Full Moon Bortle 4 vs New Moon Bortle 7/8 - Forums

Topic starter

Oct 21, 2023 · Edited: Oct 22, 2023

Hi folks:

I might have the opportunity to image during a full moon in Bortle 4 conditions. I don't really get these sorts of opportunities often, so it's beggars/choosers. Logistically this will be a difficult opportunity to take advantage of (transportation, scheduling), so I'm weighing whether or not it's worth it to try and make it happen if I can get better or comparable during a new moon in my Bortle 7/8 backyard.

If it is worth a shot, what sort of objects should I go after? I have a wide field setup (Zenithstar 61 - 360mm FL) with L-Pro & L-eXtreme filters and DSLR. I'm guessing that dark nebulae are out, would Iris be doable? Should I stick to emission nebulae with the L-eXtreme filter?

Thoughts? Thanks in advance!

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Guillermo (Guy) Yanez

Oct 22, 2023

In my opinion, you will get very limited benefits from Bortle 4 skies under full moon. In case you still want to give it a try, you may want to select a target more than 50 degrees away from the moon. That said, you will very likely still get light bandings in the background due to moon light but you may easily fix it with Siril or Pixinsight. Moonlight, however, does add some amount of noise to your images.
If it is such a hassle just to get everything packed up for traveling, maybe you should just pick a bright nebula under your urban skies and take Ha or SII subs.
In any case, just enjoy

Cheers, Guy

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Reg Pratt

Oct 22, 2023

A full moon in B4 can be just as bright or even brighter than new moon in b7/8 depending on the moons altitude . For example I live in B7 which measures mag 18.8 at the zenith during new moon. My remote site is mag 21.7 - 21.9 on average during new moon but drops down into the 16s during a high full moon.

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Michael Fleischer

Oct 22, 2023

Hello.

I live and photograph under Bortle 4 skies, but taking photos during a full moon for me is not very encouraging.
The sky is way too bright for good results.

All the best to you and clear skies.

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Björn Arnold

Oct 22, 2023

I agree with the above. I am also living in a Bortle 4 area and from time to time I try imaging broad band under full moon. It never gave me satisfactory results. The only thing that works for me is narrowband.

CS, Björn

Mark Germani

Topic starter

Oct 24, 2023

Thanks for weighing-in, everybody! Looks like I'll be waiting for better conditions. CS!

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Oskari Nikkinen

Oct 24, 2023

You can use the SQM measurement tool in ASTAP to determine the actual real time bortle rating of the sky you are imaging under to get a definitive answer on this.

Having done that i will go against the grain here and say that any amount of Moon, even full Moon high above, in bortle 4 is still better than a hard bortle 8. For a milder bortle 7, it gets closer but the bortle 4 + Moon location should still win. All light is additive so there is no such thing as "full moon being worse in bortle 4 than bortle 8", its just that bortle 8 is so un-usably bad as it is the issue of a full Moon has less of an effect.

I think you should definitely go for the bortle 4 location, i think youll be surprised how much better that can be even with a Moon in the sky. Choose a target that will be far away from the Moon while there, and you will minimize the effects. Even better would be to go for the L-extreme since Ha is very resistant to the mostly blue Moonlight (not so much the OIII band, which sucks under the Moon, but at least you'll get good quality Ha)

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Joe Linington

Oct 24, 2023

I live under B5 skies and the full moon is much brighter than the regular light pollution. I often image Ha under a full moon and Sii if it's anything less than full. Oiii and broadband is for less than 25% moon. Don't expect to hunt dusty broad band targets but something bright and in the north should be perfect. Maybe the Heart and/or Soul nebula would work out OK. It would be worth going just to answer the question.

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Reg Pratt

Oct 25, 2023 · Edited: Oct 25, 2023

Just to drive my earlier point home... Bortle 7 spans magnitude 18.5 - 18.0 while Bortle 8 <18 which is considered a white zone. With that in mind here are some SQM charts from my remote site.

Here you can see a 76% moon with a max altitude of about 35°. At max altitude SQM is around mag 19.9, so Bortle 5, not bad. Still 1 magnitude darker than B7/8 at new moon.

Here we have a 70% moon but with a max altitude of around 80°. SQM drops far below white zone levels before the moon even reaches max altitude.

