Break in star trails

Grandpa Ron

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I did not know where else to post this. I think it may be a film thing.

For some reason there is a break in the star trails, at the start of the exposure. The gap is a given length, then star trials resumed a bit thinner.

This is,
Mummy ISO 200.
4x5 format.
Tripod mounted and aimed near the north star.
"T" setting on a 162mm Wollensak lens.
f 8 Aperture.
3 hour exposure. (I opened the shutter and went to bed for 3 hours.)
The gap appears on the negative also.
If the tripod has shifted, the start trail would have shifted.

Does anyone know a reason for this gap in the star trail?

2nd Star Trials.JPG
 
Someone or some animal stood in front of the lens? Or other such is all I can think of that would interrupt the exposure.

On second look they seems to be an exposure difference in the bit at the start and the rest of the trails. Mechanical glitch of some sort, aperture closing down erratically?
 
Stars that were already visible in the sky at the start of the exposure and those that were below the horizon or obstructed by objects on the horizon at the start. As for the Iridium flare, that too.

So... there's an identical star somewhere below the horizon for every start that's above it... and they're ALL in the exact same relative position... and they all change at the same time?
 
I'm not trying to 'win'. I'm trying to understand your 'high/low' star explanation.
 
I think the issue is the break near the 'ends' of the trails. It's like the lens was covered for a few minutes. I just can't wrap my aging noodle around how the horizon and things being out of frame enter into it.
 
To me, the issue is the missing portion of the star trails as marked with the red lines.
 

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    Star trail breaks.jpg
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Seems odd that we don't have an Astrological Photography section here.
 
Sorry for the delay, but for the sake of convivence, I took several shots with my 35mm SLR. though the resolution is not as good, the is no break in the star trail.

So as previously suggest, the most likely cause was cloud cover lasting several minutes.

I am hoping that with winter upon us and the holidays behind us, I can get a few fogless, dewless, moonless, windless nights and try some more 4x5 shots.
 
Why do the small segments (after the gap) of the trails appear wider/brighter? Did the focus shift?
 
Why do the small segments (after the gap) of the trails appear wider/brighter? Did the focus shift?
Tinkerer,

The camera was sitting on a tripod in the front yard unattended for three hours. It is a manually operated Pentax. Clouds can account for missing segments of the trail.

As to increases in brightness, I have no idea.

I theory, photons of light strike the film and react with the sliver halides. In reality there are many variables. Reciprocity, spectrum sensitivity, temperature, and a host of other quirks all conspire to make film photography interesting.
 

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