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BetterSense
11-Aug-2017, 13:09
I'm building an eclipse camera from a long cardboard tube. The longest lens I can find on Surplus shed is 1400mm. I found online that the size of the sun can be calculated as f/110, which should give me a 12mm image of the sun. I would like twice this or more. Can I just put a double concave lens in combination to extend the focal length? I used to use plain DCX lenses to make macro fiters for my 35mm camera and increase the effective but not actual focal length, but in this case I don't mind if the setup is actually telephotographic or not, I just want more magnification.

Pfsor
11-Aug-2017, 15:07
Now go back to the Earth and realise that the Sun is moving while you take pictures. If you want to use 2800mm lens you will see that the Sun is running too
quickly. I was taking pictures of sunspots with 800 mm lens and magnified the slide for 80 mm Sun disc on the print, with very acceptable resolution and quality.

LabRat
11-Aug-2017, 15:17
To keep it simple, think pinhole...

Adding more FL/magnification has it's limits, as the best you will be able to do is to capture the shape outlines of the objects, and due to daytime air heat turbulence the more magnification, the more of the effect that the eclipse is immersed in boiling water (atmosphere)...

Get some proper mylar solar film for the front of your optical assembly (ONLY the front for the filter), but also realize that even with the solar film/filter there is a great deal of flare in your optics when viewing the sun (even with a well baffled OTA) so keep the camera on-axis and aimed well so the flare will be even...

But stay safe out there, I recently have met a amateur astronomer who has retina burns from a viewing "accident"...

The "Day of the Triffids" is coming again, so don't forget the Roundup and napalm, as well as your solar glasses and beer!!!!!!

Steve K

BetterSense
11-Aug-2017, 16:15
I may or may not attempt to record the event on film. I am at least trying to view it on a gg screen, but might jam in a film holder for a quick try at a photo. The sun may be moving, but at a shutter speed of 1/250 or so I would think motion blur will be minimal.

If I use a pinhole the image with be very dim. That's why I think a simple lens will be better. The 1400mm lens will give me f/45 or so.

Pfsor
11-Aug-2017, 16:23
The sun may be moving, but at a shutter speed of 1/250 or so I would think motion blur will be minimal.


Do you realize the vibration blur with a lens of 2800 mm FL? You touch the standard, you press the shutter and the picture on your gg starts to dance! Forget the 1/250s.

BetterSense
11-Aug-2017, 16:45
Well I was thinking of bolting the whole thing to my deck, and using a detached shutter. But point taken. Does anyone know a good exposure setting to use for taking eclipse pictures? Is 1/250 going to be overexposed even at f/64?

LabRat
11-Aug-2017, 17:01
Depends on the filter you end up using (and you NEED a filter)... A welding filter works for photographic use, and sticking a digital or 35mm TTL metered camera on it to measure it... (note/ DO NOT view directly with any other filter but approved visual filters as there are many IR wavelengths that might not be filtered out and harm your eyes, and ONLY mount filters on the front of the rig, so if the sun focuses on the filter inside the instrument, it does not burn or shatter exposing you to the magnified solar energy!!!!

Test beforehand with something TTL

Steve K

Jim Jones
11-Aug-2017, 20:20
I would rather enlarge an eclipse image taken directly with a long simple lens rather than degrade it with an improvised extender. Making a mediocre image larger with mediocre technique gains nothing. As for pinhole eclipse photography, I attempted it with 32 feet of 8 inch drain pipe braced by external struts and guy wires. It was too cumbersome, but worked better when shortened to 24 feet. Exposure was maybe 1/100 second with the camera braced both at the top and bottom. With a .070 pinhole (as I recall from decades ago) images were awfully unsharp. However, printed at the same size, it would have been sharper than pinhole eclipse photos from significantly shorter focal lengths. The pinhole blur did conceal any subject movement.

Bettersense, small lens apertures and short exposure times will still yield grossly overexposed solar photographs. You need a proper solar filter. There is much information online about solar and eclipse photography. Start studying it now.

Ted R
12-Aug-2017, 07:30
Yes, the problems with super long lenses are tracking (the sun will be quickly out of the frame) and vibration.

Here's some exposure data from a KODAK publication, in the text that accompanies the data they recommend bracketing +/-3stops.


168242

Randy Moe
12-Aug-2017, 08:47
Read this. Very good Canon advice.

http://learn.usa.canon.com/resources/articles/2017/eclipse.shtml

I will be near the area of longest totality. I plan to observe without trying to photograph the Sun. I expect many very professional people will make great images. Heck, the Air Force has 3 jets tracking and photographing. I am getting requests for dozens of viewing glasses. I only bought 10.

Additionally, even if overcast, I think it will still be fun and perhaps even darker.

Ignore the bright flashes in the distance...

Chauncey Walden
13-Aug-2017, 17:30
Here's what our local astrophysicist/photographer is doing: http://www.clarkvision.com/articles/photograph-the-sun/ More than half way down the page you will find tables that show how he is programming his cameras.