DSO Astrophotography with CMOS Cameras (Nikon D800 FF & D3300 Crop)

KEJ
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DSO Astrophotography with CMOS Cameras (Nikon D800 FF & D3300 Crop)

#1

Post by KEJ »

Robin hi

Earlier today I watched your video presentation on Youtube “Deep Sky Astrophotography With CMOS Cameras” – Brilliant, thank you, really good.

Could I please just pick your brains and ask for your recommendations, based on my questions below (eventually I was possibly going to buy a dedicated cooled astro camera however, now I am thinking…………..).

1) Camera or Filter
After watching your clip, I am starting to think that “Bang for Buck” I would be better off buying a light pollution filter, rather that a dedicated cooled Astro camera for the moment, especially as I am still learning as it would appear the biggest problem is that unwanted light!

2) Filters
I live on the outskirts of City, but it’s still pretty light polluted(Bortel 6) what I wanted to ask was if you were buying a Light pollution filter for street lamps and houses(back garden is realatively dark, but...), which would you recommend(I have in mind 2” round filter sitting between the camera and scope) but unsure of the type and this would be used for galaxies and nebulas?

3) Exposure Times
Recently I have been experimenting taking longer exposures and getting 5 mins with no star trails, but after watching your clip(and really wanting you to confirm), that really rather than(example):

13 X 5min exposures

I would be better off with(based on say my Bortle Class 6) BW 12secs, colour 36 secs, therefore more in the range
of:

65 X 1min
or
100 X 40 secs

Would that be a correct assumption using my DSO equip below ?


Sorry for all the questions, I did post this message earlier, but could find no trace, so my apology if a similar message is already there.

Kind regards
Keith

My Location Bortle 6

My Equip:

DSO
SW AZ-GTI
ASIAIR Pro
WO Z61 Scope F6
Nikon D800(FF) or Nikon D3300(crop) Unmodified CMOS
30F4 Guide Scope
ASI120MC-S Guide camera

Planetary
Meade LX10(old) F10 + ASI120MC-S +laptop and sharpCap
Last edited by KEJ on Wed Sep 23, 2020 8:31 am, edited 2 times in total.
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Re: Deep Sky Astrophotography With CMOS Cameras

#2

Post by admin »

Hi Keith,

I think a lot depends on your exact situation – if most of the light pollution that you have nearby is old-fashioned low-pressure sodium (orange) lighting then there are plenty of light pollution filters that will cut out that band and give you some really useful improvements. If the light pollution is largely white light then there are still options (like tri-band filters) but these are only really useful for nebula imaging – they won't really help so much with galaxies or globulars. Essentially the dual band or tri-band light pollution filters only allow through nebula light, whereas classic light pollution filters are designed to block the orange sodium light of old-fashioned street lights.

Another thing to consider is whether you are already using dark frames – if you are then there is a lot to be said for a cooled camera as you can ensure that your dark frames are taken at the same temperature as your light frames which gives the best subtraction. Here though you have to think about your budget – you already have some impressively large sensors in your DSLRs and moving to a significantly smaller sensor in a cooled Astro camera may feel like a step backwards.

Onto exposure lengths… In theory, CMOS can use much shorter exposures if the read noise is low. For most CMOS Astro cameras the read noise can be pretty-small – often down into the 1 to 2 electrons range at appropriate gain levels. For some reason DSLR seemed different. I have two sets of sensor analysis run against the Ascom DSLR driver now (fortunately I'm not sure which model they came from), but the results were unexpected:

ISO 100 - Read noise 23e
ISO 200 - Read noise 11e
ISO 400 - Read noise 6.3e
ISO 800 - Read noise 4.4e
ISO 1600 - Read noise 3.3e

Now, if those values are appropriate to your Nikon (maybe they come from a Canon, or another model, who knows!), you can use the rule of thumb

Sub length = 20 * Read Noise * Read Noise / Sky Brightness

Where Read noise is in electrons and Sky Brightness in e/pixel/s. Fortunately https://tools.sharpcap.co.uk/ will estimate your sky brightness (bortle 6, f/6, 4 micron pixels) at about 1.8 e/pixel/s, so the exposure recommendations would range from 5900s at ISO100 to 120s at ISO 1600. (See this thread for more detail on where the rule of thumb comes from - viewtopic.php?f=35&t=456).

Note that if you add a light pollution filter that (say) halves the background brightness then you need to double your exposure length. At some point you will hit the guiding/tracking limits of your mount.

Hope this info is useful,

Robin
KEJ
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Joined: Mon Feb 24, 2020 7:37 pm

Re: Deep Sky Astrophotography With CMOS Cameras

#3

Post by KEJ »

Robin good evening

Thank you for the prompt reply.

Just had a quick read through, but I will go back over tomorrow and check out the links and I also forgot to mention the ISO!

Out of interest, I am 95% sure we have the newer street lamps here now, white light, possibly LED.

Cheers
Keith

I will just gather more info, so currently for mine:

Nikon D800 QE 56% Pix 4.88

Nikon D3300 QE 44% Pix 3.89

I will now just search for the Read Noise
KEJ
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Re: Deep Sky Astrophotography With CMOS Cameras

#4

Post by KEJ »

Robin good morning

I must admit, not sure how you manage to answer lots of these questions so quickly - thank you.

I think I now have the relevant information for my cameras and I just wanted to run it by you when convenient of course please.

The one possible outstanding question was the "20" in your formula is that a constant or something that varies?

