Overview of all the fuss about getting up and running for a deep sky foto session.
It's going to be in the dark, chances are even in summer that it's going to be fresh, if not outright cold. You'll need to get dressed accordingly. Unless it's high summer, chances are you'll want winter clothing, with long (fleece or wool) underwear, woolen socks, several layers on your upper body. Don't forget to cover your head incl. ears. You'll also want bicycle gloves (those you can get your fingertips free for action, cover them for the rest of the time).
You'll want a red flashlight, preferrably a headlamp. A standard flashlight can be helpful when packing up, in case you're not disturbing others.
It may be useful to have some sort of mat to put on the ground in case you need to lay down or somesuch when fiddling around the equipment. If you've got a big Newton and/or are of small stature, a small ladder or stool can be very useful.
You may want hot drinks such as tea or coffee, or simply have some water or other softdrinks. Don't do alcohol, it will interfere with your vision, and may do so with your judgement or your driving back home - your equipment and car are simply too expensive for such indulgence.
When you're all set up and everything is running automatically afterwards, you may want to occupy yourself otherwise. Possibilities could be visual observation using different equipment (maybe a Dobson?), or using binoculars. Some people pass the time reading astro-related Facebook groups or specialized websites. Especially if it's a season for shooting stars, get down on a lounger and admire the sky naked-eye.
If, like me, you're in town with a lot of light pollution, chances are extremely high you won't want to set up in your back yard.
There can be exceptions though, when I have to get up early the day after, or conditions aren't optimal, I may choose to do so anyway. If your back yard (or whereever) is somewhat sheltered against street lights, you can still choose a bright target and take pictures of that, especially when there's no moon and the sky is quite clear otherwise. Such bright objects are the Moon and some planets or the Sun itself (using a sun filter obviously). Some deep sky objects are also quite bright, e.g. the great orion nebula, the Pleiades, the Beehive, the Perseus double cluster etc. Don't count on too much for the nebulous parts though.
An advantage is that you have other resources available that when in the field. You may be able to use an electric port, ac/dc converter, possibly a laptop etc. that you couldn't or wouldn't take along otherwise.
If you're lucky to be in a dark countryside, you can set up your stationary observatory right at home, which invalidates most of what I'm writing here.
To go somewhere out of town, you'll need to find a nice dark spot, accessible by car (so as not to have to carry heavy equipment), away from car or other lights, with a clear view to your objects of interest (no mountains, buildings, trees, power lines etc in the way). If you can, get up a mountain, as the higher you are, the less atmosphere the photons need to get through coming from the stars to your camera. Stay away from largers rivers or lakes, as there's bound to be a lot of humidity. Light pollution maps can be found, use them. Oh, and try to avoid routes of airplanes if you can. Solid underground will help, so as to avoid surprises with the mount's tripod legs sinking into mud.
Which equipment to use depends on the intended target as well as the meteorological conditions.
In case of really bad seeing, there's little point in trying for good pictures of planets. If it's windy, you'll skip using a heavy, long telescope on a mount that will not hold it strongly enough.
For small objects, you'll want a long focal length, but that will require precise positioning, tracking and guiding. For larger objects, you'll possibly have to use a different scope or even go to a tele lens.
For really wide pictures, you may have to go down to standard lenses (e.g. 50mm for all of Cassiopeia or most of Orion) or even wide-angle (e.g. Milky Way).
There's of course the option of using a Barlow lens to increase focal length, or a reducer to decrease it. Reducers may include a flattener, which corrects some distortion of a refractor. A Newton needs a coma corrector to get proper stars over all of the picture.
In any case, your mount needs to hold your equipment so as not to vibrate a bit, or else your pictures will contain star trails instead of dots. It is the mount that is the really expensive bit of it all. Most stores will gladly sell you whatever mount with a big scope - don't fall for that! If the mount is supposed to be able to take on scopes of x kg, be aware that it is an extreme figure, for visual use at best. You'll need to count additional equipments to the telescope itself, e.g. camera, additional lenses, finder scope, guiding scope, autoguider, possibly heating sleeve etc. Then, if your telescope is long, there's the angular momentum to count. You'll be well advised to not go above about 2/3 of the indicated weight capacity.
If you're having issues with transporting heavy stuff then, be aware that between battery, mount, telescope and all the additional equipment, it really adds up. For going into the field, that may be ok if you can get there with all the hardware in the boot of your car, and even then it can be tedious. Don't overdo it, or else you'll not want to fiddle with it all and end up with lots of big equipment going to waste. Better to have smaller, lighter stuff that you care to use.
