Astrophotography

Though I am into photography for practically my whole life, I am fairly new to astrophotography. There are quite a few things worth knowing when you enter the realm of stars and nebulae. You don't just take a picture like from a tree as part of a landscape nor like one of an aurora. It is quite a bit more difficult than that...

Now I don't pretend to write a course on astrophotography but describing the way I do it with the hardware and knowledge I have may help you to, just like me, make a nice start too.
Please bear in mind that this description is for any photocamera with full manual control. It is not suited for specialized astrocams for telescopes.
Just be aware that astrophotography with or without a tracking device to counter the Earths rotation will ask a certain degree of dedication from you. If you work on Windows then use Deep Sky Stacker as pre-processing software as it is free. For Mac I don't know of any free software for pre-processing. If you think that astro is something you can handle it is still early enough to go buy other software if you want.

THIS PAGE IS STILL UNDER CONSTRUCTION BUT THE MAIN PART IS READY

Weather and darkness

For good weather information and the determination of the darkness on my location I use the app Clear Outside. Find it on the right side of your screen. The darkness is in the orange field on the top and expressed as the Bortle value. The lower the better.

Sturdyness of the tripod

You best take a tripod that has stabilizers between the legs and its base and spread the legs to an equilateral triangle. If not really necessary do not extend the legs thus loosing that equilateral triangle. Extending them will have a negative effect to the sturdyness though having stabilizers will cut you some slack there, experiment with it. Extending the column of the tripod is somewhat tricky too as it adds some flexibility as well unless the column is very solid. The above parameters were the reason for me to choose the Manfrotto 058B tripod. It is expensive, it is big and it is heavy but man is it a blessing!

Levelling the base plate

The base plate is the plate on which you place the mount of the tracker. If this base plate is not perfectly level you will not be able to align the tracker with the astronomical poles. The latitude that you are on has to be the angle between the base plat and the tracker. In order to establish that angle the base plate needs to be perfectly parallel with the Earths astronomical axis. Now bear in mind that the surface of the earth is only perfectly level on a big body of water exactly in between the tides so practically nowhere and following logic: the smaller the area to level the better. Knowing this we will have to adapt a lot.



Direction: Astronomical North


Place the tripod on the ground in the most sturdy way you can at the place where you are at. Placing its feet can already pose you with a problem namely that the mount of your tracker doesn’t point in the direction of the astronomical north with a small variance. Finding the exact astronomical north is a fine tuning thing that is very hard to do with placing the tripod. For that matter the mount of the tracker has a small margin of fine tuning its direction. But that margin won’t be enough in all cases. This is why I added a turning plate to the tripod. Now I can place the tripod anywhere it gets a sturdy position and I can direct the mount in exactly the right direction during the alignment.

Leveling


To level the tripods base plate you would have to extend two of its three legs. For crude leveling this is fine but fine tuning the level is not very handy in this way. To overcome that problem I placed a leveling plate between the tripod and the turning plate. With its three screws the leveling is both easy and accurate. This order is important because otherwise the level will be off after turning the Skytracker to another direction.

So now we have a sturdy placed tripod with a mount that is perfectly level and can be headed to the Astronomical North. Just one more thing before (or maybe better during) exactly aligning the mount. Both the leveling plate and the mount have their own level glas. They will never match exactly and if they do you still can’t be sure about the level that your base plate is on. So keep this in mind as one of two variables when after alignment the stars in your photo seem to float after all. The second variable is missing the correction of your guide star (the star itself is not the astronomical centre point) to the astronomical pole being a misalignement. Have a look at the reticule of the alignment scope of the Skytracker. The center cross is the Earths center axis, Polaris has to be somewhere on the inner ring depending on the time of the day.

Calibration

Taking photos of (deep)sky objects can be done by one single image but though it is surely possible it won’t get you those astonishing nebulea. For that to accomplish you need to take many photos to stack one on the other and thereby adding to the total exposure time. But that is not all. Your camera has a sensor with its own electronic fingerprint, there is no doubt of dust on the sensor if you have a camera that has no fixed lens so the sensor gets exposed to the outside world when changing lenses. Your lens or telescope has its own vignet shape at different apertures and there is a number of forms of noise created by electronic gain (iso value), heat (thermal issues), hot and cold pixels, exposure time and darkness of the subject. All these things have to be countered and subtracted from the stack of photos (which we call lights or subs) to get a real clean contrasty image.

