• Welcome to Orpington Astronomical Society.


New version SMF 2.1.4 installed. You may need to clear cookies and login again...

Main Menu

Never in the face of human conflict has so much been asked by so few to so many.

Started by Mac, Jul 15, 2010, 20:26:22

Previous topic - Next topic

0 Members and 1 Guest are viewing this topic.


Beginners guide to imaging. Or
Never in the face of human conflict has so much been asked by so few to so many.

Step one. RTFM

So you have just spent 100K on the latest F1.4 Takashi 3000mm scope, Plus the new Losmand HGM mount. 10K on the latest STX KAI-11002 CCD Camera - Colour - Class 1 & filters, not to mention the 30 pieces of software and laptops, water cooled everything automatic observatory and Teasmade.  You plug it all together and you get nothing, so you put away and take up fishing. Sound Familiar. Well what is going wrong.

The basics.

The telescope.

Ask yourself this, do you really know how to use your telescope?
Do you really know what telescope you want?
This isn't about polar aligning, making flash things happen, its about can I set it up and look through the eyepiece and see an object.

If you are not sure, purchase a nice set of binoculars 10x50 or 12x70 and have a look round the sky, once you have become familiar with the sky then think about a telescope.

What telescope do I need. Well this is all down to wonga. Yup if you have a spare $100M you can get a nice 8 meter scope the same as the one in Hawaii, and for £100 something out of Argos, so what ones better? It's the Argos one silly, its portable. Each telescope has its own pros and cons. The smaller the telescope the easier it is to move around. A 14" telescope sounds fantastic, but when you realise that it will probably weigh in the region of  121 lb 8.5 stone! Not including the tripod! Suddenly its not too inviting. Also what it can see is also limiting. A bigger telescope doesn't mean seeing more, quite the opposite.
Think of a Camera and Lens. Most people know that a 50mm lens on a film camera will give you the same field of view as a persons eye. A 28mm is a wide angle lens giving you a nice wide field of view, and that a 300mm is a telescopic lens, which allows you to pick out details at a distance.
Telescopes are exactly the same. A nice 700mm focal length telescope will give you a wide field of view of the night sky. (Yes this is still an extremely long telephoto lens, but the objects you are now looking at a lot smaller, and this is considered a wide field of view in astronomy)
A nice 2000mm telescope will give you a much smaller field of view compared to the 700mm, and will give you fantastic detail when you look at the planets and the moon,
If you understand the concept of camera lenses, telescopes are exactly the same.

How fast is the telescope? All telescopes will do 0-60mph in about 2.5 sec's when they fall over and that's fast.
Telescopes are exactly the same as camera lenses. They have a speed known as the f-stop and a focal length. When you look at a camera lens you get three figures, the length, the speed and the price. a 300mm f5.6 lens is about £300 as this is a slow lens. On the other hand, a 300mm f2.8 is around £3000, the same can be said for telescopes. The faster the lens / telescope is the more expensive it is going to be, because it captures more light which means shorter exposures (more on this later). So once you have decided on how much you are going to spend, which one shall I get. Newtonian? Schmidt-Cassegrain? Reflector? Refractor? Hmm sound foreign translator please. Just Google for the different types, too much to explain here as its just a beginners guide.
Again this depends on what you are looking for. There is no correct answer. I have both Reflectors and Refractors, as do many of the societies other members. Reflectors tend to be shorter as the light paths are folded, but these telescopes have their own problems. The simplest one to start with would be a refractor. A straight through piece of tube with no mirrors. Nice and easy to set up.

So once you have passed you money over the counter and walked away crying at how much it's just cost you, what next.

