Astrophotography question: aperture??

05/12/2015 02:29:51 PM · #1

This is something i've been trying to wrap my head around.

I grew up as a visual astronomer. The saying was, the larger the aperture, the brighter what you're going to see in your viewfinder. Except in this context, "aperture" meant diameter of the objective lens or mirror.

Now as a photographer, I'm hearing that the faster the lens, the brighter. My old telescope is a large 8 inch cassegrain, but it's only an F10... My little 50mm Nikkor lens is an f1.8.

SO, if I were to pick out a new telescope for astrophotography, do i factor in more objective size or f number? For example, would one of those giant 200inch telescopes on top of Hawaii be worse for astrophotgraphy than a little backyard newtonian reflector if the former was an f39 and the latter was an f3? I can buy either.

I have a hunch that says focal length might play into this but i really have no idea how it works.

Message edited by author 2015-05-12 14:31:37.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

05/12/2015 02:36:25 PM · #2

Check this tutorial for an explanation of aperture in the context of cameras/photography.

The aperture for any given lens depends on its focal length -- it is not an absolute value.

Message edited by author 2015-05-12 14:41:37.

05/12/2015 02:39:06 PM · #3

Originally posted by GeneralE:

Check this tutorial for an explanation of aperture in the context of cameras/photography.

Naw Braw, that doesn't touch on my question.

Message edited by author 2015-05-12 14:40:09.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

05/12/2015 02:43:29 PM · #4

Yeah ... on you camera the aperture is a fraction of the focal length, so the same size "hole" will be a different "F-number" on a 50mm lens and a 300mm telephoto lens.

05/12/2015 02:43:32 PM · #5

Originally posted by GeneralE:

Check this tutorial for an explanation of aperture in the context of cameras/photography.

The aperture for any given lens depends on its focal length -- it is not an absolute value.

But aperture in diameter IS an absolute value.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

05/12/2015 02:46:12 PM · #6

On your telescope it refers to the diameter of the lens/mirror, which determines how much light it can gather. It is independent of the focal length of the lens system.

Tamron SP 24-70mm f/2.8 Di VC USD Canon Mount

05/12/2015 02:47:05 PM · #7

Maybe another way of thinking about my question is: why doesn't f/ratio matter for visual astronomy?

Mike

Canon EOS-5D Mark II

05/12/2015 02:51:58 PM · #8

Originally posted by LanndonKane:

Originally posted by GeneralE:
Check this tutorial for an explanation of aperture in the context of cameras/photography.

The aperture for any given lens depends on its focal length -- it is not an absolute value.

But aperture in diameter IS an absolute value.

fstop is a ratio and thus the open diameter is dependent on focal length

a 50/1.8 requires a lens opening of 27mm

a 135/2 requires 67.5 mm

a 300/4 requires 75mm

this is why you rarely see long lenses with wide apertures, unless they are huge and expensive

Message edited by author 2015-05-12 14:53:06.

Mike

Canon EOS-5D Mark II

Tamron SP 24-70mm f/2.8 Di VC USD Canon Mount

05/12/2015 02:55:19 PM · #9

Originally posted by LanndonKane:

Maybe another way of thinking about my question is: why doesn't f/ratio matter for visual astronomy?

because magnification is usually more important.

the fstop is going to determine your exposure time in astro photography. if you want a shorter exposure time, get a faster lens, or use a higher ISO.

Message edited by author 2015-05-12 14:57:03.

Canon 12x IS USM (S3 IS built-in Lens) -- 36-432mm (35mm EQ)

05/12/2015 02:56:23 PM · #10

Thanks Mike.

The discussion in this thread might help. It also might help if we knew the focal length of your telescope ...

kirbic

Canon EOS R5

Canon RF 24-70mm f/2.8 L IS

05/12/2015 05:05:16 PM · #11

Your question has a complicated answer. In short, though, astrographs, which are optimized for photography, are often fast focal ratio instruments, and therefore shorter focal length, since a short focal ratio at a long focal length gets somewhat... large. The exception is for planetary photography, where long focal lengths are used in conjunction with small imaging sensors and special techniques like acquiring thousands of frames of video, sorting through for the sharpest, and then averaging those.
Another consideration when shopping for an astrograph is whether it will cover the desired sensor size. A lot of 'scopes will cover APS-C, but much fewer will cover a 35mm frame.
Finally, consider whether a field flattener will be required for the chosen instrument. They are often required for designs which are intended primarily for visual work.

