SharpCap Forums

Hey All , Here is M81 from January 4th/5th , taken with a QHY183m on an ES127-FCD100 and unguided CGX mount . 4 hours of 6 second frames in SharpCapPro , and then repaired with StarTools . I am learning and having tons of fun . Thanks for looking https://drive.google.com/file/d/1Nk7KG- ... p=drivesdk

Nice.. congrats.. i'm still learning sharpcap myself.. scope is in the shop, ED127, but i'm using an ST80 for practice...

That is some image for such short exposures.

Hi,

I couldn't help but do an analysis.

Your telescope's focal length is 952mm and your camera's pixel size is 2.4um, so your pixel scale is 0.52 arcsec. Very few of us are fortunate to have 1.5 arcsec skies so if your skies are "OK" like mine then the CCD Suitability Calculator says you are oversampling. That means that the image is spread over too many pixels. Each pixel is receiving too little light, plus f/7.5 is slow for deep-sky. When you combine that with 6-second exposures then this explains the need to stack for a total of 4 hours. If you doubled or tripled the exposure you could have gotten a better result in 2 hours.

I have reason to believe that you employed 2x2 binning either on the camera or in StarTools in order to reduce noise. I deduced that by the following: the dimensions of M81 are 24.9 arcmin major-axis x 11.5 arcmin minor axis. That is (24.9*60/0.52) x (11.5*60/0.52) pixels = 2873 x 1326 pixels. The sensor is 5544 x 3694 pixels. Fine. But when I downloaded your image and right-clicked properties, Windows reports 967 x 1213 pixels. Calculate the hypotenuse. It is 1326 pixels. That is the major axis of the galaxy on the image. Double that and you get 2652 pixels. This tells me that you used 2x2 binning probably to reduce noise. Next zoom into the image. There is a lot of quantization which indicates to me that you probably did use 6-second exposures and got a sub-optimal result. Next time try doubling or tripling the exposure. Also you might want to consider a camera with 5 or 6 micron pixels. It is better suited for your scope.

Brian

Thanks Brian

Hi,

There is a raging debate on this forum and elsewhere regarding short exposures. The decision to use one over the other is multi-faceted. Here are some reasons to consider short exposures over longer ones:

1. Your mount isn't up to the task.
2. You live in a zone where aircraft frequently buzz by.
3. You have clear skies but with intermittent clouds.
4. You are a EAA practitioner.

If none of these apply to you then you should open yourself up to the benefits of longer exposures. I'd like to present two examples. The first one has both signal and noise so it easily applies to imaging:

1. Imagine it is the dead of night and the world is asleep. You wake up suddenly and whisper to your partner: "Did you lock the front door?" Their reply is "Yes, now go back to sleep." This conversation was possible due to a low-noise environment. Now consider a high-noise environment like Niagara Falls and try to whisper. Nope, doesn't work. You need to raise your volume. Some people might interpret this as the reason why you need longer exposures with CCDs. This is true but please understand that CMOS and CCD both benefit from longer exposures. Imagine that you are back at Niagara Falls and your partner finally finds a voice level that you can just make out, barely. Doesn't it seem reasonable to ask for louder voice levels just to be absolutely, positively clear what they said?

This next example is an analogy. Take it for what it is worth but I think it does a good job of explaining why increasing exposure is beneficial even when it means that you are recording high levels of sky glow:

2. Imagine that you are at an automobile drag strip. One car can travel at a maximum speed of 100 feet per second and the other at 110 fps. Assume that when the light turns green that they can immediately accelerate to their maximum speed. After 1 second they have traveled 100 ft and 110 ft from the starting line, respectively. Only 10 ft separate the two. After 2 seconds they traveled 200 ft and 220 ft. Now 20 ft separate the two. You can see that the distance separating the two vehicles steadily increases as time goes by. This is the same with imaging. After 1 second you have only 10 units of brightness separating galaxy from sky glow. But after 2 seconds you have 20 units of brightness separating them. And so on.