Follow the list below and you wont be far off of enjoying your telescope. Don't run before you can walk. If you cant see anything, start again. Lens cap? Is it off, sounds silly, but everyone been there.

a)   Have you set your finderscope up with the telescope? Sounds simple but many of the finderscopes that come with the telescope are not aligned. This will save on the "Dad I cant see anything howl from the back garden at 2am when its –5"
b)   So how do I do it? Easy, DURING DAYLIGHT, Build your telescope in the back garden, this is so much easier then trying to do it for the first time at night, place the finderscope onto the telescope and put an eyepiece in. Look through the telescope and focus on the furthest object on the horizon that you can see. Once this is in focus and in the middle of the eyepiece, look through the finderscope and see if the same object is in the middle of the finderscope. If not, RTFM and find out how to adjust the finderscope so that the object that is in the middle of the telescope is also in the middle of the finderscope, this is normally done by adjusting a couple of screws to align the finderscope up. Each finderscope will probably be different so just follow the instructions for your one. Once this has been done you will find that the object in the finderscope is in the middle of the telescope. Makes life a lot easier later on.


The Mount.

There are so many different mounts available that this is going to be just an overview.
Did the telescope I purchased come with a mount? Sounds stupid, but you can buy telescopes with and without mounts. This is the same as the telescope problem, how much can you afford. Most of the society have either EQ5 or EQ6 mounts some are the pro versions, again as this is a beginners guide, I suggest that you Google to find the different types of mount available. Assuming now that the scope you have just purchased came with its own tripod.

Questions to ask yourself, am I doing visual astronomy or photographic astronomy? If you are doing Visual, you DO NOT NEED to polar align your telescope. Depending on your mount, it will be one of the following
Powered with tracking
Powered with tracking and goto.

So what are the differences.

Manual .Well with a manual one you have to do all the work, once you have the object in the eyepiece, it will slowly drift out of view as the earth rotates. So you will have to manually move the mount to follow the star, this will be done either by twisting a couple of knobs, or by pressing the direction buttons on the keypad.

Powered with tracking. This makes life a lot easier, you put the object in the middle of the eyepiece and the telescope follows it across the sky.

Powered with goto. Same as b) but it has a built in library of different objects that you can select and the telescope will goto those objects.

Normally a powered mount has a preset setup routine that you have to follow, in order for the telescope to know where on the planet it is. Again RTFM to make sure you know how to set this up for your scope. This is normally the following, point the tripod so its facing north, level the tripod, turn on, and follow the handset instructions.


Imaging visual.

Even the good imagers have bad days and bad imagers have good days.

So by this stage you have obtained your telescope, mount, aligned your finderscope, know how to set up and follow the instructions on how to use your telescope. Now the fun really begins.
During daylight there is by some weird coincidence a lot of light. So finding the little bird sitting in the tree, or the bee at the end of the garden was easy.

At night there is no light, don't worry about street lights. You look up and all you see is stars points of light. Really as you have just bought your telescope expect about 3-4 weeks of cloud.

So getting your first image. Stuff guiding, who cares about field flatteners, Barlow's, star trails, filters. No image means exactly that. NOTHING.

Use your finderscope to place that dot in the sky into the middle of the cross hairs. Look through the telescope and what do you see, If you are lucky you might see a circle of light, now the fun begins, Focus the telescope. The critical focus point for the stars is probably 0.1 of a mm, which means the difference between being in focus and out of focus is only 0.1mm of  the movement of the focuser.

So what will you see.

Don't expect to see exotic colours of strange worlds, all you will get is the same dot that you can see when you look up. The difference is you can see the other stars around that star that you couldn't see before.
For example M13 ( A large cluster of stars in Hercules), you look up, if you are lucky to have dark skies you might see a faint fuzzy (for that is exactly what it is), look through a smallish telescope and you will see a large fuzzy object and you will probably be able to make out individual stars within that cluster. Change to a higher magnification eyepiece and you should be able to more detail within the cluster, more stars appear as stars. Use a larger telescope and again, you will get more detail within the cluster, some of the stars will have colour, but they will still be dots.
This is basically it.

Don't forget your eyes will take 30mins to be truly dark adaptive, and only 0.1 sec's to have it ruined by white light.

Faint fuzzies are just that, faint and fuzzy. YOU WILL NOT SEE IN THE SKY THE PHOTOS THAT ARE ON THE INTERNET. Look learn and enjoy.

Ask questions, but first learn the basics. Everyone was once at the stage you are at, take one step at a time. Walk don't Run.


The dark arts.


