A rough M42

[JohnH]

I had to try out the new scope on something, the moon was bright and there was a fair bit of high level cloud about but I took a few shots anyway. This is the best of them, single unguided 1 minute shot at iso 1600 with the Canon at prime of the VC200L which was (very) rounghly polar aligned. Cleaned up. levelled and re-sized in Elements. I know it is'nt much but when I comapare this single shot to what I could do with the ETX I am blown away...very happy Jan....:zzz:

[h0ughy]

nice John!

[davidpretorius]

hey, i don't have tracking, and i am getting an appreciation of polar alignment.

I like this shot very much.

1minute!!!

roughly how do i equate iso 1600 to say gain or contrast. ie what does iso mean???

[TidaLpHasE]

:)Well done John, i am trying to polar align my EQ6 with the ED80 for some long exposures of M42.

I would be blown away as well with this attempt, keep em coming;)

Dave, [quote]
ISO sensitivity expresses the speed of photographic negative materials (formerly expressed as ASA).

Since digital cameras do not use film but use image sensors instead, the ISO equivalent is usually given.

What ISO denotes is how sensitive the image sensor is to the amount of light present. The higher the ISO, the more sensitive the image sensor and therefore the possibility to take pictures in low-light situations.

And, where you would have needed to physically change to a different roll of film if you wanted a different ISO speed, digital technology allows you to simply dial one in. In this way, you can record images taken at different ISO speeds on the same memory card.

ISO Speed & Exposure

ISO speed affects the shutter speed / aperture combinations you can use to obtain correct exposure.

Suppose your digital camera's light meter warns you there is not enough light to correctly expose a scene. You could use the on-board flash, but let's suppose again it's not allowed (like in a concert or indoors recital).

You would then need to use a higher ISO. Set on "ISO Auto" mode, your digital camera will automatically select a higher ISO. Otherwise, you can manually select the next higher ISO and see if the increased sensitivity allows you to obtain a correctly exposed picture. If it does, you can now take a correctly exposed picture.

Similarly, if you find the camera is using a shutter speed that is too slow (1/60 sec. and slower) to handhold the camera steady and shake-free (thus resulting in blurred pictures), and you cannot open up the aperture anymore, and you do not have a tripod or other means to hold the camera steady, and you want to capture the action, etc. etc. -- then you might select the next higher ISO which will then allow you to select a faster shutter speed.

ISO Speed & Noise

However, all this increase in sensitivity does not come free. There is a price to pay with your image appearing more noisy.

See, when you boost the sensitivity of your image sensor by selecting a higher ISO, the image sensor is now able to record a fainter light signal. However, it is also true now that it will record fainter noise, where noise is any signal that is not attributed to the light from your subject. Remember that an image sensor is still an analog device and it generates its own noise, too! The increased sensitivity allows the image sensor to record more light signal and more noise. The ratio of light signal to noise (S/N ratio) determines the "noise" in your resultant image.

An image sensor is usually calibrated so that it gives the best image quality (greatest S/N ratio) at its lowest possible ISO speed. For most consumer digital cameras, this value will be expressed as ISO 50, ISO 64 or ISO 100. A few digital cameras use ISO 200 as their lowest ISO speed.

Just as with its film counterpart, an image sensor will exhibit "noise" (comparable to "graininess" in film) at the higher ISO speeds. Unlike film, where graininess can sometimes contribute to the mood of the image, noise produced by an image sensor is undesirable and appears as a motley of distracting coloured dots on your image.

ISO Speed & Image Sensor Size

The size of the image sensor determines the ISO speed range that a digital camera can use without suffering from undue noise. One reason for this is because the pixels on the larger image sensor can be larger and therefore receive more light, and thus have a greater signal-to-noise (S/N) ratio (for more information on noise, see our tutorial: What Is... Noise? (http://www.photoxels.com/tutorial_noise.html)).

If we take two image sensors, each with 4 megapixels resolution, but of different sizes, the 4 megapixels image sensor that is smaller will exhibit more noise at higher ISOs than the larger one.[quote]

[atalas]

One minute on M42 brings out quite a bit of detail John,well done.

[Itchy]

Hi TidaLpHasE
A great exposition on ISO. Well done.

Just one point about digital ISO though. The sensor itself does not change its sensitivity. It responds to photons in the same way regardless of the ISO setting. Its actual sensitivity is a direct result of its quantum efficiency, which does not change with ISO. What the ISO setting does do is apply a gain to the signal that is read from the sensor to mimic sensitivity. The end result is exactly as you have so well described.

Cheers

[JohnH]

Stacked and re-processed - total 5 mins. Tracking off, think I had better put a focal reducer in...

[TidaLpHasE]

;)Great pic John, you are well on your way.

Itchy, i can't take credit for the ISO explanation, i searched good ol' Google for the answer, i should have posted the source, here it is.http://www.photoxels.com/tutorial_iso.html

[davidpretorius]

thanks for iso explanation, great stuff.

great image again john

[atalas]

Nice John,thats much smoother !