First Guiding Effort - M33

[steviemac500]

Quite pleased with the guiding tonight apart from the errors I made at the start. Lots to learn with processing yet and I way over exposed - I did 10 X 10 minutes to check the guide camera really. ISO 800. Any comments appreciated.

[Leveye]

I think it looks great. It has a lot of detail in there that can be brought out even more using levels and curves. That said when zoomed in a bit it looks that your focus was slightly off. If your camera has live view and a zoom feature I find that most helpful in focusing. That or using a Bahtinov mask to assist in focusing. Well done and your on your way!

[steviemac500]

Thanks, I've a bahtinov mask coming next week and an upgraded focuser too so that will help. It was great just to get the guiding working tonight though!

[Leveye]

Feels good yes I know. Oh and sooner or later you will want to invest in a coma corrector for your Newt. My Baader MPCC mkII works great for that.

[geordie85]

Nice image. Just curious, did you start taking your first image whilst PHD was calibrating? I notice a couple of your brightest stars have a little L shape under them

[steviemac500]

Yes, I was still playing about with settings at the time and I had the wrong camera option selected so it wouldn't track. As I was just testing it I just left it but its really obvious where the camera started guiding!

[ollypenrice]

It's not over exposied. We can see detail in the core. I think you first setp might be to check out the net for what's known in imaging as the Levels and Curves routine.

Olly

[steviemac500]

Sorry, over exposed is probably the wrong term. The histogram was nearly at 80% so the camera was saturated. If I was taking it properly, I would have brought the exposure time back to 5 minutes to bring it to better levels.

[cjdawson]

Or, you could just turn the ISO setting down, therefore meaning that you are recording less noise and more signal.

[steviemac500]

I thought it didn't work like that in astro photography?

[cjdawson]

When you take a photo, you open the shutter and expose the sensor to the object being images.  The photons of light will enter the sensor and be collected.   There is always a certain amount of rubbish that is also collected as well - this is the noise.  For normal daytime photography, you tend to capture a huge amount of light.  This is also true for the moon - Let's use the moon as an example, so that I can talk about magnitudes.  

So, the full moon is about Mag -12.6.  The sun is at -26.7.   So using this information you could say that the sun appears about twice as bright as a full moon (I'm being simple just to explain the photography info, not to be accurate)

So, if you doulbe the exposure time on your camera everything is good, and you'll get an image.  This can be seen as when you photo the full moon, you'll get exposure times somewhere around 250ms, in the range of daytime photography - that's why you can do it without a problem on a not tracking mount as the exposure time is just like taking a normal daytime shot.

However, M42 the Great Orion Nebula is Mag 4.  This is a very dim object in comparison to the moon or the sun.  So you need much longer exposure times.

As the shutter is open for longer, there is more opportunity for stray photons to hit the sensor - they're not desired, but you can't stop them.  There is also false positives that happen because of noise generated by the camera itself - heat, magnetic forces because of the electricity flowing, etc.  All of this adds up to degrade the image.  The longer the shutter is open the more of this noise is generated.

So, this is where the signal to noise ratio comes in.  As you lengthen the exposure time, you increase the noise collected.  But you also collect more signal as well.   If the signal to noise ratio is good, you'll get more signal than you will noise, so an image will start to form, the bigger the signal to noise ratio, the faster the image will form.  Or to put it another way, the brighter the object, the shorter the exposure time.

At this point, what I've just said is completely true for any kind of photography. They don't just just because you are pointing the camera at a different place.

Where astrophotography becomes fun is in the issues that you get in capturing the image.

The sky is a moving target - so as the exposure time goes up, you need to track and guide the camera to make sure that it stays on target.

As the objects are very dim, you'll need to have extremely long exposures.  Ideally, many hours of image capture to produce a single image.  As the planet rotates, you can't simply leave the shutter open for 24 hours, the sun would destroy all the nights hard won photons.

So, we take lots of shorter images, and stack them.

Capturing images for stacking, is again another thing, so the idea there is you take a sub frame - which would be the correct exposure for a image. So the sub frame makes the image bright enough to capture all the detail that you can get.  The longer the exposure you can capture the better - provided that you don't over expose the brightest parts of the image, as close to, but don't overfill it.   Then you take another and another.

The key here is that in each image, the signal (M42 for example) will be the same.  But the noise will be different in each sub-frame, by averaging the images, you can double that all important signal to noise ration.  Do this over and over and you can increase the signal to noise ratio even further.

I hope this helps explain things a little.  I know that I went off on a tangent.