Exposure question

Started 2 months ago | Discussions
Tom59 Senior Member • Posts: 1,327
Exposure question

Been having a hard time with processing my deeper astro images. Want to make sure my initial data is ok to start with.

Took 30 lights last night of Pleiades & Mars. (Should of uses the 135 mm)

Made some darks, bias and flats to be stacked in DSS.

Shooting from bortle skies so LP is an issue.

A screen shot of 1 image. Does the exposure look ok or over/ under?

2 minutes at 3.2 io 250.

Thanks.

Alen K
Alen K Senior Member • Posts: 1,278
Re: Exposure question

I think your background level is fine. Even at ISO 250, the noise due to the high background level from light pollution should swamp the read noise. I think also that a 200mm lens does give a good field of view for this event; no need to go wider. However, you could have rotated the framing for a better fit so that Mars to Pleiades runs lengthwise.

Saying "Bortle skies" doesn't actually specify what level of light pollution you have. The Bortle scale is a very coarse numbering system for light pollution, hence all skies everywhere are "Bortle skies." I'm going to assume you have a Bortle 5 sky but that's just a wild guess. The higher the sky brightness, the more total exposure you need to pull out the faint stuff. With your skies, your hour at f/3.2 will I think give you some decent nebulosity. But to get the really faint stuff surrounding the Pleiades, you would need much more, even in darker skies.

OP Tom59 Senior Member • Posts: 1,327
Re: Exposure question

Alen K wrote:

I think your background level is fine. Even at ISO 250, the noise due to the high background level from light pollution should swamp the read noise. I think also that a 200mm lens does give a good field of view for this event; no need to go wider. However, you could have rotated the framing for a better fit so that Mars to Pleiades runs lengthwise.

Saying "Bortle skies" doesn't actually specify what level of light pollution you have. The Bortle scale is a very coarse numbering system for light pollution, hence all skies everywhere are "Bortle skies." I'm going to assume you have a Bortle 5 sky but that's just a wild guess. The higher the sky brightness, the more total exposure you need to pull out the faint stuff. With your skies, your hour at f/3.2 will I think give you some decent nebulosity. But to get the really faint stuff surrounding the Pleiades, you would need much more, even in darker skies.

Thanks Alen.

Am in a bortle 6 zone, forgot to input that somehow.

I could not rotate the camera for a different orientation  the way it was setup on the tracker. If I had used the tripod collar I could have.

Will stack and process (best I can, that's my weak spot right now!!) this week and see how it comes out. I would be happy to get something similar to what Primeshooter posted recently. With  36 minutes, more likely from darker skies though.

Thanks again.

Alen K
Alen K Senior Member • Posts: 1,278
Re: Exposure question

Tom59 wrote:

Alen K wrote:

I think your background level is fine. Even at ISO 250, the noise due to the high background level from light pollution should swamp the read noise. I think also that a 200mm lens does give a good field of view for this event; no need to go wider. However, you could have rotated the framing for a better fit so that Mars to Pleiades runs lengthwise.

Saying "Bortle skies" doesn't actually specify what level of light pollution you have. The Bortle scale is a very coarse numbering system for light pollution, hence all skies everywhere are "Bortle skies." I'm going to assume you have a Bortle 5 sky but that's just a wild guess. The higher the sky brightness, the more total exposure you need to pull out the faint stuff. With your skies, your hour at f/3.2 will I think give you some decent nebulosity. But to get the really faint stuff surrounding the Pleiades, you would need much more, even in darker skies.

Thanks Alen.

Am in a bortle 6 zone, forgot to input that somehow.

I could not rotate the camera for a different orientation the way it was setup on the tracker. If I had used the tripod collar I could have.

Will stack and process (best I can, that's my weak spot right now!!) this week and see how it comes out. I would be happy to get something similar to what Primeshooter posted recently. With 36 minutes, more likely from darker skies though.

I am also in Bortle 6 (edging toward 7). I have spent the last few nights imaging this conjunction as well. The event was really just an excuse to get out and image, even if "out" was only to the backyard. In the winter, snow means my usual imaging sites in Bortle 4 skies less than an hour away out in the country, are practically inaccessible.

But boy is it frustrating to think how much time I am wasting taking exposures in the city. In my case, there is a minimum exposure ratio of eight times. In other words, I need to expose at least eight times longer at home to achieve the same SNR as I would at those sites in the country. It's "at least" because that particular ratio is based on SQM numbers at the zenith. The affect of light domes, which are worse in the city, increase the ratio depending on the location of the target.

For me right now, that means the 6.5 hours of data on the Pleiades I have collected is no better and probably worse than under an hour in the country. Astrophotographers nowadays do sometimes spend many hours on one target, sure, but usually for a bigger payoff.

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