60% moon again dropping SQM far below white zone levels before the moon reaches max altitude.

Now lets remember that a full moon is in the sky the entire night. so there is no gradual increase or decrease in sky brightness. A high altitude full moon WILL render a Bortle 4 sky a completely white zone, which is significantly brighter than a Bortle 7/8 sky during new moon. Given this, IMO it isn't worth a trip into the field but everyone values their time differently.

I don't have a chart for low altitude full moon but will grab one and post it in a few days when we are back at 100% illumination.

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andrea tasselli

Frequent contributor

Oct 25, 2023

I beg to differ and with good reasons. I live in 6+/7- Bortle location heavily affected by low transparency most of the year, just to me it clear. Now, exception made for the high Winter Moon (either side of the Winter Solstice, when is highest in the sky in the N.E.) then there is no good reason NOT to image with the usual precaution of using a LP suppression filter and imaging targets at least 2 hours away (3 better) or equivalent in Dec from Full/Near Full Moon. Some of my images BROADBAND were taken in the aforementioned conditions and whilst they are far from ideal (but then my skies are rarely so) they are nothing to sneeze at.

As for the effect of the Full Moon in Bortle 4 (or better) skies I had the chance to observe it this summer and I could still pick up visually (at least) magnitude 4 stars 2/3 hours away, something I can only dream of doing under my skies. Let's not forget that the Moon illumination follows an inverse quadratic distance relationship while high LP (Bortle 8) is just basically bright sky anywhere except around the zenith.

Blaine Gibby

Oct 25, 2023

My backyard is Bortle 4. I almost never shoot under a full moon. You might get some useable Ha data though depending on the target.

Dave Erickson

Oct 25, 2023 · Edited: Oct 25, 2023

Seems worth the risk to me. Here is the imapact of the moon from 3rd qtr to full from Bortle 1-2 skys . Verticle axis is SQM in magnitude per arc sec. Horizontal axis is time in minutes, about 7 hours of data. each graph line is conditions for a different night. From the begining of 3rd quarter to Full moon. The wavey lines at and near full moon are due to atmospherics, clouds, etc. This is data from a perminatly mounted SQM pointed at polaris. Data collection is automated.

Magnitude 18 to 18.5/arc sec, as good or slightly better than your home site. Clearly local atmospherics are a major factor. I'd plan sessions well away from light domes, and the full moon... Lots of objects available using narrow band filters...

Best,
Dave

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Joe Linington

Oct 25, 2023

This is all some fantastic data. Thanks guys for the empirical measurements. This will really help me plan, full moon in winter looks pretty bad for me at B5, even with NB filters but a summer full moon has less effect than I thought.

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Reg Pratt

Oct 29, 2023

As promised. Yes the sky brightness falls off some the further away you get from the moon but enough to make a trip into the field worth it? No, not in my opinion.

Mark Germani

Topic starter

Oct 29, 2023 · Edited: Oct 29, 2023

I love the wealth of information presented here - thank you all (great charts @Reg and @hbastro!)

I did not end up going. I had no transportation and was busy with family commitments, and it would have been a challenge to get out even in favourable conditions. I've been taking the rig outside for the past few nights and the humidity has been AWFUL - I've had to take 120s exposures down to 30s. But it looks like anything below 75% illumination might be worth it, so the next time the team goes up I'll try and make it work.

Thanks again, all, and CS (and no moon!)

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Brian Boyle

Oct 29, 2023 · Edited: Oct 29, 2023

I am lucky enough to live in a Bortle 2 area. I frequently observe in full moon, but I am just as frequently disappointed with the result.

It is not only only the increase in sky brightness, but also the poorer guiding [less SMR on the guide stars] and the scattered light. Even with a baffle at the top end of the tube, I still find variable illumination across the field in different subs that is impossible [at least for me] to flat-field out.

Occasionally it works, but really not enough to justify the effort. As I speak it is full moon and perfectly clear. I am not observing, and I don't feel guilty. [Although I did take some sky flats at sunset!].

I realise that I have the luxury of living under dark skies [when the moon is down], but I think even under brighter skies the systematic effects of lunar pollution might also have an effect equal to, or larger, than the statistical decrease in SNR due to sky brightness.

CS Brian

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