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03-06-2017, 06:54 PM   #1
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What Focal Length for the Total Solar Eclipse?

Just wondering what I should be getting for this event in the summer. I have a 1100-1200mm mirror telescope which is amazing but I'm guessing this thing is way too long for this (unless it's not but I don't have a t-mount adapter atm to test).

My next longest lens is 210 which is way too short for even regular moon shots. What is the sweet spot for the sun+moon? I'll be shooting APSC.


Last edited by ZombieArmy; 03-06-2017 at 07:03 PM.
03-06-2017, 07:02 PM - 1 Like   #2
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Try the following

Image size = subject size X focal length / distance

Image size is the sensor size , or the portion of it you want to fill

Subject size is the diameter of the moon,

Distance is the distance of the moon to the earth.

Rearrange to solve for focal length

Focal length = image size X distance / subject size
03-06-2017, 07:05 PM   #3
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QuoteOriginally posted by Lowell Goudge Quote
Try the following

Image size = subject size X focal length / distance

Image size is the sensor size , or the portion of it you want to fill

Subject size is the diameter of the moon,

Distance is the distance of the moon to the earth.

Rearrange to solve for focal length

Focal length = image size X distance / subject size
And adjust for crop correct?
03-06-2017, 07:13 PM   #4
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QuoteOriginally posted by ZombieArmy Quote
And adjust for crop correct?
No need to adjust for crop see my comment on image size, I.e sensor size or any portion of it you want to fill.

I consider, for example that a total eclipse has the sun and moon almost equal in size (as observed by their relative distances)

Depending on camera, the diameter will appear on the shortest dimension of the sensor, either 16mm for APSC, and 24 mm for full frame.

To consider the approach to eclipse you may want to have the sun and moon almost separate , and in this case you might want to think about the long length of the sensor as the image size, and then look at the subject 2 times the moons diameter


Last edited by Lowell Goudge; 03-06-2017 at 07:19 PM.
03-06-2017, 07:15 PM   #5
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QuoteOriginally posted by Lowell Goudge Quote
Jon need to adjust for crop see my comment on image size, I.e sensor size or any portion of it you want to fill.

I consider, for example that a total eclipse has the sun and moon almost equal in size (as observed by their relative distances)

Depending on camera, the diameter will appear on the shortest dimension of the sensor, either 16mm for APSC, and 24 mm for full frame.

To consider the approach to eclipse you may want to have the sun and moon almost separate , and in this case you might want to think about the long length of the sensor as the image size, and then look at the subject 2 times the moons diameter
Ok thanks this is really helpful. Seems like the telescope will be useful for what I need it for. I could run the Q7 for some other shots too.
03-06-2017, 07:40 PM   #6
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To prepare, just adapt your camera to the telescope with a T-mount adapter (I prefer the 2" models if you have a 2" drawtube or baffle (most Celestrons and Meades have an adapter that is perfect). This will give you a prime-focus image that should be of the highest quality. With your focal length, it will be just about right. See this tool to emulate the field of view with film or other cameras:

FOV Calculator - 12DString

You can chose the target (Sun or Moon) and the sensor size (film 35 mm). Obviously, take precautions about the sun prior to the eclipse with a mirror telescope. The elements can be destroyed and or melt if exposed to direct sunlight without a front filter (I recommend a baader solar film filter). Even with the sun partially eclipsed, there is a lot of intensity. I have melted the crosshairs right off of a finder when removing the lenscap for a second or two.
03-06-2017, 07:43 PM   #7
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Don't overdo the magnification though - the corona is at least 2-3x the diameter of the solar/lunar disk! You definitely want to leave a decent amount of space when practicing with the moon.
03-06-2017, 07:58 PM - 1 Like   #8
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Consider just watching the eclipse. You don't want to miss witnessing the event because you are too busy figuring out camera settings. Having said that, if you still want to move forwards with the fun and frustration of astrophotography...

Here's an online simulation for a camera + telescope on a variety of astronomical objects. You can manually enter a focal length, or select a telescope model from the dropdown list FOV Calculator - 12DString

1200mm is usually good for showing details on the surface of sun (with solar filter to avoid eye/camera damage) or moon in the frame. During eclipse totality (the 2 minutes or so when you don't need a solar filter), though, faint details of the solar corona extend far past the edges of the sun, and less magnification can work well.

My tentative plan is to have 2 cameras going. #1 will be on a fixed tripod, wide angle lens, unattended intervalometer snapping landscape and crowd shots.

#2 will be on a tracking mount following the sun so I don't have to keep reaiming, 500mm lens, shooting 11-stop brackets at totality so I can process later. I hope to achieve something like this APOD: 2015 March 31 - Corona from Svalbard

I am doing a lot of planning. I plan to setup well before the eclipse begins, and the photo taking will be mostly automated, letting me pay attention to the eclipse.

---------- Post added 03-06-17 at 09:59 PM ----------

QuoteOriginally posted by Pentax Syntax Quote
FOV Calculator - 12DString
I see I'm not the only one who knows about that website.

---------- Post added 03-06-17 at 10:04 PM ----------

QuoteOriginally posted by Lowell Goudge Quote
To consider the approach to eclipse you may want to have the sun and moon almost separate , and in this case you might want to think about the long length of the sensor as the image size, and then look at the subject 2 times the moons diameter
Maybe I'm misunderstanding. Do you mean the moon next to the sun at first contact? The sun's glare will hide the moon. A long exposure to try seeing the dark side of the moon facing us = sensor damage from the sun.

03-06-2017, 08:17 PM   #9
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QuoteOriginally posted by DeadJohn Quote
snip......



