Figuring out correct exposure times?

[bluesilver]

Hi, I am just trying to figure out how to know if I am under exposing or over exposing my images.
I have the SharpCap license.
I am in around a Bortle 3 area.
I am using a asi2600mc camera and using a F7 Skywatcher Esprit 150
I generally just go with 180 second exposures, but want to fine tune this to see if I am over or under exposing.
I have used the smart brain to do a sky brightness test and get some exposure times, will these times get me pretty close to being not over or under exposed?
Main question is that I have now got a C14 with a Hyperstar and it is F1.9
So I was planning to give this a try but want to figure out how to best get my exposure time correct to start with.

I have to say though, that I don't actually use SharpCap for my main imaging program, but find it very good at getting correct readings.

[admin]

Hi,

the exposure calculations in SharpCap concentrate on one particular aspect - working out the combination of gain and exposure needed to ensure that your final stacked image will not have excessive noise due to the read noise of the camera (which will be a problem if your sub exposures are too short).

If you use an exposure *longer* than the one SharpCap suggests then the quality of the final stacked image (in terms of noise level) will not improve noticeably - for instance if SharpCap recommends 60s and you use 180s, the final stacked results from 60x60s frames vs 20x180s will be almost indistinguishable in terms of image noise level. On the flip side, 180s frame put more pressure on tracking, guiding, etc and more chance of satellite or airplane trails in each frame.

If you use a *shorter* exposure than the one SharpCap suggests then the final stacked image will be more affected by noise - following the same example above, with a 60s recommendation, using 20s sub exposures and taking 180 of them in an hour will most likely lead to more noise being visible in the stacked image at the end of the session compared to the 60x60s or 20x180s ways of imaging for an hour on the same target.

If SharpCap recommends a very short exposure, you may want to go longer anyway - to reduce the number of frames captured (and speed up processing) or to make stars more prominent in the frames if you are having trouble getting alignment to work.

You will find when using your new telescope with Hyperstar that you can use much shorter exposures - going from f/7 to f/1.9 will allow you to reduce your sub length (and total imaging time) by a factor of about 13 and still get similar results.

Hope this helps,

Robin

[bluesilver]

Appreciate the reply and it dose make sense in what you are saying.
I was just trying to out if there is a way to know if I am exposing loo long or too short.
So if I take a 180 second sub, how to find out if that sub is over exposing or under exposing?
I hear people mention histograms and measuring things, but at the moment. I am not really sure what I am suppose to be looking at histogram wise and as for measuring, no idea there.

[turfpit]

bluesilver

I am not really sure what I am suppose to be looking at histogram wise and as for measuring, no idea there.

Try this:

[url]https://docs.sharpcap.co.uk/4.1/#Correc ... 0Histogram

This is what you need to focus on (no pun intended), aim for a small gap between the left hand vertical axis and the histogram.

histograms.JPG (61.08 KiB) Viewed 4071 times

With a hyperstar, your exposures will be substantially reduced.

Search for 'hyperstar' on Astrobin, you might find some images that interest you.

Dave

[bluesilver]

Ah, That was what I was looking for, I remember seeing that once and forgot all about it.
I will go back and take another read of it,
Greatly appreciated.
I did take a look at some of those Hyperstar images on Astrobin.
I think some of the sub numbers must be wrong though.
Doubt you could do a 600 second exposure at F2 without over saturating.
I am thinking around 30 seconds would pull it up, that was what I want to find out using the histogram.
Although not that familiar with imaging using Sharpcap Pro,
Is it worked on once you have taken an image,
Or is it just set the exposure slider say to 30 seconds and then wait 30 seconds or so and the histogram will move?
I use SharpCap Pro to take flats, just adjust the exposure until the white curve is close to 50% but I am thinking the histogram graph may me a bit different to this?

[turfpit]

bluesilver

I pointed you to the Astrobin search as a likely source of information on Hyperstar exposures. The IOTD and Best Picks are the ones worth looking at as they are deemed to be decent images and one must assume that those posters know what they are doing.

This is a histogram in FITS Liberator of a 10s L frame of the Double Cluster in an LRGB session.
viewtopic.php?t=8322

histogram.JPG (11.56 KiB) Viewed 4055 times

I would consider this frame to be correctly exposed because there is a gap at the left hand side of the histogram (value=40) and the right hand side has a value less than 65535 (actually 4095 in this case).

These are the steps I would use:

• Think of an exposure

• Capture a single frame as FITS

• Look at the histogram of the frame in (say) FITS Liberator

• If A=0 then faint data would be lost - increase the exposure and/or offset/brightness/black_level depending on the camera QHY/ZWO/Altair and retake the frame

• If B=65535 and the histogram only slightly touches the right hand side then probably OK but beware of any very bright stars as they can be over-saturated on the image. If the histogram hits the right hand side hard then bright data is lost (over-exposed/saturated).

• If A>0 and B<65535 then the exposure is probably good

• If A and/or B not right then adjust exposure and repeat

These are guidelines as no-one can tell you exact figures because of all the combinations of equipment, Bortle numbers, street lamps etc.

With experience, you will establish exposures that are good for equipment/sky conditions.

Hope this helps. At the end of the day you have to get out there and try out these settings.

Dave

[bluesilver]

Appreciate you taking the time to go over this.
I will have to look up FITS Liberator.
I found some data from my Skywatcher Esprit 100 ( F5.5 ) with 300 second subs

Esprit 100 reduced.jpg (160.41 KiB) Viewed 4052 times

And this is the same data but with a Skywatcher esprit 150 ( F7 ) also with 300 second subs

Esprit 150 reduced.jpg (159.37 KiB) Viewed 4052 times

Camera is the color camera asi2600mc and was set at gain 0
To be honest, I can't really tell the difference, the F 5.5 from the Esprit 100 should be more exposed due to being F 5.5, but then again it has the smaller aperture of 100mm vs the 150 of the Esprit 150
I think both images are exposed correctly as there appears to be a small gap on the left hand side of the histogram.
Dose this look to be about what I should be looking for?
I will also take a look at FITS Liberator and see what that is also.

[turfpit]

Those histograms look ok. Are they without the Hyperstar?

FITS Liberator v3 https://esahubble.org/projects/fits_lib ... nload_v30/ used in the above picture. There is a v4 now.

Dave

[admin]

Hi,

there doesn't seem to be a big difference between those two histograms, but it's worth noting that as well as the f-ratio of the telescope, sky conditions will play a part in the brightness of the background of the image (if you have any significant light pollution then a 'crystal clear' night will have darker backgrounds than one with a bit of haze/murk).

cheers,

Robin

[bluesilver]

Appreciate the replies and advice,
Yes I did forget to mention that I am in a Bortle 3 area and these were taken with no moon, so pretty dark
No these are not from the Hyperstar
The first one is from a Skywatcher Esprit 100 and 300 second subs, it is a F5.5 and 100mm aperture
The second one is from a Skywatcher esprit 150 and 300 second subs, it is a F7 and 150mm aperture.
Dowloaded fits liberator v4 and v3, v4 doesn't seam to show the histogram as weel, but only just downloaded it and yet to really take a look at it to see how to enlarge the histogram
Below is the fits liberator verion of the Skywatcher esprit 150 at 300 second exposures

fits liberator.jpg (120.47 KiB) Viewed 4015 times

And tis is the one of the Skywatcher esprit 100 at 300 second exposures

fits liberator.jpg (120.47 KiB) Viewed 4015 times