05/12/2015 05:23:55 PM · #12

Originally posted by kirbic:

No offence guys, but I really just was waiting for Kirbic to swing around. Great answer.

05/12/2015 05:25:39 PM · #13

Originally posted by Mike:

Originally posted by LanndonKane:
Maybe another way of thinking about my question is: why doesn't f/ratio matter for visual astronomy?

because magnification is usually more important.

the fstop is going to determine your exposure time in astro photography. if you want a shorter exposure time, get a faster lens, or use a higher ISO.

I disagree that magnification is usually more important. Maybe for planetary observation, but the deep sky buffs are all about huge apertures even though the objects have large apparent diameters in the night sky.

MelonMusketeer

Nikon D7000

Nikon MF 50mm f/1.4

05/12/2015 11:45:57 PM · #14

With an SCT, you have to figure in the area of the correction plate blocked by the secondary mirror when determining aperture.

The apparent f stop will change with magnification if you plan to shoot using an eyepiece and projecting the image directly on the sensor.
That works like using an extender with a tele lens. A 2X extender spreads the light over 4X the area, so you lose 2 f stops. The exposure time would be multiplied by 4 to get the same EV unless you change the iso.

In any case, with a "faster" scope, you would be able to shoot more images in the same time period at night.

Canon RF 24-70mm f/2.8 L IS

05/14/2015 03:55:17 PM · #15

For anyone still interested, I finally found an answer that I'm satisfied with. I had a hunch along these lines.

//www.lonelyspeck.com/lenses-for-milky-way-photography/

"A physically larger aperture for any given focal length will help us achieve more detail in any given portion of the night sky. This is why the worldâ��s best telescopes have huge diameters: to collect more light.

The light gathering capability of a lens is directly proportional to the area of the clear aperture. Since the area of a circle is proportional to the square of the diameter, the clear aperture area increases quickly with lens size. For example, when you look at equal portions of the night sky between the two lenses, the 100mm f/2 lens collects over 16 times more light from that portion of the sky than the 24mm f/2 lens due to its much larger clear aperture. (excluding the rest of the 24mm/2â��s field of view, sort of like cropping the 24mm/2â��s image to the same field of view as the 100mm and then making the comparison.) But wait, didnâ��t I just finish saying in the last section that we wanted a short focal length so we can use longer shutter speeds? Which one do we actually want?

The 24mm f/2 lens collects light from a comparably wider field of view than the 100mm f/2. Since theyâ��re both f/2, they both capture light at the same â��speedâ��. So for equal shutter speeds, they should provide the same illuminance at the sensor. So in terms of exposure value, the 24mm lens will produce equivalent brightness images for any given ISO and shutter speed because itâ��s pulling light from more of the scene than the narrower 100mm lens, hence the identical f/number rating. The long lens collects more light at a time from a smaller area of the scene while the short lens collects less light at a time from a larger area of the scene. Without being able to track the stars with an equatorial mount, the limiting factor of the 100mm is then its field of view which will only allow us a 5 second exposure before the stars start to trail. Thatâ��s a two stop (four times) disadvantage to the 24mm. Looks like we want the shortest focal length lens with the largest clear aperture."

kirbic

Canon EOS R5

05/14/2015 05:42:57 PM · #16

Yep, that page has a lot of good info. If you really want great images, you will have to track, there is no way around it. One more thought: with today's high-resolution sensors, it doesn't take that long a focal length before your resolution on the sky (in arcseconds per pixel) out-resolves the seeing conditions. Recall that really, really good seeing approaches 1 arcsecond, but in most areas about 4 arcseconds is more normal. With the high resolution of the D800, you probably are limited by seeing above 500 or 600mm.

ETA:
Yup, by my calculations for the D800 at 500mm focal length you have 2.0 arcseconds per pixel, which for the most part will equal or exceed what the seeing conditions allow.

Message edited by author 2015-05-14 17:48:56.