Maybe I'm misunderstanding. Do you mean the moon next to the sun at first contact? The sun's glare will hide the moon. A long exposure to try seeing the dark side of the moon facing us = sensor damage from the sun.
We are discussing here focal length. When it comes time, we also need to discuss the filtering. I used to polarizing filters back to back, to shoot the transit of Venus across the sun a few years back, with this it may be possible to see the two spheres, one bright one dark
03-07-2017, 04:53 PM   #10
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Here's are a couple sites for photographing the solar eclipse: How to Photograph a Solar Eclipse from Nikon,
Total Solar Eclipse 2017 - Total Eclipse Photography


It is recommended to use at least a 300mm lens. Keep in mind the movement of the eclipse will be harder to keep in your viewfinder the longer lens you use. If you are using a telescope you are going to want to have a clock-drive on it.
03-07-2017, 05:35 PM   #11
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You will not be able to see anything on the moon during the partial phases of the eclipse! The dynamic range necessary is way beyond that of any DSLR.

Check my album at Transit of Venus for some pictures of the sun.

For those pictures, I used a 10-stop ND filter along with both 1.4x and 2x teleconverters. The net effect is at least 13 stops of attenuation compared with my 300mm telephoto lens by itself.

These exposures used f11 and 1/2500 seconds, at ISO 100.

To compare to an exposure of the full moon, one could use the “sunny 16" rule (the full moon is basically in full sunshine, just like high noon here at home), although the surface of the moon is rather darker (a stop or two) than the earth. Thus, you might try ISO 100, f16, and 1/100 second exposure.

This is at least 16 stops down from the sun (13 for the TCs and filter, -1 for f11 versus f16, and 4+ for 1/2500 versus 1/100 sec). And, that is for full moon. During the eclipse, we are looking at the DARK side of the moon, and that is really dark, although a bit of earthshine lightens it up. I’d say allow another 4 or 5 stops at least.

So, the side of moon that we can see will be some 20 stops or more darker than the sun during the partial phase. That is a factor of a million!! (2 to the twentieth power)

No camera any of us owns has anywhere near that dynamic range. So, all you will see of the moon until, perhaps totality (check out that Astro pic of the day referred to above, for the earthshine result), is a silhouette!!!!!!!!!!!!!!!!!!!!!

As to the focal length you want for the eclipse, here is a full-frame (significantly reduced in quality - all I want to show is the size of the solar image) image from my Venus transit series referenced above. This is for my Pentax 300mm with both a 1.4x and 2x TC, or about a 900mm equivalent focal length lens, mounted an a K5D (i.e. APS-C) camera. If you want a nearly full-frame image of the sun, you need a long lens (or some TCs)! If this was on a K1, the image of the sun would be 1.5 times smaller in the full frame.

Only at totality do you want/need a wider field view, to get the corona. In that case, you will want a wide range of exposures, again as noted above, with reference to the Astro pic of the day.
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03-07-2017, 06:50 PM   #12
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My only question is, unlike the transit of Venus, the moon is a lot closer and there will be at some point an appreciable earth glow that will illuminate the dark side.

I would be hoping to see the potential of at least, in the extreme with one bright disk and one dim but not dark disk

The illumination should be better than a transit of Venus and as a result lower contrast
03-07-2017, 09:52 PM - 1 Like   #13
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QuoteOriginally posted by Lowell Goudge Quote
I would be hoping to see the potential of at least, in the extreme with one bright disk and one dim but not dark disk
No - that is the factor of a million I am talking about. Unless you have a camera with that dynamic range, or are willing to sacrifice your camera sensor by way over exposing the sun, you can not see ANYTHING on the moon until totality.
03-08-2017, 02:51 PM   #14
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QuoteOriginally posted by AstroDave Quote
No - that is the factor of a million I am talking about. Unless you have a camera with that dynamic range, or are willing to sacrifice your camera sensor by way over exposing the sun, you can not see ANYTHING on the moon until totality.
Are you talking of the brightness of the moon, or from your previous example the trainsit of Venus.

Note Venus is 25million miles from earth, or 100 times further than the moon. Assuming light falls off at 1/distance squared, the illumination from earth glow, on the surface of the moon, will be 10000 times that of Venus, and then an additional 10000 times brighter for the re-reflected portion back. In total the surface of the moon, would appear , to be 100 million times brighter than Venus, in your shot of Venus transiting the sun.

Why I questioned the point is because you started with the transit of Venus.

Last edited by Lowell Goudge; 03-08-2017 at 03:12 PM.
03-08-2017, 09:19 PM   #15
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QuoteOriginally posted by Lowell Goudge Quote
Are you talking of the brightness of the moon, or from your previous example the trainsit of Venus.

Note Venus is 25million miles from earth, or 100 times further than the moon. Assuming light falls off at 1/distance squared, the illumination from earth glow, on the surface of the moon, will be 10000 times that of Venus, and then an additional 10000 times brighter for the re-reflected portion back. In total the surface of the moon, would appear , to be 100 million times brighter than Venus, in your shot of Venus transiting the sun.

Why I questioned the point is because you started with the transit of Venus.
I am talking about the brightness of the moon, illuminated by earthshine. In my calculations, I am comparing the exposure for the sun compared to an exposure for the earthshine-lit side of the moon, and, I think, being very generous as to how bright the moon is under that condition. That is my factor of a million.

As soon as the next new moon is a few days old, I will go out and shoot an earthshine exposure, and I will be able to give a more accurate comparison of brightness.

Bottom line, though - you will not be able to get a simultaneous shot of the partially-eclipsed sun and moon with any details visible on the moon. During totality, with a long enough exposure, you might be able to get some earthshine details on the moon.

The brightness of Venus is irrelevant to this discussion. I used my Venus shots only as an example of a typical exposure for the un-eclipsed sun.
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