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Seeing is the critical factor for those of us imaging with longer focal length setups. Currently, I need to set up my scope and run the guiding and imaging for a few minutes at the start of an evening to see if the seeing is going to be acceptable. I try to ascertain this ahead of time through the typical "watching the stars twinkle" (naked eye and binoculars), and through on-line seeing predictors for my area, but neither of these have proved to be very helpful. Does anyone know of a tool (laser, etc.) or some other method which does a good job of assessing seeing? |
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| I have been trying to think of something for years. Something that you could hold up to a star and read the twinkle ahead of setting up a full scope. I am hoping to retire on the funds from the sales. However, up until now, I haven't been able to come up with anything. You just have to setup the scope, put in an eyepiece and see if you can pick out a diffraction pattern. Or just say forget it, it's clear, I'm setting up so damn the torpedoes. |
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| There are seeing monitors out there, basically based on scintillation assessment rather than wavefront assessment (which also exist but I suspect just not at the right price). I'd still think that Mk.I eyeball is a good proxy of the former but not of the latter. If is twinkling too much it is usually the case that there no reason to bother with high resolution. In over 20 years in this hobby I rarely found the assessment wrong. The reciprocal is also true, if it ain't twinkling then it is the case to rush out for a shot. Given the weather in the UK is mostly crappy anyway you get what you get and be happy that you gonna get something at all! |
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There are several methods of determining seeing conditions. the Differential Image Motion Monitor (DIMM) and Polaris Scintillation Monitoring method. I was fortunate enough to have picked up an SBIG seeing monitor a few years back based on polaris scintillation. It works very well and being pointed at polaris offers a consistance useful for comparason. Here is an image of the device. I modified it to include a Unihedron sky quality meterr as well, and added dew heaters for those cold nights below dew point.  Here is a graph from the installation showing an evenings seeing.  This provides a data output and a graph useful during imaging sessions. Hope this helps. Regards, Dave |
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| Dave, that is exactly the type of thing that I was looking for, thanks! I was hoping to spend less than $1000 for such a capability, but from a quick search it looks like most of these types of monitors are well above that. In any case, you have given me something to think about. It is nice to know that this capability is out there. |
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Hi Gary, Great glad you found it useful. Yes even used the SBIG seeing monitor was a bit more than 1K. Dave |
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Hey Gary. I've been thinking about this one. Seeing monitor It's not cheap but looks cool. |
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| I've personally never been overly accurate at guesstimating from visual star movement but maybe it is just my not so great eye sight! I do find that following the jet stream and wind direction is pretty accurate to an extent. If the seeing is looking quite reasonable then the longer focal length telescope gets to see the dark skies and the jet stream looks to be stronger or the wind isn't blowing in the "right" direction for the time of year then the wide field instrument comes out. |
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That's an interesting question Gary. My system out at DSW runs at an EFL of almost 4 m so it's pretty sensitive to seeing conditions. Just by watching the guide star and the tracking signal, I've gotten pretty good at judging how good my FWHM numbers will turn out. I also find it interesting to see how my numbers correlate with the numbers reported by the observatory seeing monitor. Unfortunately, agreement is hit and miss. Sometimes the observatory monitor reports superb conditions, which out there is typically a bit over one arc-second and I can watch my guide star bouncing all over the place. Other times, the agreement is closer. I haven't kept records but my impression is that it's right only about half the time--and it almost always reports better than actual conditions. To be fair, I believe that sometime earlier this year, they installed a newer seeing monitor that may be performing a bit better. Regardless, I've almost completely given up on relying on the numbers from the monitor. The monitor will tell you when things get better or worse, but in my experience, the numbers themselves are typically meaningless. I wish you luck in finding a monitor and I hope that you experience with it is better than mine; but, don't get your hopes too high. John |
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| Wow! Excellent work Dave! |
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| @Tim Hutchison That does look cool! My neighbors would probably call the police out on me. But as cool as it looks, it is a lot of money to pay for a cool looking object. @John Hayes John, I really appreciate your wise counsel. As I look more into the cost and practicality aspects of a dedicated seeing setup, it is starting to make my current "setup and see" process seem not so bad, especially if I can't have 90% or so confidence in the results. |
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| Not a ton of experience here, but when I started doing collimation I couldn't help but notice that the airy disk image when de-focussed is sometimes quite lively, other times more stable. I put it down to variations in seeing... |
