Skyfog challenge!

[Ags]

Get out your DSLR and take part in the Great Skyfog Challenge! Set your camera to F4 and ISO 800 and take an exposure of the zenith from your observing/imaging site. How long can you make your exposure before the histogram goes past the middle of the graph? It took my camera 45 seconds, meaning my limiting magnitude is 3.5 or Mag 17.5 (per square arcsecond) skies.

For more info: Measuring Skyfog from Camera JPEG photo - Samir Kharusi photos at pbase.com

If you are impatient you can use a higher ISO - e.g. shoot at ISO 1600 but multiply the time by two to get the ISO 800 equivalent. You can also shoot at F2 and times by four to get the F4 result.

[garethmob]

might give this ago tonight how do you see the histrogram? in the camera?

[Ags]

Yes apparently you must use the camera histogram. If you click the View button on a Canon DSLR it cycles through view modes for the image - you want the view mode showing the composite histogram (we are not interested in the separate RGB histograms). What camera do you have? EDIT: Duh! You have a 300D. I can't advise on that one.

[garethmob]

i think iv seen a hitrogram before in the image setting, ok i dont have live view but i have a timer to give it ago:)

[Ags]

You probably need to set the camera to bulb mode and then hold the shutter down for 45 seconds or so.

[garethmob]

thas what i always do, but i got a external timer that plugs into my cameras shutter (intervalve port?) its got a button for manual which i can press and it dosent introduce shakes to the mount one of the best things iv bought

[Ags]

Yes I love my programmable remote too!

[Ags]

I'm hoping someone will post a time... This may be the least answered poll in SGL history at this rate!

[gooseholla]

Perhaps because it seems complex and people have better things to do?!

[garethmob]

not nessasaryly complecated, maybe a little time consuming but a fun thing to do, i wonder if it could be used to get a sort of bortal scale thingy magig

[RikM]

This was fantastic. Thanks so much. I used my Canon 1000D f/4 ISO800 with a nifty fifty pointed straight up. I voted 120 sec. It was actually 150 sec so Mag/sq arc-sec = 19.2 and VLMag = 5.0, which squares nicely with my own estimates using Ursa Minor (which sits a little lower towards Gloucester's LP and I typically estimate at VLMag 4.2 - 4.5. It was quite a clear night though so I approximated down rather than up.

[Ags]

Perhaps because it seems complex and people have better things to do?!

You may have a point - I misread the logarithmic scale on Saamir's page :-) Silly Ags, 45 seconds is not 0.45 minutes... It's more like 0.7 minutes, so that means I have mag 4.0 skies. To be honest, I believe the old incorrect figure more.

I repeated my measurement at midnight, and found that the histogram only hit the midpoint after 70 seconds - quite a difference. I guess that means that any imaging I do should be after midnight.

[RikM]

I am going to compare before midnight (like last night about 2200) to after midnight because the dim our street lights after then and are moving to part night lighting soon

[Ags]

Post your findings - interesting to see what effect streetlight dimming actually has.

[Cath]

Thought I'd have a go

Canon EOS 500D, F4, ISO800, 18-55mm lens at 18mm.

at 8:45pm it was around 60 seconds.

at 9:05pm it was around 70 seconds.

So it's still getting darker, I'll try again a bit later if I can for you.

It's nice and warm outside this evening, sky is very clear, perfect night to get my telescope out, but can't tonight

[schulzep]

Hi, I am a new member. I do AP from my home observatory 60 miles SW of FT Worth.  I currently take photos with a modified Canon 600D connected to a Televue NP-127is.

I checked my DSLR reading against my Apple iPhone App DSMMap and get good agreement. With the moon at 38% but having set in the West, I get SQM = 21.63 / Mag 6.9.  I am 60 miles from Ft Worth and I can just see the light dome low in the sky but it rises up to about 15 degrees.  I used a modified Canon 450D with a TC-60N3 Canon remote timer (modified to be used by the timer).

[DaveS]

Good God, this thread is over 2 years old! Zombie thread alert!!

Actually I wonder how I missed it the first time round, must have been while I was off-forum for a while. I'll definitely give it a go when there's no moon around. Be interesting to see what my actual LVM is and how it varies with dust and clag in the air

I don't believe Stellarium's Bortle values as they don't agree with where I am and what I see. Where I am come up as Bortle 9, what I see is Bortle 6-7 according to Stellarium's sky simulation. I don't believe either.

[Stargazer33]

Does this test not depend on what lens you are using? Wouldn't someone using an 18mm lens will get more light hitting the sensor quicker than someone using a 70mm lens. Also a 50mm EF lens isn't equivalent to a 50mm EFS lens due to the 1.6x field of view crop factor of the EF lens on a DSLR body.

[DaveS]

When I do the test I'll use my 90mm f/2.0 Summicron on my Canon 5D. Should give a reasonably tight field. I'm not sure though if i should defocus to get an overall value.

[Scooot]

I just took a few shots at different speeds and couldn't understand why the histogram wasn't changing. Then realised I had the lens cap on!

It eventually came out at 90 seconds which if my maths is right is a visual mag of about 4.6 which I'd say is probably a bit better than I'd guess. Although I did hand hold the camera so don't know if this would make much difference. Moon was starting to rise as well.

[Ags]

Does this test not depend on what lens you are using? Wouldn't someone using an 18mm lens will get more light hitting the sensor quicker than someone using a 70mm lens. Also a 50mm EF lens isn't equivalent to a 50mm EFS lens due to the 1.6x field of view crop factor of the EF lens on a DSLR body.

Its true you want to use Telephoto not a wide-angle lens for this. The longer lens will grab the zenith while the wide-angle will catch more sky from lower down the sky, polluting the results.

[jonathan]

Would like to give this a try, so what mm should my lens be?  I have the kit telephoto lens (18mm - 55mm or something like that) and a longer zoom to choose from.

[Thalestris24]

I think I'm depressed after reading Samir's page.... But I'm not giving up imaging! Not yet, anyway.

Louise

[Ags]

Would like to give this a try, so what mm should my lens be?  I have the kit telephoto lens (18mm - 55mm or something like that) and a longer zoom to choose from.

What zoom do you have?

[jonathan]

What zoom do you have?

Canon EF-S 55-250 1:4-5.6 IS