To get images like the ones you see on the internet will take you 100's of hours of HARD work. Not to actually get the image, but to get to the level of knowledge required to get the image in the first place.

By now you should be able to get your telescope set up and be able to use your telescope to visually see objects. So what next,

I know I'll photograph what I see.
Click hmmm no image, so what went wrong?

There is a huge amount to learn in a very short space of time. So step by step you need to do the following.


Stuff guiding, who cares about field flatteners, Barlow's, star trails, filters, dark frames, flat frames, bias frames, field rotation, polar alignment. You might as well be talking German.

No image means exactly that. NOTHING.

When you look through the eyepiece your eye is collecting light from the stars and you can see an image, cameras / CCD's work in exactly the same way (camera from now on) so you need to do exactly the same for the camera as you do for your eye.

a)   First of all do you have an image? Simple answer Yes or No.
b)   As the light from these stars if very faint, you will need to take a long exposure, start with 30s and work your way up. (this gets into the realms of guiding but we will cover that later)
c)   Now you have an image? is it in focus? Don't forget the focus point on the camera will be different from your eye, as it is in a different position to where your eye would normally be.
d)   Focus your image.
e)   So now you have an image and its in focus. (You can use things like Hartman masks to help in achieving focus, Google to find info.)
f)   One other thing to remember. It may take you 30sec's to get an image of say M13, other fainter objects will take longer. Exposures of 20mins are not uncommon, but for the time being stick to about 30s until you are happy
g)   Don't forget a blurred image is the same as no image, useless. Don't worry about star trails, worry about getting an image, Star trails will still be in focus.
h)   Work on getting the basics first. One image that's in focus.
i)   Once you have your image go on to the next step.

What does iso. mean?
If you are using a DSLR it will have on it somewhere the words ISO. This refers to the speed of the camera or more simply how quickly it can gather the light.
The higher the number the faster it can gather the same amount of light.

So if your images at ISO 100 are taking 30 seconds to gather enough light for an image.
Changing the iso to 200, makes your camera more sensitive to light. so it will only take 15s to gather thesame amount of light
(Simple version)

So the table below should help

Iso             Length of time

100              64s
200              32s
400              16s
800              8s
1600            4s
3200            2s
6400            1s

so why not stick your camera on ISO 6400 all the time.
Simple NOISE.
at 100ISO your sensor will produce the cleanest image.
as you increase your iso speed you also increase the amount of noise in the image.
at some point there is a trade off. Speed v Noise.
This is dependant on your camera.

Step 2
Polar Alignment

The next thing would be to polar align correctly. Why. Simple, it will remove the star trails. For this step to work you need a mount that is equatorial and also motorised. The principle is as follows. When you look up all the stars will rotate around the pole star (Polaris), the same for the southern hemisphere, although they don't have a star above their pole.

If you can set your telescope up so that the Axis's of the telescope are aligned with the axis of the earth, then the telescope will track the stars across the sky and the stars will not leave trails. So how do you do this. Each Mount that is available will have its own method for setting up polar alignment. You will need to follow their steps. RTFM.

One method that is independent is as follows.
1)   Set your telescope up level and pointing north, select a star from the handset and the goto that star.
2)   Centre the selected star in the eyepiece (an illuminated eyepiece makes this easy) and then sync on that star. (RTFM for this)
3)   Select Polaris from the handset and goto Polaris. Note where Polaris is in the eyepiece
4)   Using the manual adjustments on the mount NOT THE HANDSET, move Polaris closer to the centre of the eyepiece. DO NOT MOVE IT TO THE CENTER, JUST MOVE IT TO ABOUT ONE THIRD DISTANCE
5)   Select the original star you chose at the beginning and goto that star again.
6)   Centre the selected star and sync again, then repeat from 3) until there is no movement between Polaris and the centre of the eyepiece. This will take about 10 mins and will probably take about 7-8 iterations, this will then allow you to do unguided images between 100 & 300 sec's. Depending on how accurate you want to get your alignment.

