Using the histogram

[carnbrea]

Im a bit of a beginner and need some help with the correct length of exposures for imaging please.

 

Im using a Canon Modified 600D on a Orion ED80 Telescope being guided ok with PHD on a Celestron CPC1100 Deluxe and I'm shooting after Astronomical Twilight ends

When I start imaging M31 for example I try and get the exposures so that the three channels align roughly with the first bar on the left in the histogram, however after I have done this I can see that after awhile the peaks gradually move to the left on the histogram and move past the left bar indicator as the telescope rises during the evening session, exposures are the same length at about 420 secs so do I need to increase the exposures to peak again on the first bar ?

Is this the correct method to use please.

Any suggestions would be most helpful

Thank you

[astroman133]

First, you can choose how many vertical bars are displayed in the histogram (0, 1, 2, or 3), so your description is a bit vague.

 

It would not be unexpected for the histogram to shift during an imaging session.  The higher in the sky you point, the darker the sky would tend to be since you are farther away from terrestrial light pollution sources. Also the moon may rise at some point, and affect your imaging.  As long as the left shoulder of the histogram is well away from the left edge of the graph then you are capturing all the data.  However, it is better to shoot fewer long exposures than lots of shorter ones. Since the long exposures do a better job of elevating the signal away from the noise.  As long as your guiding is good and you are not overwhelmed with sky glow, you would be better off to go longer.  But you don't want the right shoulder of the histogram to approach the right hand side of the graph.

 

For now I would just try to center the histogram bump at the start of your session.

[carnbrea]

Sorry Rick I was talking about the vertical bar 1 and didn't want to encroach any of the channels on to bar 0 on the far left as I know thats not good.

Thank you that's been most helpful and I will put that in to practise.

Just got to wait for the clouds to disappear now.

[s3igell]

However, it is better to shoot fewer long exposures than lots of shorter ones. Since the long exposures do a better job of elevating the signal away from the noise.  As long as your guiding is good and you are not overwhelmed with sky glow, you would be better off to go longer.

 

I'm going to have to disagree with Rick regarding the wisdom of always Exposing Long.

Noise - both Skyglow and Heat (and other Shot) Noise - are going to INCREASE the longer the Exposure.  Heat-related Noise is especially a Concern for DSLRs compared to TEC'd CCDs.

 

Stacking will do the Grunt Work for Reducing the Noise - Noise is reduced at the Square-root of the Image Count.  And then Stretching will bring up the Image Signal - as long as there was actual separation between the Data curve and the Left wall of the Histogram.

 

As well, one can readily Saturate all the Foreground Stars with Long Exposures.  And Brighter DSOs like M31 or M42 will also Saturate their Cores if the Exposure is "too long". 

[dts350z]

And you increase the chance of a plane or satellite or something else going wrong in longer exposures.

 

But you do decrease the total amount of time dithering, etc.

 

So it's a trade-off.

 

You mentioned 420 seconds. I use 480 (for non narrowband anyway), I can do six of those in an hour (with dithering).

[jimwc6491]

All

The length of exposer time is governed by the "F" ratio of the scope and the brightness of the object you are photographing.

example with my F/4 scope I rarely get over 90 seconds if I get over that the center M31 would be washed out, most of my shots are around 60 seconds.

I use trial shots to see what it will look like then check the histogram just to make sure it is away from the left side of the graph.

if everything checks ok then start my capture plan with that amount of exposure time. if the histogram is not off the left side then I tweak the exposer time a little.

[astroman133]

As you can see, everyone has their opinion.  It is influenced by their equipment and experience, the ambient conditions, the sky conditions, and the target brightness.

 

I disagree that the exposure time is governed by the f-ratio and target brightness. I would go so far as to say it is influenced by it.  That influence contributes to how much the histogram bump moves right or left as you change the exposure.

 

I am currently shooting an LRGB image M33 from a reasonably dark sky with a CCD camera cooled to -10C.  Thermal noise is non-existent with that sensor at that temperature. My scope is 5" refractor with a focal ratio of f/5.2. I am also using a 0.8X focal reducer giving me an effective focal ration of f/4.2. My luminance exposures are 7.5 minutes each. The focal reducer is new. Without it my exposures would be 10 minutes. The histogram of my raw lights is about right at 7.5 minutes..

 

Does this mean that others are wrong? No, it doesn't necessarily mean that. What it does mean is that while there are some guidelines that make do sense and you can get close by following them. You should not be afraid to play around, make adjustments, be prepared to make mistakes and throw images away if you are not happy with them.  As long as you learn from the effort, it is not a failure. Have you ever used WD-40? What happened to WD-1 through WD-39?  They learned 39 ways not to make the stuff.  I doubt that they would say that they failed.

 

i will also say that having a plane or satellite in a single exposure should have no effect in the final image if you select a good stacking algorithm (min/max excluded average or sigma clip should reject the abnormal pixels from the trails in a single exposure).

 

I hope this helps. Enjoy the journey.

[carnbrea]

Thank you everyone I appreciate the helpful recommendations I am a bit wiser now as what to do with a bit of trial and error

I have been pausing for 15 seconds between taking images does this make any difference at all to heat noise?

[s3igell]

I have been pausing for 15 seconds between taking images does this make any difference at all to heat noise?

15 sec is probably too little time for much of the Heat generated during the Previous Exposure (a Bit of energy is expended throughout the Exposure and then a bigger burst at the end due to Reading the Sensor and then Processing the Results and Writing to SD and/or USB).

It will depend upon many factors - which Model (different Processors and different Frames and different Mass) and your Length of Exposures and whether you also have a significant Dither Settling Pause and even whether you have a Scope with Open Optics.  AND of course the Ambient Temp and Humidity...

 

In the Dry but Hot Desert Night, I find that a total Dither Settle and Pause of about 1.5min will allow my 5min Exposures to reach a "Steady State" that is only about 5-7*C above the Ambient Temp.

 

If Heat Noise is an Issue for you, make sure that you are doing the appropriate "Heat Control" actions: Start with a Cool Camera (don't let it get Pre-Heated" while sitting in your Car or on a Table - keep it in a Cool Storage location until Needed); Limit LiveView use to Bare Minimum; Turn the LCD Display OFF; and Use an External Battery Replacement to power your DSLR.