Saturday was great. It was 54 degrees in Chicago, in January. I was otuside in just a sweatshirt and no gloves getting things prepared. In testing I found some play in the gears in both Dec and RA, and promptly fixed it in the daylight. Collimation was dead-on, mirrors and lenses and sensors were all perfectly clean. Despite the large moon, the night was set to be clear until dawn. The sun finally set and nothing worked. From 6:15PM until 2:30AM I fought to get exactly 10 usable 3 minute exposures.
I spent nearly four hours trying to get my polar alignment sorted out with drift aligning. A quarter turn of either the azimuth or altitude adjustment knobs sent the guide lines way off, to a spread of several hundred pixels. I would stop, and restart the drift without adjusting and have a 2 pixel error, then stop and restart again and have a 600 pixel error.
I had refocused the guide camera earlier and rotated it to allow for better cable routing. Yes, I ran the calibration, but it didn’t seem to stick even though it said it did. I reran the calibration and received the message that my calibration had changed significantly, and was asked to replace the old calibration. So now it worked, and I thought I was able to reasonably drift align.
Just when I thought I had it straightened out and started taking exposures, the guiding was all over the place, jumping eight or more acrseconds suddenly every 20 seconds or so. It couldn’t be backlash. I really had everthing down to zero play in the gears. So I ran the guiding assistant, and was informed by PHD2 that my polar alignment was 34 arcseconds off! This is after fighting with drift alignment to tell me I was roughly one arcsecond off.
Despite the size of the moon, I was shooting away from it into a clear and relatively dark sky (as dark as it can get near Chicago). However, from my back yard, I have to drift-align to the south and east… right into the moon. It was about midnight before the moon had moved west enough that I was able to align and was confident that it was OK enough to image.
Guiding was still all over the place, but I noticed the guide stars fading in and out. Without a cloud in the sky I realized I was chasing the seeing. Seeing was forecast to be average all night, and I noticed minimal “twinkling” of bright stars, but it must have been poor at best. I don’t have the chart for Saturday night, but see the chart below from cleardarksky.com It’s the most accurate sky forecast I can find. It’s not perfect, because it’s nearly impossible to determine all atmospheric conditions for a specific location, but it will definitely help you.
Seeing goes from a scale of 1-5. 1 being bad, and 5 being impossibly excellent. Good (3) comes around here once in a while and rarely do I see an Above Avergae (4) come around. The forecast for tonight is bad-to-poor and it looks like it will slip into poor tomorrow night. This has been typical all Winter in my area this year, but I’ve actually imaged in these conditions and gotten OK results. Generally, reading forecast for Cloud Cover, Transparency and Seeing can be seen as white=bad, and the darker the blue gets, the better. You can hover over individual squares for the detail.
The easiest way to define seeing conditions is that it is related to atmospheric fluctuations. It’s what makes stars twinkle, shifting light ever so slightly as it travels through our atmosphere to our eye, or camera. The forecast is an estimate of what is expected and a guide, but not always accurate. There are just too many factors to really accurately forecast beyond a few hours out. Cloud cover and transparency are easier to forecast, and generally more accurate.
When people talk about chasing seeing, it means that your guiding software is trying to track on a star that is dancing around. Your mount might not be off, but all the guide camera can do is report on the star that’s shifting in the atmosphere and try to move your mount to counteract, which it can’t do. It’s a nasty circle that makes you want to throw things!
So back to the night. At about 12:30 AM the seeing problems eased a bit and I was guiding within a couple arc seconds. . I decided to wait a half hour for M81 & M82 to cross the meridian. I slewed over, got them framed nicely and got in three 180 second exposures with perfect stars. then the infamous red flashing PHD2 telling me the star was lost, and a white fuzzy screen… Clouds that weren’t supposed to be there. I checked out a high-resolution infrared radar that showed they should pass within 30 minutes, and they did, until more crept North 20 minutes after the first round passed.
That’s when I called it a night – eight hours after setting up with only 30 minutes of exposures captured. My lesson learned is to accept what the night is giving me. There are times you just can’t fight through it. Sometimes what looks like a good sky just isn’t manageable. I should have packed it in by 8PM and enjoyed the evening with my wife instead of half-paying attention to the computer screen and half to our binge-watching of American Horror Story! If you see your guide star fading in and out and you are 100% certain that no thin clouds are around, the night might be a lost cause.
These nights happen to everyone in this hobby. It doesn’t have to be the seeing conditions. Sometimes there are thin clouds you can’t even see with the naked eye, or too much moisture in the atmosphere, which affects seeing and transparency. Even though I live in an extremely low crime area, sometimes a raccoon keeps setting off the neighbors one million watt LED “security” lights. And sometimes equipment just doesn’t work and you never figure it out, because it works the next time out.
The best thing you can do is to realize early enough that some nights just won’t work, and the sun will go down the next day. It might be clear, it might be cloudy, but night always comes and there’s always another clear night… some advice I need to start taking myself.
Clear Skies – KA