NOTE. There are a couple of stars you MUST NOT USE. Hamal, Aldebaran, Capella, Betelgeuse, or Rigel

One other method which is independent is to drift align.
The method is as follows.
1)   Set up telescope level and pointing north.
2)   Using an illuminated eyepiece choose a star on the southern horizon and place it in the center of an illuminated eyepiece
3)   Rotate the eyepiece so that the star moves along the horizontal line when you move the telescope east/west.
4)   Observe the star for a period of time, it will move in the eyepiece.
5)   This movement is caused by the misalignment of the axis's, you need to correct for one movement at a time. Either N-S or E-W.
6)   Correcting for N-S first. If the star drifts up with respect to the horizontal in the eyepiece, then use the mechanical adjustments on the mount move the star back down to the horizontal line, If it drifts down move it up. IGNORE any LEFT – RIGHT movement this will be corrected later.
7)   Once you have observed this star for 5-10 mins and it doesn't move  up or down then move on the E-W alignment
8)   Select a star in the Eastern horizon center this star and the observe the movement. If the star moves UP then you need to move the star RIGHT, don't forget to re-center the star after each movement with the keypad. If it moves DOWN then you need to move the star LEFT. If you choose a star on the western horizon, you need to reverse these movements.

Again once the telescope is aligned then you should be able to image for a much longer period of time before star rotation is observable.

Once you have mastered getting an image, and polar aligning the next thing  to consider is filters and Barlow's.


Filters and Barlow's.

Ok why filters and Barlow's. Filters first.
We have been talking about camera's and most people will probably assume that all camera are colour, well yes and no. Astronomical CCD's tend to be monochrome, where DSLR's are colour, so filters will normally be used with the astronomical CCD' cameras.
So why filters. Well a black and white camera will take a black and white image, No trickery there. So how do we get a colour image. Well, if we fit a colour filter in front of the CCD, then the black and white image taken will be for the colour that the filter represents. So a RED filter in front of the CCD will only photograph the red portion of the Image. Blue for the blue component, and Green for the green component. Once you have these three colour black and whit images, you can use software like Photoshop to combine these images into a colour image. So what happens when we put a filter in front of the camera.

If you place a filter in front of the camera you need to go back to the very basics. GET AN IMAGE. Not all filters will focus the image at the same location, they will need small amounts of focus adjustment, unless you buy par focal filters. So for each filter, check that focus a wasted 900second blurred image is a pain. Also you need to remember that as the filters are now introducing an extra colour layer, the amount of light that is passing is reduced, so your exposure will go up accordingly, This is dependent on the filter. A narrow band filter is just that, it allows a narrow band of light through, so you original image of 30s with no filter might now take 300s with a narrowband filter fitted.

A Barlow is a special lens that doubles or triples the length of your telescope.
So instead of having a nice 1000mm telescope it now becomes a 2000mm or even 3000mm, Barlow's come in a range from 1.5 through to 5.

Advantages. Allows you make the focal length of your telescope longer.
Disadvantages. Quite a few.
The field of view that your telescope will decrease, by the square of the size of the Barlow you are fitting, and the exposure will increase by the same amount.

If your field of view is represented below on a 1000mm telescope.


By fitting a 2x Barlow your telescope becomes a 2000mm and the  field of view now becomes the following


This means that you get more magnification of an object. The downside is now that the image is ¼ of the size of the original.
So if your original exposure was 10 seconds, It now becomes 40 Seconds

The same is true of a 4x Barlow. Your telescope is now a 4000mm and the field of view is such 1/16 of the original image


The exposure time is now 16 times as long as the original exposure 160s.

Combine this with filters, and you can see that the original 10s exposure when fitted with a 4x Barlow and a narrowband filter and easily change to 300+ seconds.
Always remember the basics, GET AN IMAGE.

Field flatners work in the opposite way i.e., a minus barlo.
your image goes from                               to

      XXXX                                            XXXXXX
      XXXX                                            XXXXXX    
      XXXX                                            XXXXXX
      XXXX                                            XXXXXX

Like wise your times for images come down, so a 10 secs image will only take 6 secs.

If it worked before you fitted something, chances are what you fitted has caused the change, Prove it first. Go back a step.



