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Michael, thank you very much. Just what I needed to understand. I’ve been doing this hobby for almost 5 months and have yet to a decent (shareable) image. I have many useful nights by too strictly following the Astrospheric app!!? Tom

--- mherman346@... wrote:

From: "Michael Herman" <mherman346@...>
To: [email protected]
Subject: Re: [Losmandy_users_io] Astropheric Question
Date: Sat, 16 Jan 2021 08:21:53 -0800

Wow...good question, Tom and Barbara.?

The answer depends on what you are trying to image.??

As you said: if the sky is perfect, you can do anything you want.? But some things are possible even when the sky is not perfect.? And...the sky is always changing so you can have perfectly useable conditions for part of the night, not necessarily all the night.??

I use the web based app "clear sky chart" like this one for my area.? ?The same "clear sky chart" can be found for many places around the?world. I use this version:?

Clouds matter but so does atmospheric turbulence (called the "seeing" conditions).?

That chart shows multiple atmospheric conditions and also explains what the color coding of these parameters means for different imaging.??

In the notes for the different factors, the app explains why the telescope aperture matters as well.? Smaller aperture scopes generally are looking through a smaller diameter tube of air, and this small volume of air is less affected by air disturbance than experienced by larger diameter aperture scopes.???

If you are doing planetary imaging, you are trying to focus on a bright but tiny region of the sky, at high magnification.? In that endeavor, the key factor is "seeing" or atmospheric fluctuations.? It helps if the sky is cloudless, but sometimes you get very good "seeing" on nights that turn cloudy.??

If you are doing long exposure deep sky imaging (wide field of view compared to planetary), then "seeing" is less important a factor.? Factors affecting your long exposure results are the quality of your telescope mount (periodic error and autoguiding) and stability of your optical system (mirror flop in reflectors)?

However, if you use infrared imaging, with a special filter like a hydrogen Halpha filter, then you may get very good results even if the atmosphere is humid, hazy and seeing is poor.? That is because infrared long wavelength light passes through the atmosphere with less scatter than the shorter wavelengths (blue is the most scattered of the visible wavelengths...which is why the sky appears blue...UV would be worse).??

Of course, if a really thick sky cloud passes in front of your object, you will not capture anything but a blur.? Same if your lenses get coated with dew droplets.? Same trouble when satellites or planes fly through your image.? So using a stack of shorter "subs" allows you to throw away damaged subs and still get the results of a very long exposure perfect image.??

Anyway, this is an experimental hobby.??

The more you use your equipment, and try different settings and exposures and stacking methods, the better your imaging technique will be.??

Even on a night of poor imaging potential, just tracking (not autoguiding) a star near the meridian and equator intersection can be used to measure the RA drive components and to help you further perfect your mount performance ("PE").? (See the free program PECprep from the EQMOD group...it can be set to Losmandy mounts and analyse their PE.)

Try your luck and have fun.? And....remember to sleep some times!

Best,

Michael

On Fri, Jan 15, 2021, 7:25 PM Tom & Barbara Coverdale <tcoverda@...> wrote:

I try to follow Astropheric imaging reports for Cloud %, Transparency and Seeing. My question is what is the target? I know 0% clouds, Excellent Transparency and Excellent Seeing is GO time but what about 1-2% Clouds, Above Average Transparency and Good Seeing? What is an appropriate range of conditions to be out trying to get everything working and imaging?