Like I said not everyone can run longer exposures for the reasons that I listed above but there is one other consideration to be mindful of. Foreground stars are the brightest objects in your field of view. The longer you keep your shutter open the more the risk of saturating the brightest stars. The saturation level is very much dependent on your camera. CCDs are very good at this because they generally have much deeper 'wells' than CMOS and can hold more electrons (i.e. photons). But this is all a matter of taste. Personally I don't like saturated stars so I try to crop them out when I can.

Finally, what about stacking? Stacking helps increase signal-to-noise in both CMOS and CCD, however keep in mind this inequality: 100x 1-second exposures does not equal 1x 100-second exposure. You will always get better results in less total time when increasing exposure. You don't have to go crazy. Just try doubling and then go from there.

Brian

The fallacy of the perfect exposure:

Your choice of exposure is akin to a multi-lane highway -- both have boundaries. Highways have shoulders -- veering one way sends you into a gully, and the other way into oncoming traffic. Likewise with imaging, too short of an exposure records nothing of interest, and too long saturates stars and potentially damages the frame by passing aircraft and clouds. What lane you travel in depends on your skill level and risk tolerance.

SharpCap's Smart Histogram (or Brain) is the lane nearest the shoulder with the gully. It recommends exposures that will get you to your destination but will take you some time to get there. In my opinion it provides a great starting point. On the other extreme is the lane nearest oncoming traffic. Your best images come from here but beware that your equipment and sky conditions may not support it. Being a multi-lane highway, there are lanes in between as there are with imaging. Let the Brain start you off but then experiment as your skill improves.

Brian

I've not yet used a guiding system , but will purchase that soon .

Actually, I thought that your tracking was very good.

I don't guide. I have a 50-year old mount having just a stepper motor on the R.A. axis. I rely on good polar alignment thanks to SharpCap. I can get 200-second exposures. Star roundness isn't perfect but it works for me. A new mount is in my future!

Brian

oopfan wrote: ↑Sun Mar 24, 2019 7:11 pm Hi,

I couldn't help but do an analysis.

Your telescope's focal length is 952mm and your camera's pixel size is 2.4um, so your pixel scale is 0.52 arcsec. Very few of us are fortunate to have 1.5 arcsec skies so if your skies are "OK" like mine then the CCD Suitability Calculator says you are oversampling. That means that the image is spread over too many pixels. Each pixel is receiving too little light, plus f/7.5 is slow for deep-sky. When you combine that with 6-second exposures then this explains the need to stack for a total of 4 hours. If you doubled or tripled the exposure you could have gotten a better result in 2 hours.

I have reason to believe that you employed 2x2 binning either on the camera or in StarTools in order to reduce noise. I deduced that by the following: the dimensions of M81 are 24.9 arcmin major-axis x 11.5 arcmin minor axis. That is (24.9*60/0.52) x (11.5*60/0.52) pixels = 2873 x 1326 pixels. The sensor is 5544 x 3694 pixels. Fine. But when I downloaded your image and right-clicked properties, Windows reports 967 x 1213 pixels. Calculate the hypotenuse. It is 1326 pixels. That is the major axis of the galaxy on the image. Double that and you get 2652 pixels. This tells me that you used 2x2 binning probably to reduce noise. Next zoom into the image. There is a lot of quantization which indicates to me that you probably did use 6-second exposures and got a sub-optimal result. Next time try doubling or tripling the exposure. Also you might want to consider a camera with 5 or 6 micron pixels. It is better suited for your scope.

Brian

Hi Brian

Your analysis was very helpful to me in understanding resolution in terms of sampling. You mentioned binning as a way to reduce noise or to improve an oversampling situation. Since this can be done in either the camera which in my case in a ZWO 533 or post processing software such as Star Tools is the any virtue in doing it through my camera via the Sharpcap software rather than Star Tools? Seems to me doing it through Star Tools would be simpler since it would not require another Dark needing to be created in SC.

Thanks
Stewart