Therefore, with my limited knowledge and based on the information I have, would you say this example(even if you could confirm any one please) for the Nikon D800 is correct ?

ie: Pretty much optimal times are

ISO 800
20 X 2.6 x 2.6 /3.86 = 35 secs

ISO 1600
20 X 3.55 x 3.55 /3.86 = 65 secs


Based on the information I have would you say this example for the Nikon D3300.

ISO 800
20 X 1.2 x 1.2 /1.93 = 15 secs

ISO 1600
20 X 2.1 x 2.1 /1.93 = 45 secs

My conclusion
Pretty much, stay with what I have, forget maybe for the moment upgrading the camera or buying filters.

Go for higher stacking numbers.

The astro camera option for DSO maybe later as the smaller pixels or smaller sensor might make some of the smaller DSO larger in my Z61(61mm, 360mm FL, F5.9)........maybe.

Also currently I only use Lights(no darks, bias etc).......could explore.



Read Noise(providing its the correct measures or graph).
Read Noise 2020-09-21 at 06.18.25.png
Read Noise 2020-09-21 at 06.18.25.png (171.12 KiB) Viewed 2325 times

D800
D800.png
D800.png (225.08 KiB) Viewed 2323 times
D3300
D3300.png
D3300.png (265.47 KiB) Viewed 2323 times
Last edited by KEJ on Mon Sep 21, 2020 10:07 am, edited 1 time in total.
KEJ
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Re: Deep Sky Astrophotography With CMOS Cameras

#5

Post by KEJ »

***************** My Results ***************

So looking at my results and graph, I conclude that I might have the wrong graph!

It's just based on your initial comments(providing I have the right end of the stick), my numbers are almost/are in reverse of maybe what I expected.

Unless I am going Mad of course.
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Re: Deep Sky Astrophotography With CMOS Cameras

#6

Post by admin »

Hi,

I'm not really sure about that graph – it's certainly not read noise in the sense that the equations that I use think about read noise. To feed into the equations to calculate the optimum exposure times you need the read noise to be measured in electrons, and that almost invariably drops as you increase the gain (ISO) value, so the highest read noises are found at the lowest ISO values. I suspect that the figures on that graph may be measured in ADU rather than in electrons, although it's not entirely clear.

Certainly you would expect to get shorter suggested exposures at higher ISO values.

The 20 in the rule of thumb above comes from this equation (which assumes you are prepared to accept a 5% increase in noise above the theoretical floor that would be achieved by taking one exceptionally long exposure).

[math]

In particular, this bit

[math]

Is approximately 20 for a QE of 50%.

Robin
KEJ
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Joined: Mon Feb 24, 2020 7:37 pm

Re: Deep Sky Astrophotography With CMOS Cameras

#7

Post by KEJ »

Thank you once again for a prompt reply and explanation of the 20 value.

Regarding the results, that was my gut feeling after I did my calculations, just didn't look right and I remember from your talk, something along the lines of if you increase the exposure time by a factor of 100, the noise would only increase by a factor of 10(Hope I have that right).......better go back and check.

I will do some more digging and try again, but all the same thank you.
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Re: Deep Sky Astrophotography With CMOS Cameras

#8

Post by KEJ »

Revised.

Clicking on the graph, yields the electron values, providing I am looking at the correct measures/graph.

I took the relevant values before taking a screen print.

Again I have doubts, as it it would appear the better camera D800, has shorter times?

However, all that said, the cameras were launched as follows:

D3300 Jan 2014

D800 Feb 2012

So whilst the 3300 is a cheaper camera, it's 2 years newer, therefore in theory better tech in the sensor!

OR

Is it telling me, the optimum settings are reached sooner?

Nikon D800

ISO 800
20 X 2.58 x 2.58 /3.86 = 35 secs

ISO 1600
20 X 2.33 x 2.33 /3.86 = 28 secs


Nikon D3300.

ISO 800
20 X 2.44 x 2.44 /1.93 = 62 secs

ISO 1600
20 X 2.25 x 2.25 /1.93 = 52 secs
Screenshot 2020-09-21 at 16.18.32.png
Screenshot 2020-09-21 at 16.18.32.png (538.08 KiB) Viewed 2297 times
Screenshot 2020-09-21 at 16.24.37.png
Screenshot 2020-09-21 at 16.24.37.png (58.26 KiB) Viewed 2297 times
Last edited by KEJ on Mon Sep 21, 2020 4:43 pm, edited 2 times in total.
KEJ
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Re: Deep Sky Astrophotography With CMOS Cameras

#9

Post by KEJ »

Robin hi

Very quick question, else I will continue looking.

Regarding the revised graph and details I found, would you say judging by the numbers/curve/data and title more of what you would expect?

Thanks
Keith
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Re: Deep Sky Astrophotography With CMOS Cameras

#10

Post by admin »

Hi,

the read noise values are much closer to the ones that I would expect for a CMOS sensor, and are giving calculation results that seem reasonable. The measured values that I have for the DSLR driver are much higher, but I have no idea if that came from a Canon or Nikon camera or even what model. It would be interesting once I get more data of this sort to try to start understanding the patterns behind it.

I think the biggest difference between your two cameras is the much bigger pixels on the D 800, meaning that it collects sky background at a higher rate which in turn immediately leads to a lower suggested exposure. I would suggest that overall the combination of the larger pixels and the higher QE of the D 800 sensor will probably give it the edge.

Hope this helps, Robin
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