Night sky movement..

[peter k.]

Somewhere I read that taking a shot of a dark night sky, that ones exposure should not be more than 30 sec, or you will see movement in the stars.
Is this correct?

Thinking of trying some 4x5 HP5+ @ 1600 ASA in the upcoming Oct 8~9 Draconids meteor shower.

 

[avalladares7439]

I know that this is true for 35mm the amount of time it takes for movement depends on the length of the lens. I'm not sure this applies to large format but I think it should

 

[Richard S. (rich815)]

I think it us moving not the stars.

 

[peter k.]

+1

 

[Xmas]

If you are after trails then they are straight and distinguishable from star rotation (around Polaris).

If the background is street light contaminated that is a limit.

If it is clean you may get our galaxy background (the Milky Way) as a limit.

 

[peter k.]

Yes, I wanted the trails, but did not want the star rotation in it.
The direction I am shooting has very little background light contamination, so that's why I wondered if the 30 sec was accurate.
Can't seem to find anything about this.. as my search question must be phrased wrong.

 

[Nodda Duma]

You'll get straighter star trails to the east or west, rotation to the north or south. Not sure if that's what you're asking.

For little or no movement at normal focal length 30sec is probably right. Assuming little reciprocity failure in your film. For longer focal lengths the time will need to be shorter.

 

[Gerald C Koch]

I think it us moving not the stars.

E pur si muove. Galileo

 

[peter k.]

Nah.. don't want straight star trails.. don't want any trails but the meteor trail...

 

[RobC]

the earth rotates ((24x60)/360) = 3 degrees a minute.

However, the latitude you are at and where you point your camera makes a difference. If you know the field of view you lens focal length gives across your film format and direction the stars will move across it, then you can do some rough calculations based on the resolution you require.

Lets assume the stars would move horizontally across your 4x5 film in landscape orientation. A worse case scenario with you standing on equator looking straight up. Your field of view would be appox 40 degrees. So it would take 40/3 = 13.333 minutes for a star to pass across full width of film.

Now suppose you want 60 lppm resolution on film. 5x25.4= 127mm film width.
13.333 minutes = 800 seconds

(800/127)/120 = 0.0524

So I rekon with my rough calculation off the top of my head that you will want a shutter time of no longer than 1/20 of a second to get really good sharp stars with around 60 lpmm on film. Anything longer and they will start to trail.

But at your latitude and depending where you are pointing the camera you may get away with longer shutter time.

Someone please check my figures becasue I doubt they are perfect but I think they cover the worse case scenario.

If you want the stars to remain stationary with just the meteor trail then you will need a motorised camera mount set correctly for your latitude and camera direction if you use a longer time than I have suggested.

 

[Buje]

the earth rotates ((24x60)/360) = 3 degrees a minute.

Actually, the earth rotates one degree in 4 minutes. Your equation is upside down.

 

[BrianShaw]

No wonder I'm dizzy all of the time.

 

[RobC]

Actually, the earth rotates one degree in 4 minutes. Your equation is upside down.

Oooops. Good job someone has their finger on the pulse.

corrected calculation follows:

the earth rotates (360/(24x60)) = 0.25 degrees a minute or 1 degree every 4 minutes.

However, the latitude you are at and where you point your camera makes a difference. If you know the field of view you lens focal length gives across your film format and direction the stars will move across it, then you can do some rough calculations based on the resolution you require.

Lets assume the stars would move horizontally across your 4x5 film in landscape orientation. A worse case scenario with you standing on equator looking straight up. Your field of view would be appox 40 degrees. So it would take 40*4 = 160 minutes for a star to pass across full width of film.

Now suppose you want 60 lppm resolution on film. 5x25.4= 127mm film width.
160 minutes = 9600 seconds

(9600/127)/120 = 0.63

So I rekon with my rough calculation off the top of my head that you will want a shutter time of no longer than 0.63 of a second to get really good sharp stars with around 60 lpmm on film. Anything longer and they will start to trail. So that would be 1/2 second shutter speed but 1 second would probably be OK for your purposes.

But at your latitude and depending where you are pointing the camera you may get away with longer shutter time.

Someone please check my figures becasue I doubt they are perfect but I think they cover the worse case scenario.

If you want the stars to remain stationary with just the meteor trail then you will need a motorised camera mount set correctly for your latitude and camera direction if you use a longer time than I have suggested

 

[peter k.]

the earth rotates ((24x60)/360) = 3 degrees a minute.
Now suppose you want 60 lppm resolution on film. 5x25.4= 127mm film width.
13.333 minutes = 800 seconds
.

Hmmm ok getting educated... Please correct if wrong.
60lppm = 60 line pair per millimeter, using 4x5 film, camera, this would equal a 127mm lens being used?

 

[RobC]

Hmmm ok getting educated... Please correct if wrong.
60lppm = 60 line pair per millimeter, using 4x5 film, camera, this would equal a 127mm lens being used?

Note: I made an error which has been corrected in later post.

127 is the width of film (5 inches) in millimeters. I am assuming a 150mm lens is approx 40 degree field of view on film.