Obviously, the choice of target and choice of equipment are linked. If you can't use your “big” scope because of wind, there's little point in choosing a very small target. If conditions are good, and you want to go for a small target, do use the big equipment, otherwise the small object will hardly be visible at all. If you want to picture, say, the rosette nebula, using a 1000mm focal length will show you only its center - if you want all of it, a focal length of about 500 will be fine.
Some objects can be seen only during certain seasons, rather close to the horizon. If the sky is playing along, you can try for those objects. Also interesting for good skies is the more difficult, less bright objects. If the conditions aren't good, you may want to go for a target closer to zenith, to have as little light pollution as possible, or at least brighter objects.
Find a patch of solid, non-slippery ground. See where north is (or south, if you're below the equator). Set up your tripod accordingly, make sure it is at least approximately level. Put up the mount, make sure it does point towards north (south), add the eyepiece holder plate (which provides more stability to the tripod and mount).
Now, use the polar finder for precise orientation towards true north (south). Polaris is close to true north, but not quite - it is actually circling true north. To find the proper location for Polaris on that circle, I find the Kochab method to be useful - check the relative position from Polaris to Kochab (the brighter star of Ursa Minor's “box”), the same direction is where Polaris needs to be on that circle. This should be done with high precision.
My polar wedge is set for my approximate location. Since its mechanics are known to be weak, I tend not to touch it. I make sure to have the alignment correct left/right, then fiddle with the tripod's southern leg until I'm good with the height.
Make sure all screws for the tripod legs and connection to the mount are tightened.
Once the mount is there, you can put on the telescope. Make sure it's approximately balanced on both axes, but be aware that you'll be adding weight when putting on finder scope, guiding scope, camera. Be sure your dovetail is properly set in the clamp.
Using a crosshair eyepiece, make sure your finder scope is properly aligned with your telescope.
If your mount features only tracking (e.g. Skywatcher EQ-5 or EQ-6 Syntrek), you can skip this part.
If you have GoTo features (e.g. Skywatcher Synscan versions), you'll need to to a star alignment. You need to first put the scope into “parking position”, which typically is pointing towards north. After turning on the mount, you need to provide it with basic data such as position coordinates, date and time. Then proceed with star alignment - 3 star will be the most precise, and since you're in the field rather than in a fixed observatory, that's what you'll need. The GoTo will propose a star to navigate to, depending on your location it may sometimes be necessary to change the proposal if a star is not visible due to some object being in the way. Once the mount is done with going to that star, you can fine-tune the positioning - make good use of the finder scope for starters, then of the crosshair eyepiece once you get close. It's the same exercise for the second and third stars, but it should already be much closer once the first star is done. If all went well, the GoTo will inform you of successful alignment.
Now, you can use the GoTo to select an object and navigate there, you should at least get close to the mark.
Once the GoTo is set up, navigate to a bright star and replace the crosshair eyepiece by your camera. I'll write about a DSLR as that's what my experience is with.
Case Newton: I have a 2“ focuser, fitting a 2” coma corrector featuring a T2 connection towards the camera. Using a T2 adapter, I can put on my Canon camera on there.
Case ED refractor: I use this with a 2“ 0.8x reducer/flattener which features a T2 connection. Again, using a T2/EOS adapter, I can fix my camera there.
Case FH refractor: my coma corrector also corrects some of the chromatic aberration of the FH refractor, so I tend to use that there, too.
Once the camera is connected, I can visually look at the bright star and try focusing for it. Once I'm in the ballpark, I can turn on LiveView and get quite close to a good focus - close, but not perfect. Use a Bahtinov mask (you can buy one or make it yourself) in front of the scope - it will transform the star into a bright dot with 3 lines on each side. When focusing, you'll notice how the middle line moves around - focus so it's smack in the middle between the other two lines, now you have perfect focus - tighten any screws fixing the focus to stay in that position.
This should be easy if the GoTo setup was properly done. Select the object you want from the GoTo object list, and have GoTo get you there. Taking a picture, say a shortish exposure at high ISO (e.g. 30s ISO1600 for my f/5 scopes) should show you if you're in the right place. You may want to move around a wee bit to have the object in the proper place of your frame - I usually try this looking through the camera, if there's bright enough stars for reference, or looking through the finder scope, repeating until I get a good test frame. It may even be necessary to turn the camera a bit, but you'll have to re-focus, i.e. it may be necessary to move back to a bright star and back to the object of interest.
Note that if you want to get a couple of targets onto the same picture, you'll have to point at some point in between them. Getting prepared using e.g. Stellarium, you may find a star there that's in the SAO catalog and brighter than mag 8, in which case the SynScan will have it in its database - so you can point at that. Otherwise, you can note the RA/DEC coordinates, and navigate to there via “user object” - this may also help with objects from catalogs not known by the your mount's database (in the case of SynScan, the Sharpless catalog is such an example).
