In this series of articles I try to answer the question "How do you make these photographs?" which I am often being asked. Because of the complexity I thought perhaps it was a good idea to write a series of articles covering the topic. There are a quite huge amount of material available on the net explaining the theories behind, so I decided to provide a step-by-step description of what I do, and at each step I will explain the reason why I do it in that way. My main focus in astrophotography is deep-sky imaging, being the field I feel to have enough experience to write about. Although Solar System or planetary imaging is another area that could be of common interest, I do not have yet enough experience in it to be the proper person to give description about.
Creating photos of deep-sky objects is a complex and lengthy process which begins with planning and preparation, and finishes with the publication of the result. In this part of the series I concentrate on field work right after arrival until the telescope is set up completely to be ready for taking pictures.
Thermal equilibrium
To achieve good image quality with lot of details and small stars in the picture, your telescope has to have the same temperature as the air around or at least it should be as close as possible. A large lens or mirror may need hours to cool down, especially in winter times when outdoor air temperature is low. Therefore my first task right after arrival is to take the scope out from the car and let it cool freely, without any packaging while I do other things. Sometimes even 2 hours of cooling is not sufficient for my 10" F/4 Newtonian, when outside temperature is below 0 °C. It also worth to put all the elements of your optical train out for cooling. Lenses may change their focal length as they cool and contract and you will have to chase the right focus continuously until they reach thermal balance.
The tripod and the mount
The OTA is cooling, so I can go for assembling the mount. I place the car battery on the location I plan to install the scope, then I assemble the tripod above it. Despite common believes, the platform of the tripod does not have to be perfectly horizontal, however leveling it may help during polar alignment. Newest SynScan firmwares contain a polar re-alignment function which indeed requires you to level the platform, so does drift alignment methods, but if you are in a hurry and satisfied by the accuracy of your polar scope, you don't need to level the tripod at all. I also put wooden discs under the legs of the tripod to increase contact area when I set the scope on loose ground like sand or grass, otherwise as the load increases on them they tend to sink in the soil, ruining polar alignment.
Polar alignment, balancing, collimating
Next I put the mount on on the tripod, and do a quick but accurate polar alignment using the polar scope, without having the counterweights and the OTA installed, because my OTA is quite heavy and it is difficult to set the elevation of the RA shaft when it is under load. Then I put the weights on approximately at their correct position, and I install the OTA. Next I put all the accessories on the OTA, exactly the way I'll use them during the night: the guide scope with the guide cam, dew cap, all the cabling, etc... even the camera with the coma-corrector. I need them all to be able to precisely balance the mount. First I balance the declination shaft, then the RA. When everything is in balance I remove the dew cap and the camera, and collimate the mirrors. It is important to do the collimating after the balancing, because the movement and forces being applied to the OTA may distort the tube slightly. The collimation method you choose doesn't really matter, given you can set your mirrors almost 100% true. Personally I prefer CatsEye Collimation System.
Star alignment
When I finished collimation I get my eyepieces and do a very accurate 3-star alignment, centering all three of the alignment stars using my guiding eyepiece with the reticle. At this point the scope has already cooled for an hour or a bit more, but usually air disturbance over the mirror is still noticeable. Accurate 3-star alignment is necessary to easily re-frame your subject after a meridian-flip. After successful star alignment, if I plan to have the scope installed for more than one night, or the mirrors are still not cooled down, I do the polar re-alignment provided by SynScan to minimize the polar-alignment error of the mount. Then I do another accurate 3-star alignment.
Composition, focusing
Now the telescope is ready for use, except if the mirrors are still warmer than the air. You can see that in the eyepiece watching the picture of a de-focused bright star. Anyway, I put the camera with the corrector in the focuser, locate a bright star and set the focus using Bahtinov mask. Then I locate the object I plan to photograph and set the FOV according to the plan I created during preparation. To set the FOV I do the following cycle: Take a test exposure and check it, move the scope if needed, take another test exposure, check again, rotate the camera in the focuser if needed, check, etc... When the intended framing is achieved I recheck the focus by taking test exposures with the Bahtinov mask. I use a small 10" netbook to check test exposures, because the small display in the camera can't show all the small details I have to pay attention to. I also avoid touching the camera when the focus is set, to prevent any displacement. I use a USB cable to connect the camera to the netbook, and I only touch the PC end of the cable. Camera is controlled by the PC during test exposures.
A few additional words on focusing
During focusing the goal is to move the detector in the Critical Focus Zone, which is a small range of distance around the focal plane of the telescope. The size of the range depends on the focal ratio of the telescope and the pixel size of the detector. With my F/4 telescope and Canon camera the range is less than 40µm wide. Optimally, to get the sharpest image the centre of that range have to be reached and maintained during the imaging session. Fortunately with the high precision focuser on my scope and the Bahtinov Mask I can do that. Related article: Comparison of focusing masks
The whole procedure described above takes about one and a half hours for me. After having everything set properly the imaging session may begin. I think none of these steps can be omitted, because it can easily ruin the whole night, resulting several hours of unusable recording due to collimation error, polar alignment error, being out of focus, etc...