Light frames, Dark frames, Bias images, Flats, Stacking, ect,ect. What do they all mean. Eyes down and I'll see you at the end.

Light frame.
This is your picture. Simple. If it is a 10s B&W CCD or a 900s colour DSLR image, this is a light frame. Label them correctly,  DSW_1446.jpg, means nothing. M16_ha_300s.jpg means a lot more.

Dark Frame.
This is an image of exactly the same length of time as your light frame, except you have the lens cap on.

Bias frame.
This is similar to the dark frame in the fact that you have the lens cap on, except you expose the image for the shortest length of time. I.e. 1/4000 or higher.

Flat frame,
This is an image of a white piece of paper / object. This takes a photo of all the dust and debris in your optical path.

So how do we use them.

A light frame contains the following data.
Image data
Noise created by the CCD itself
Readout Noise.

A Bias frame contains only the readout noise

A Flat image contains
Noise from the CCD
Readout noise
Image data for the dust ect.

A Dark image contains only
CCD noise
Readout noise.

By using various pieces of software deep sky stacker, registax, or various other programs, they will take the correctly labeled images and process them to produce an image.
So how many do I take. And why more then one.

Here is a piece of string. ----------------------------------------------------------
How long is it?

To produce a colour image you will need  a minimum of 4 images. Red, Blue, Green & Luminace (white light no filter)

For every light frame you need a flat frame, so for an absolute minimum image you need the following 10 images.

1 Bias
1 Dark
4 Flats one for each RGB&L
4 Lights RGB& L

So why do we take more then 1 image.

1 image of a faint object contains the image and the random noise.
10 images contain the same faint image and random noise

So if we stack the 10 images, we will increase the image data and the random noise will cancel out.

So 10 30sec images is the same as one 300 sec image. But it will have less noise in the end.

So when you see an image that is 12 hours, you can be sure that they are multiples of shorter lengths.
By taking multiple images and stacking, the picture quality increases, more images better quality.


Now this makes the dark art of obtaining making the image look stunning.
There are so many ways to skin a cat, I would worry about getting the images first.
Use Deep sky stacker and registax first and once you have got to grips you can then start to delve deeper.

Above all, Enjoy.


Been there, done that and bought the T-shirt.


Great stuff Mac, a useful slap round the back of the head to bring back reality.
Growing Old is mandatory - Growing Up is optional


We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan


i'll get round to doing a beginners guide to guiding over the next few days.
I cant realy thing of anything else to add to the beginners bit.



Thanks for doing that Mac, the stuff about the filters and Barlows will be useful.  Prior to doing imaging I only ever used an automatic digital camera which is why I had/have problems understanding some things like speed. 




"Never worry about theory as long as the machinery does what it's supposed to do."  Robert A. Heinlein


Brilliant document Mac!   I love your sense of humour (e.g. how fast is a telescope)

It even told me a few things I didn't know before. 

There are a few other useful thing that could be added - would you like me to send a few sentences/paragraphs when I find time?



I can highly recommend the book:

The New CCD Astronomy: How to Capture the Stars With a CCD Camera in Your Own Backyard - Ron Wodaski

It has everything you would ever need to know about astrophotography in it.
We live in a society exquisitely dependent on science and technology, in which hardly anyone knows anything about science and technology. Carl Sagan


Quote from: Mike
The New CCD Astronomy: How to Capture the Stars With a CCD Camera in Your Own Backyard - Ron Wodaski

Does he also do a book for Nebulae and Galaxies?


     As a non-imager, that information is priceless for me, Mac.  Perhaps I can now start to understand what everybody is doing  and in the future have a go.  I shall try and print out your information and use it as a very fine helper.  Thanks from a no-no imager.

     Doug.   :D
Always look on the bright side of life ...


QuoteThere are a few other useful thing that could be added - would you like me to send a few sentences/paragraphs when I find time?

yup fire away.

Thanks for all the comments.



I think Ian or Rick was going to make this a sticky and remove / replace the old one. with this.
So at least you can always find it at the top of the list.



I'll go back over the text in the next few days, and adjust the grammar and spellings.
That the problem of typing on one of those small eyboards you tend to miss letters and add other letters.