Astrophotography - Procedures

This page describes typical activity related to imaging sessions. This is how I conduct imaging, it is likely far from optimal, but nevertheless I hope it may be useful for somebody ...

A few things need to be taken care before the imaging begins. The scopes need to be properly collimated and the mounts aligned. Optical collimation of a Newtonian can be done during the day using a collimation laser. I always do it during the day if I can to save precious clear sky hours. I use a barlowed laser with 2" barrel, as shown on the accessories page. Some people would then proceed to the more accurate star collimation, but I found the this level of collimation is sufficient for typical imaging. C14 is a different story. At present there are no devices allowing proper calibration of an SCT telescope in the daylight, maybe except for an artificial star. Unfortunately artificial star must be placed very far from the scope, the longer focal length the further, which means several hundred meters for C14. It is unfortunately impractical. Therefore I conduct star-based collimation on a slightly defocused real star using short focal length eyepiece. Diffraction rings in focus give the most accurate collimation, but in typical Northeast seeing and with 14" aperture it is not possible to do in-focus collimation. The collimation is done with an eyepiece, before any imaging equipment is attached. I collimate my imaging scopes about 2-3 times a year, after all they are permanently mounted in the observatory.

What I wrote above does not apply to Hyperstar collimation. Hyperstar collimation must be checked and adjusted (if needed) each time it is installed, but fortunately it is easy to do. Hyperstar must be installed along with all imaging equipment first, and the scope must be focused. Then I take a short (20-30sec) image of a star field. The image is stretched  a lot, such that the faint details show up (and bright ones might be burned out). If the stars on the edges and in the corners do not show any evidence of elongation or coma, all is fine. Elongation generated by wrong collimation changes direction and magnitude throughout the image (usually nothing in the center, a lot in some corners), while the one caused by guiding or mount error is the same everywhere. It is easy to collimate Hyperstar using the collimation knobs - but one should make only small adjustments, and tighten the locking screws to prevent rotation. Hyperstar keeps collimation fairly well, but still it needs to be checked each time since it is a bit unpredictable. On the other hand, secondary mirror collimation is preserved very well when installing/removing Hyperstar.

The next step is to focus and (optionally) align the mounts. I use Bahtinov mask for focusing, the process is described on the separate page. Pointing errors gradually grow over time, since the mounts point to the area on the sky far from the original alignment stars. From time to time, about 3-4 times a year pointing errors reach the level I don't like and the mounts are aligned. At the very beginning, just after the scopes were installed, I carried out full 2+4 alignment (full six star alignemnt), but each subsequent time I do just 2 star alignment. When alignment is not needed I choose "Last Alignment", since I never "Hibernate" the mounts. When slewing to the imaging target it is a good idea to use "Precise GOTO", it allows to increase intervals between mount alignments and generally produces more precise results. "Precise GOTO" slews to a bright star near the target, the star is centered (using finderscope and then main optics, with short test exposures) and then the final go to is executed to the imaging target.

All the connections between my scopes, imaging equipment, control room and computers are described on "Imaging Equipent" page. Connecting the scope control via the hand pad port allows to use both laptop and the hand pad. However, in this case, computer can only be used after alignment is done, otherwise runaway slews may result. I don't use Celestron NexRemote program, since it replaces hand controller, and then only computer control (and alignment) is possible. The plus of my approach is that I can always control the scope just standing next to it with the hand controller; the disadvantage is that I need to walk between scope and computer when doing mount alignment (since only hand controller works at this time), but this is quite rare. One could use wireless gamepad for remote control - using Bluetooth connection to the computer - but I never tested this option.

Once the target is acquired the guider program (PHD) is calibrated, final framing adjustments made and imaging begins.

I usually try to choose targets in such a way that they don't cross meridian during imaging, and therefore I don't need to do meridian flip. Of course it is not always possible or practical ... If meridian flip is necessary I set a timer to remind me of the event - I tend to forget about things when observing with Obsession. CGE stops tracking after the meridian  and then waits for an operator, when enter is pressed it will slew to the other side. After meridian flip the target framing is upside down (180º rotation). Meridian flip involves a slew, so some framing will be necessary since the target position will be close, but not identical as before the flip. In order to position camera as close to pre-meridian orientation relative to the target I use Nebulosity's neat feature that allows to mark any point on the images. Actually up to 3 markers can be placed by using ctrl+right_click - and they can be all removed by shift+right_click. I take the last frame from before the flip, rotate it 180º, and place markers on 3 bright stars. The markers persist even when new images are acquired, so as long as the image zoom stays the same, the only thing to do is to adjust scope pointing in such a way that the 3 bright stars are very close to markers.

The same trick can be used for multi-night imaging, in this case the last frame from the previous night is used - and of course no rotation is necessary unless the new session is on the other side of the meridian.

When working on framing I use TheSkySix telescope control to move the scope.