Another M101 and some thoughts on total integration time.

[tomato]

Much as I like imaging small, little known galaxy targets, I can't let the season go by without having a go at one of the blockbusters. Inspired by @ONIKKINEN's wonderfully deep and detailed M101, I have collected 102 x 2 mins Lum and 94x 2 mins RGB over the last couple of nights to add to 352 x 2 mins RGB from Feb 2022, all captured with the Esprit 150/IMX571 dual rig, captured at 0.74" per pixel.

So at 18.25 hrs integration I might have thought that was enough, but if you compare it to Oskari's 25 hr image, it's found wanting. For a start the lack of Ha data is a big minus, I don't have a Ha filter for the mono camera but I could try and get some hours with the OSC and the NBZ dual band. Also, the faint outer spiral arms are less distinct and small objects are lacking in detail. For example, the tiny barred spiral galaxy that I admired is a just a mangled blob in my image. Now some of this may be due to the different OTAs used to gather the data, and my heavy handed processing, or my sky conditions are inferior, but I'm hoping that 18 hrs is just not enough.

 

 

[ONIKKINEN]

Yours is quite nice too, those fainter spiral arms are a pain to pull out of the noise and here they are readily available.

I think we could calculate an "equivalent time" between our scope/sky combos to see how big a difference there should be in SNR. Purely in aperture my 200mm is around 50% larger in area (roughly considering the obstruction and reflection losses from mirrors) but our imaging scales are almost the same (mine is 0.76''). The skies mine was taken from were probably around SQM 20.7 on average for the 7 nights i spent, some nights better than 21 and a few closer to 20, and integration time needed to reach some chosen SNR increases by roughly 2.5x per magnitude of sky brightness increase. So if you want to crunch the numbers you'd only have to figure out your sky brightness using ASTAP's sky quality measurement tool and then plot in the numbers to see how many hours would you need to reach what i got in 25h.

[tomato]

So I have obtained an SQM figure of 19.83 from ASTAP, but that was with with a sub captured about 40 degrees way from an 70% illuminated moon, but all of my recent M101 data was similarly moon affected. I'm assuming all of your data was collected on moonless nights so assuming your SQM was 20 and if I was to image under the same moonlit conditions,  I would need 1.425 times the integration time so around 35 hrs, about double what I already have.

Now I know why I went for a dual rig.😉

[ONIKKINEN]

5 hours ago, tomato said:

So I have obtained an SQM figure of 19.83 from ASTAP, but that was with with a sub captured about 40 degrees way from an 70% illuminated moon, but all of my recent M101 data was similarly moon affected. I'm assuming all of your data was collected on moonless nights so assuming your SQM was 20 and if I was to image under the same moonlit conditions,  I would need 1.425 times the integration time so around 35 hrs, about double what I already have.

Now I know why I went for a dual rig.😉

Im getting a different number for the factor, here is how i calculated mine:

For our scopes we get a multiplier of 1.47 times already, your Esprit 150 has an aperture area of 17671 square mm, my 200mm aperture newtonian with an obstruction of 70mm and mirror coatings of 97% (OOUK Hilux coatings) is an effective aperture area of 25937.

I went ahead and checked 3 subs from all my nights, 1 from the beginning one from the middle and the end and got an average SQM of 20.73 for the UV/IR nights and 20.43 for all the nights. I think the UV/IR data is doing most of the work based on the weighting calculations from Siril's plot drawing fucntion so lets call it something like SQM 20.6 as the average.

So with an SQM difference of 0.77 multiplied by the 2.5x per magnitude factor we get a 1.925x increase in required integration time.

In the end we get my baseline of 25h * 1.47 * 1.925 = 70,74h. So you would have to image for 70,74 hours with your kit and sky to reach the SNR of my 25h image, in theory of course. Not sure how well practice would follow this theory but safe to say your conditions would require significantly more integration time than with mine.

If you got some pristine moonless data i think it would not require nearly as much time since a 70% Moon probably adds somewhere between 0.5-1 magnitude to the sky if not more. Doable, since you get twice the data with 2 scopes at the same time.

Edited April 20 by ONIKKINEN

[tomato]

Thanks for that, your figure of 20.43 makes quite a difference! I'll be on it again tonight, albeit with a 92% illuminated moon.😟I might try to capture some Ha instead.🙂

 

[TiffsAndAstro]

currently trying to use dss to stack two different evenings on m101. maybe 5 hours total

i notice my previous session subs score about 1100 (120 seconds) and the ones i did last night score around 600 (60 seconds).

are those numbers relative to each other or for each session?

Edited April 20 by TiffsAndAstro

[tomato]

If it is Astro Pixel Processor’s sub analysis quality score, I think the numbers are relative to the data being analysed in that session. Recently the numbers have been in the 300-400 for 2 minute subs under a moonlit sky.

[tomato]

So this one has two hours of Ha, using NBZ dual band filters with the mono and OSC cameras. Total integration is now 20.3 hrs. I'll stop posting these now until I have 30+ hrs, assuming that's possible this side of losing astro darkness.