SharpCap Forums

I just recently realized that the Smart Hist calculator allows manual entry of sky brightness, so I had play with it today. I started by using the calculator at https://tools.sharpcap.co.uk/ and used my sky's Bortle number. This worked fine, but I use two setups - one with a 0.5x reducer and one without. The only way to account for this is to halve the F-number for the setup with the reducer, which I did. As expected, the sky brightness increased (by a factor of 4). What I did not expect, however, was that the calculated faintest object level increased. This seems counter-intuitive. In general, I have heard that a 'faster' scope (i.e. low f-stop) is better for deep-sky AP. Why would an increase in F-stop also increase my ability to capture faint objects?

Also, from recent experience, my photo stack of the Bubble Nebula of over 200 15-sec photos had very little nebula detail. I admit my post-processing skills are limited, but I just could not bring out much detail without getting into the noise. The calculator predicts that 4.3sec is plenty long and no point in a longer exposure.

Is there a way to predict whether my setup can capture a specific DSO if I know the reported magnitude?

Hi,

I think maybe you didn't spot that the scale on the vertical axis of the graph changes when you adjust the sky background, so although the curves may *look* lower down the graph if you set a higher sky brightness, the faintest detected object brightness value does actually decrease at any particular exposure length as the sky brightness increases.

For the DSO question, I think if you can find the surface brightness of your target (in magnitudes per square arc second) then you can put that into the tools.sharpcap.co.uk sky brightness calculator (as if you were using a sky quality meter that reads out in mag/arcsec^2). That will then give you an e/pix/s rate for that target, which you can compare to the detection threshold simulation in the smart histogram graph.

cheers,

Robin

>if you set a higher sky brightness, the faintest detected object brightness value does actually decrease

Well, not on my software (SC 4.0.8149.0). See attached screencaps. With sky brightness of 2 e/p/s, it looks like the faintest object is about 2.3. With a sky brightness of 4 e/p/s, the faintest object is about 3.3. Actually, I don't see how it could work any other way. As the sky gets brighter, I would think you would lose the ability to see faint objects since they would get washed out by the background. I am just surprised about what it implies about a lower f-stop.

It also seems to go against my basic experience when I switch between my two setups (with and without reducer). I notice that the exposure needed to get a decent image drops in half when I use the reducer, which is what I would expect. So I was thinking the reducer would help find fainter objects for a given exposure.

Hi,

I honestly can't see where you are getting the 2.3 and 3.3 figures from on those screenshots. The correct way to get the faintest object value is to click the mouse over the graph to read off the figures - like this
This shows how the detection drops as the sky noise increases, which is how I would expect it to work.

Hope that makes sense,

Robin

>I honestly can't see where you are getting the 2.3 and 3.3 figures from on those screenshots.
Sorry, I was only quoting the most significant digits and was estimating. Should have said 0.023 and 0.033.

>The correct way to get the faintest object value is to click the mouse over the graph to read off the figures - like this
Thanks for that tip. Did not notice the mouse function. However, your own graphs show that the faintest object detectable is 0.036 for a 4 e/p/s sky while the 2 e/p/s/ sky shows 0.028 (rounded). How does that show the brighter sky allows detecting fainter objects. Am I reading this wrong?

Hi,

0.036 (the value for 4 e/p/s) is brighter than 0.028 (the higher the value, the brighter), so the faintest object for 4e/p/s is brighter than the faintest for 2 e/p/s

cheers,

Robin

Sorry, I still don't get it.

You said above: "the faintest detected object brightness value does actually decrease at any particular exposure length as the sky brightness increases."

Then you said: "so the faintest object for 4e/p/s is brighter than the faintest for 2 e/p/s"

Isn't that a contradiction?

Yep, now that I go back to read it, it's clear that the point I was trying to make did not come out at all well

What I meant to say was either

the faintest detected object brightness value does actually increase at any particular exposure length as the sky brightness increases.

or perhaps

the ability to detect faint objects does actually decrease at any particular exposure length as the sky brightness increases.

but instead I got a mixture of the two that was plain wrong... Apologies for that.

So, going back to the figures, I still think the numbers generated by the smart histogram back up what I meant to say. Are we in agreement yet?

cheers,

Robin

YES!

However, this still leaves me puzzled about the benefit of having a 'fast' (i.e. low F-ratio) telescope for AP. Not really a SharpCap question at all, but goes against what I have read since a lower F-ratio would lead to LESS ability to detect faint objects at a given sky brightness.

Hi,

not quite - if you move from an f/5.6 to an f/4 scope (with the same aperture), the sky brightness will increase by a factor of 2, but so will the surface brightness of any target. Notice that when we increased the sky brightness by a factor of 2, the detection threshold went up from 0.028 to 0.036, so by less than a factor of two. That means that more faint stuff will be detectable in the fast scope because the factor of 2 increase in the surface brightness of the target is bigger than the increase in the detection threshold.

cheers,

Robin