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Not much to aid Gary but i have some experience with bad seeing, to say the least. First off it's obvious that having a part of your gear somewhat permanently in place would make it faster to set up enough to determine go or no go for the night. My current set up is a pier with the mount and telescope in place 24-7-365. I can see while doing a PA in 30 seconds what the night is probably going to be like, all other things like clouds being equal. Also I think for many of us as mentioned above, guiding is our canary in the coal mine. If my guiding star image is not very good and my graph numbers are weird, I am assured to be disappointed in my images. I find Astrospheric app to be very accurate for overall prediction with only occasional misses + or- in hourly timing. Astrospheric is North America only I believe. Scott 24-7-365.jpg |
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| @wsg Thank you Scott for the recommendation on Astropheric. I have loaded it and will give it a try. |
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I've found more success by taking a larger sampling of sources on seeing predictions, as they can definitely vary. In fact, I check 5 different seeing reports (plus 3 additional ones for clouds) to judge the night ahead of time. Occasionally, one or more of them can be very far off, which is why I use so many. However, usually the mean judgment between them all does very, very well. Here they are: Meteoblue- This doesn't always give results that reflect how my imaging will be, however it contains one of the most important metrics anyways, which is the jet stream speed. Higher speeds (into the red), and it will definitely be worse. Lower speeds, better seeing. Being near mountains, the jet stream is almost always fast, and the seeing almost always terrible. ClearDarkSky- One of the best, bookmark your location and I say its seeing estimates are good 80% of the time. Apps: "Clear Outside"- Great for weather, not heavily-weighted for my seeing predictions though. "Astrospheric"- Pretty good seeing estimates as well. Discussed above. "Astro Panel"- Okay predictions for seeing. Viewing the cleardarksky and astrospheric predictions, coupled with the jet stream information, I can usually know what to expect. I'll say though, it's not an easy science; some of my best seeing was during mediocre forecasts. If you're feeling it, try anyways! I also think looking at the stars does an excellent job of predicting how well the guider would do from seeing. Every single night I do multiple seeing assessments by rating the sky between 1 and 5 mentally, by looking at it, and I think I could reliably predict my RMS error values on PHD2 just by looking at the stars (only look in the area you're imaging; it can vary widely throughout the sky!). Also, always image as high in the sky as is possible. Always. |
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My experience with the setup imaged above using an SBIG seeing monitor and Unihedran SQM has been excellent for more than 9 years now. I have relied on it for instruments up to 1500mm focal length. Reliability is dependent on many factors some of which can be controlled by proper installation, others like natural airflow along the ground, trees, and weather cannot. My device is set 16' above ground on the north facing side a tower. There is a ton of literature, studies and models for atmospheric turbulence, while interesting none is as good as local measurements. Watching guide stars and judging FWHM over time is an excellent local, at the scope, measurement as well Dave |
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John Hayes: John, Does DSW also have a transparency meter? We have a both the SBIG pole star seeing monitor and the Alcor Zenith seeing monitor at SRO. But I found the transparency meter to be more correlated with getting good data. I do find the seeing monitors to be a good indicator of my FWHMs numbers but If the seeing is good, and the transparency dips below the norm, I get soft stars. Kevin |
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| We've always found that nothing beats understanding your local environment. We can look at a few weather models are have a fairly good guess if it is going to be above average (longer focal length) or below average (shorter focal length) for the night and be quite accurate on that. Before astronomical darkness we have a fair idea of what the seeing is going to be like for the night and pick targets accordingly. To add complication some parts of the sky have better seeing than others and that changes as the night progresses. |
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Kevin Morefield:John Hayes: Hi Kevin, DSW does not have a transparency monitor. I’ve given up asking, but for a long time, I’ve been advocating for a much better sky cameras so that I can assess both transparency and cloud locations. The northern NM sky is rarely totally clear, calm, and transparent. (In the winter, it’s definitely better than central Oregon, but central Oregon totally wins in the summer...except for the shorter nights!) I’ve considered attaching a wide field, astro-camera/finder-scope to my scope so that I can use it to have my own wide field view of the region of the sky that I’m pointed at. I may experiment with that idea on the 20” scope—although it’s destined for a location that generally has much better sky conditions. It’s interesting that you see soft stars when the sky transparently dips. I don’t have hard numbers but I often notice that it’s not uncommon to have very little turbulence when the sky is a bit hazy. The haze often generates gradients, which overwhelms the advantage of tight stars that I usually get under those conditions. John |
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John Hayes: My terminology may be a bit sloppy here - the softness is a bit of a glow as the star is diffused through the haze. And yes, the real issue is gradients. The glows seem worst on the blue end of the spectrum as well. Regarding the Summer in Oregon; I often laugh at myself in June after I drag my equipment out to Goldendale or OSP and only then remember there’s about 3 hours of full darkness. |