Hopefully, you'll be equipped with a good atlas, and are familiar with star hopping. A finder like a Telrad will help you get into the proper region, the finder scope will show you quite some objects (depending on conditions), and you can then “hop” from known references to close other stars until you're in the right region for the object of interest.
As for the GoTo case, test frames will show you whether you've navigated correctly, if you can't make out the target visually in the finder scope.
Turning the camera, refocusing will of course mean you'll have to re-do the navigation, which can be quite tedious if you aren't yet familiar with that particular target.
If you're not using guiding, the precision of your tracking is the critical limiting factor. Obviously things depend on the focal length of your scope. I've found that with my 8” f/5 Newton and my NEQ6, I'm limited to no more than 4 minutes of exposure on a good setup, more likely 2 minutes. At that rate, chances are I'll have to go to ISO1600 to get half-decent results, despite higher noise levels.
If you can use guiding, the limiting factor will be the brightness of the background. You'll want to expose so that your histogram will show signal as high as possible, without getting cut off at the right. You can lower ISO values so as to have less noise, but there's little point in going to low values such as ISO 100 or 200. It is preferrable to have a higher number of slightly shorter exposures rather than few long ones, so stacking with sigma can properly compute noise etc. to get a proper picture. That being said, long exposures are preferable to shorter ones if you want to get decent “depth”. Very short exposures will give you essentially no signal, multiplying nothing by a great many exposures will still give you nothing. So the trick is to get a proper balance. With the sky I usually have around here, 5 to 10 minute exposures at ISO 800 tend to be approximately right, but can be too much if the target is close to the horizon or conditions are bad. With luck I can go down to ISO400 for less noise, but I'll have to get more “lights” too. With longer exposures, I'd be in trouble getting a sufficient number of “lights”, and would increase the chance of lights being polluted by airplanes or satellites.
Besides the actual “lights”, the stacking software needs more for proper results:
The flat frames must be done on the exact same setup as the lights, so they are best made immediately after the lights. The bias and dark frames can be made afterwards, when the camera already has been disconnected. The trick though is to have the camera at the same temperature as when taking the lights, so plan enough time especially for the darks, as you'll need some.
The guiding equipment is set up along with the telescope, making sure all is properly balanced.
I go with an autoguider, the Lacerta MGEN II, as I don't want to lug around a laptop in addition to all other equipment - though if that's an option for you, a relatively simple cam with the proper software can take the job (I read PHD guiding is good).
Around the time you use a bright star for focusing is a good moment to turn on the autoguider, to give it proper parameters for date/time etc., and to run the calibration (so it knows how to correct the two axes). Don't forget to take the cap off the guiding scope! On first use, make sure to properly configure the focal length of the guiding scope.
When the target is properly framed and the exposure parameters are determined, you can activate the guiding, supposing you can find a bright enough star to hang on to. It may be necessary to fiddle somewhat either with the parameters of the guiding cam (e.g. gain), or slightly reorient the guiding scope.
The MGEN also allows for controlling the DSLR, including mirror unlock. In this case, you'll need the proper cable from MGEN to the camera. A nice option to use is “dithering”, i.e. the MGEN will slightly move the mount between individual light frames. This is done so as to eliminate some of the noise, and can apparently make darks unnecessary - though I haven't yet got around to trying that properly.
If you get a message that contact with the camera is lost, chances are the MGEN isn't getting enough juice. I'm not yet sure if I can simply “up” the setting on the DC/DC converter in such a case. I've had the mount continue to work quite a while after the MGEN had this issue, the battery sure wasn't empty yet.
I tend to run series of exposures, using the same parameters. Such a series, à la n exposures of time t, with a delay d between exposures, possible with a mirror lock wait m, is configured either in the MGEN, or in a programmable remote switch (mine doesn't so mirror lock, unfortunately), or if you've got a Canon EOS better or equal 500D, in MagicLantern (additional Firmware).
In between series, I can check whether the parameters and focus are still good. I also may change to another target.
You'll need to keep an eye on your equipment while it's running - make sure the autoguiding is still working, that no clouds have come up covering your target, that nothing's frozen over, that the focus is still good (as it can wander with equipment changing temperature). I tend to run series of x pictures, and check in between series whether all is still good.
If you target something around zenith, a meridian flip may be necessary - otherwise, the scope may hit a leg of the tripod. Essentially, it means the mount will flip the scope from the western to the eastern side, requiring both the RA and DEC axes to be turned by 180°. I didn't implement this yet, the SkyWatcher SynScan hand provides for it. Obviously, this needs to be done between individual pictures, also I'm wondering whether the pictures will be 100% aligned (which may be solved using plate solving, a technique I haven't looked into yet).