Its all pie in the sky if you want stars static with no trails and need a long exposure time to catch the trail of a meteor over time. You will need a motorised camera mount for that.

But if you want stars static and the meteors own tail static on not over time then I think my figures are approximately correct but others who know better may correct me.

 

[peter k.]

Ok.. rest of the story
Latitude is 34.8600* North
At this time plan to shoot horizontal.. between 7~8pm Mountain standard time. Azimuth Direction 316* w/Altitude appox 50.5*

Don't know which lens I'm going to use.. so don't know the angle of the dangle, will find out tonight on a test shot.
But for education... lets say I use both lenses of a double Protar .. (Two 35cm/12.5 Protar Ser. Vll cells) that together make a 205/6.3 lens. I have no understanding of what the field of view for this lens is. How many degrees. Let me see if I can find it in the web for this lens.

Meteor Shower Reference:
http://www.timeanddate.com/astronomy/meteor-shower/draconid.html

 

[RobC]

Lf lenses don't usually tell you becasue they are more about angle of film coverage. But I think you'll get about 32 degrees on film with a 205 lens.

Again I'm doing quick and dirty calcs so do check what I'm telling you makes sense.

 

[peter k.]

Yeah..couldn't find it if its there... also some of the info was in German...

 

[RobC]

Whether half or 1 second is long enough to capture stars and/or the meteor is debateable.
I think most people would just use a long exposure and capture the star trails too if they don't have a motorised camera mount.

I guess you should try vaious exposures. The short time for static stars and long time (30 seconds) for short star trails and the meteor trail and tail if it has one which is bright enough.

 

[Jim Noel]

Yes, I wanted the trails, but did not want the star rotation in it.
The direction I am shooting has very little background light contamination, so that's why I wondered if the 30 sec was accurate.
Can't seem to find anything about this.. as my search question must be phrased wrong.

Not possible! if you are going to get meteor streaks, you are going to have star trails, plus any man made satellites which happen to pass by.

 

[RobC]

I would use a try using a 400 film if you want everything static. Delta 400 and use a speed increasing developer such as DDX or even Microphen stock which will pick out the faintest objects.
Don't push the film though as that will lose faint objects, especially if you are using only 1 second exposure.
Pushing mostly loses shadow detail and lifts highlights so if stars are very faint(shadows detail) you'll lose them and only get the brightest objects.
I would also try a slower film too (Delta 100) and use DDX stock. Again don't push it.

 

[peter k.]

Not possible! if you are going to get meteor streaks, you are going to have star trails, plus any man made satellites which happen to pass by.

Yep... plus planes flying by.
Beginning to grock the issue, need to use a motorized camera mount .. if I want the stars still and meteor streaks.
So will see tonight if I can get an image with just still stars.. and then at shower time will do a time exposure and get both.
The advantage is, if the meteorite streaks are coming down, the stars will be going around more horizontal, depending on the how much time the shot is exposed. Ss the shot will be towards the northwest.

Thanks all for the clarity...

 

[RobC]

If you think about it, a 30 second exposure will move a star how far on the film? Very little, they will be tiny little dashes not great long trails. So meteorites will easily stand out from the stars. You'll only get long star trails with half an hour or more exposures.

 

[peter k.]

If you think about it, a 30 second exposure will move a star how far on the film? Very little, they will be tiny little dashes not great long trails. So meteorites will easily stand out from the stars. You'll only get long star trails with half an hour or more exposures.

Just been doing the math to learn, and I'm dizzy... go shoot de ole man says... see what ya get...
Couple a weeks ago I did a 20 minute, exposure with Arista 400.. and developed it... it was so faint.. after fixing.. I didn't even finish the rinse... later aftr it dried I scanned it, there where star trails.. amazing...

I only have D-76 and Hc110 for developer.. and and only HP5 and Arista. So it makes sense about what you say about pushing, but what is lost.. one sheet of film.. so have never ever pushed... so where going to try that to... what fun... next... (Its great being retarded.. mean retired )

 

[JBrunner]

I have a little gadget from a company called Ioptron that functions as a compact equatorial drive for wide field astrophotography. You mount it between your camera and tripod. It requires being aligned to Polaris (or Crux if that is the case). Not really all that hard. After that it tracks to your hearts content. Expose until Mr Reciprocity kicks you in the tenders, or, if you really want to get weird, read up on film hypering.

With out either of those, I use the 500 formula in which you divide focal length into 500 to arrive at the maximum exposure in seconds. So with a 24mm lens, 500/24=20-ish, so about twenty-ish seconds would be the maximum before stars begin to be noticeably blurred. Of course what is noticeable blurring is subjective and also depends on where and how big you are displaying, etc., so some folks use 400 or 600 depending on personal preferences. The result of the formula does nothing to guide exposure beyond simply giving you threshold on blur (or rather your tolerance of blur, there probably will always be some with any useful film based non tracking astro exposure). The actually exposure will be determined by how big the other two numbers are, or as usually is the case, how big they can be.