For all frames use RAW as filetypes.

So there are 4 types of calibration frames:

1. the bias frames (50-100): Lenscap on, fastest shutterspeed you have. Iso needs to be equal to the light frames and temperature is irrelevant.
2. the dark frames (20-25): ISO, temperature and exposure must be absolutely equal to those of the light frames (the photos) but this time leave the lenscap on. You can build a library of these but your sensor ages so very 6-12 months you need to make new ones.
3. the flat frames (20-25): make these with an even lightsource that is a bit dimmed. DO NOT change the lens or even take the lens off during making these and the lights. Also DO NOT change the orientation of the camera or take the camera of the tracker. If so you can (re)move dust on the sensor or even add dust to the sensor which leaves your flats worthless. By dimming the lightsource you can prolong the needed exposure time to give the flats time to gather all of the dust. And… use the same aperture and ISO as your lights because the aperture greatly influences the visibility of the dust on the sensor.
4. the dark flat frames (20-25): these take out the same things as the dark frames but then specifically for the flats. The setting should be the same as for the flats. Though for the flats temperature doesn’t matter… for the dark flats it does so take them while out shooting. And don’t forget to leave the lenscap on! The exposure time should be over a second otherwise leave the darkflats as they can cause more problems then they solve.
 Though some advise to take some 75 darks and 75 flats it seems that all the extra over 20-25 doesn’t add more calibration info and just takes time returning very less extra useable data. My thoughts (not yet tested) are that the more pixels the sensor has the more flats can be useful as the dust particles spread over more pixels on the sensor. Experiment with that!

To be complete: the amount of light frames… the more the better. The deep sky is the limit. Just keep an eye on your capacity of storage and backup and if your computer can handle the amount of gigabytes.

Exposure of the light frames

Now exposing the light frames needs some thought. To short will end up with loss of details because the image is to dark. To long will end up with blown out details because of too much light. So expose for a histogram somewhere around the 20-25% mark on your histogram leaving a good margin on the left (black) side.

Three very good explanatory videos on this subject, each with its own details, that are absolutely worth watching:

Tim Richter calibration frames

Astro Escape calibration frames

Michael Martin exposure and calibration

Pre-processing

When all files are complete they must be pre-processed before they can be processed. Logical but crucial. All the calibration frames have to be stacked to create their respective master frames. These master frames are then subtracted from the stacked light frames which obviously result in a master light frame. To create this master light frame all the stacked lights have to be aligned first in order to exactly fit all those images together. For this a program looks at certain stars and takes care of fitting one on the next image by those guiding stars. This all will result in a clean master image with which we can process in any good photo editor like Photoshop or Gimp.

Stretching

The first thing to do now is stretch the image. This stretching is literally stretching the histogram like a rubber band. This can also be done with a single file astrophotograph of course. The only limiting factor is that a single shot image will contain (much) less information but it will certainly work.
Before you start this part: many stackingprograms also do stretching and they do a failry well job so you may want to skip this part if you have such software and know how to stretch within that program.

Stretching the image is done by adjusting levels of he image in Photoshop.

1. Load the file into Photoshop
   
2. go to Image -> Adjust -> Levels
   

   Start by sliding the left slider to the left foot of the graph and the right slider about halfway the right foot of the graph and save the result. Go by these halfway steps because going all the way up to the right foot of the graph will result in early loss of data. Keep repeating this until details emerge from the image where they weren't before. This process is one of learning by doing.
   

For the best result working in 32 bit RAW on a 40+ megapixel camera is the way to go. But it is absolutely no necessity for nice first time results. Stretching the file is as simple as it is: data, loads of data! the more data you have the more you can stretch it. Following are 3 graphs with the effect on 32-bit raw files of a 42 megapixel file and 3 8-bit jpg files in sRGB from a 10,5 megapixel file. Each series representing steps 1, 3 and 5 of the process. The difference shows a loss in remaining data being details and tones and contrast in the resulting image. You can see that in the shape of the graph and the jagged line of it.

After the stretched image has been given its contrast back there is a difference in details.
On top the 32-bit 42mPix raw and under the 8-bit jpeg sRGB 10Mpix.