of 1600mm

Using a 16mm eyepiece:

The 8" scope will produce x100 magnification with an exit pupil of 2mm

The 16" scope will produce x100 magnification with an exit pupil of 4mm, so the image will appear brighter

alternatively, with an 8mm eyepiece, the 16" scope will produce x200 at an exit pupil of 2mm. This means it will give you the same brightness of extended objects but at twice the image scale of the 8", so the objects will be more easily detected.

Does that make sense/help?

 

 

 

[Stu]

Taking your scopes as examples

ST120 f5 has a focal length of 600mm

ED80 f7.5 has the same focal length of 600mm

Using the 32mm eyepiece, you get the same mag in each ie x19, whilst the exit pupil is 6.4mm in the ST120 and 4.3mm in the ED80

Using the 20mm eyepiece, again you get the same mag in each at x30. The ST120 gives you an exit pupil of 4mm, and the ED80 gives you 2.7mm, so in every case, the ST120 will give you the same image scale but with higher surface brightness for extended objects (and the sky background)

Working it the other way, a 21mm eyepiece in the ST120 gives you an exit pupil of 4.2mm, with a magnification of x28 ie you get a larger image scale whilst maintaining the same image surface brightness of the ED80 at x19. These effects become much more apparent when you move up to larger scopes, but the benefit is still there with the 120, plus you have the additional resolution given by the larger aperture.

[parallaxerr]

4 minutes ago, Stu said:

Does that make sense/help?

Very much so, thankyou Stu, I didn't fully appreciate the difference between starts and extended objects wrt surface brightness.

Your examples help and as it happens, I now have two scopes of the same FL but with 80mm & 120mm apertures, so I can test the theory.

So, with my existing eyepieces the view in the ST120 will be scaled the same as the ED80, but brighter and I have the option to increase image scale in the ST120, whilst maintaining the same or larger exit pupils than in the ED80 and likely improving contrast

 

[Stu]

Just now, parallaxerr said:

Very much so, thankyou Stu, I didn't fully appreciate the difference between starts and extended objects wrt surface brightness.

Your examples help and as it happens, I now have two scopes of the same FL but with 80mm & 120mm apertures, so I can test the theory.

So, with my existing eyepieces the view in the ST120 will be scaled the same as the ED80, but brighter and I have the option to increase image scale in the ST120, whilst maintaining the same or larger exit pupils than in the ED80 and likely improving contrast

 

Excellent, glad it made sense  

The only other variable in your comparison is the figuring of the objective. The ED80 has a very nice apo doublet objective, whilst the ST120 is an achro doublet. Larger and faster lenses tend to show more spherical aberration (where the light from all parts of the objective is not focused at the same point). You also have more CA to contend with vs the ED80. This MAY (or may not!) have an impact on visibility of fainter stars and planetary detail so the comparison is not totally 'fair' on the ST120. However, on extended objects, nebulae etc you should get some interesting side by side results which hopefully prove the theory  

[parallaxerr]

Lets hope the 120 is significantly bigger as to negate some of the effects of SA & CA then, otherwise it may be a pointless purchase!?

I've been eyeing up UHC & semi-apo filters, do you think they will be of any benefit?

[Stu]

20 minutes ago, parallaxerr said:

Lets hope the 120 is significantly bigger as to negate some of the effects of SA & CA then, otherwise it may be a pointless purchase!?

I've been eyeing up UHC & semi-apo filters, do you think they will be of any benefit?

I'm sure you will see a benefit, particularly on DSO's. You will get better resolution on things like globulars too.

@cotterless45is better placed to advise on semi apo filters on achros, I know that he rates them on his 150 f8, not sure which one though.

UHC filters do help on specific targets ie emission nebulae, but don't help on galaxies or reflection nebulae. They are great for planetary nebulae such as M57 and M27 and also useful on targets like M8. There is much debate about the benefits of OIII filters vs UHC. Many people say that the OIII is not useful on scopes under 8". I disagree with that. Under a dark sky, and OIII can be fantastic on the Veil or NAN for instance in a 4" scope or even smaller.

On other targets, the UHC will tend to show the wider extent of a nebula, whilst the OIII shows the brighter parts with better contrast. The true answer is that you probably need to try both to see which suits you best. A UHC however is the less aggressive of the two and is useful on a wider range of targets. The OIII shows better results on a narrower range of targets. There are plenty of people who this the OIII is worth having just for it's ability to show the Veil Nebula to it's best, I agree with this!