Is it worth shooting through whispy clouds? - Forums

Topic starter

Jul 18, 2022 · Edited: Jul 18, 2022

For the last 4 nights the weather forecasters said the clouds would be clear.

On each occasion I have had constant whispy clouds in front of my target all night.

I can guide at 0.7 arc secs RMS despite this (my pixel size is 0.865 arc secs).

Is it worth my while capturing frames through this cloud or should I wait for clear skies which we very rarely get?

I'm just wondering if I add these whispy cloud frames to my M51 image will they improve it or make it worse when I integrate them?

and this is an example of a cloud passing by

Engaging

Jul 18, 2022

If you can image the target through them, give it a go, but only integrate the data into data from other nights with those wispy clouds.

M31 from last night with lucky (459 x 0.7s) imaging and a few normal exposures through those same types of clouds you discribed at 0.64"/px and 0.5" guiding:

This is ~14 minutes of integration.

andrea tasselli

Frequent contributor

Jul 18, 2022

Did the same (after all we are in the same island…). So yes, if the subject is sufficiently bright relative to aperture and sampling. And as long as you apply proper weighting in the final integration. Wispy clouds are no bar to imaging, IMO.

Christian Koll

Jul 19, 2022

Andy,

as long as you don't have bloated stars, shooting through whispy clouds should not do any harm.
The longer you expose (expecting the clouds to pass over your entire field of view), the less brighness gradient is introduced.

Have a look at the data to disregard the worst frames and all of those with bloated stars.
In the integration, weigh the frames quality wise and use local normalisation correction (choose a reference frame that has no clouds).

I imaged narrowband through whispy clouds with the almost full moon up, using a 3nm Ha filter with almost no apparant difference to the frames without clouds.

Chris

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Michael von Berner-Purgstall

Jul 19, 2022

Spare your time! ANY clouds would harm the result - you'll have to deal with less resolution, less colour but a huge gradient and bloated stars, not worth, better go to bed.

John Hayes

Frequent contributor

Jul 19, 2022

Thin clouds can cause all kinds of problems but if you are very careful, you might be able to use the data. As Michael pointed out, you can get gradients and the clouds can make color calibration more difficult. Clouds alone shouldn't affect image sharpness but they can introduce a "glow" (bloat) around the brighter stars, which can be difficult to remove if you have too much data taken through wispy clouds. You will also have a lower SNR from data taken during periods of lower transparency. You can get an image through high, thin clouds, but it's a good bet that it won't be very good.

John

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andrea tasselli

Frequent contributor

Jul 19, 2022

In an ideal world we all have our remote observatories high in the Atacama desert. Reality, alas, is Britain is a poor choice as far as transparency is concerned never mind clear skies and you get what you get. Either that or move the telescope elsewhere.

GalacticRAVE

Jul 19, 2022

Well, according to common wisdom it takes 10000h of training to become really good at a complex task - like mastering an instrument - and this is probably true also for our beloved astrophotography. So even if the data may be suboptimal, it makes you more familiar with your equipment, your software, the sky, gives opportunities to try something new … and at least in European climates north of the Alps, life may be too short to get 10000h of excellent nights… So if you feel like it, go for it!

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Andy Wray

Topic starter

Jul 19, 2022

To be honest, I think I will in future look at the FWHM figures for a few frames and decide if it's worth continuing. Last night was really poor with FWHM over 4.0 when I had had figures of around 2.8 on a recent previous whispy sky night. Last night's frames will be binned, but I may keep the ones from a few days ago.

Rick Veregin

Jul 20, 2022

It might be clouds, seeing, wind, moonlight–whatever disfunction there is, you need to make a decision.

To make a decision you need data. You need to get to know each target, based on brightness, altitude range and detail that it contains. If the target is low in the sky FWHM will be much higher than a target overhead, and skyglow will be much worse. So what is good for FWHM and skybackground or a low target is bad for a high target. If there is little detail, FWHM is not so critical–but getting enough exposure is. If the target is faint, you will not worry so much about detail and FWHM, but clouds, full moon will kill what you are trying to do. If the object is bright with detail, worry about seeing and wind, skyglow and light clouds will not be a huge issue–though you might need to fix halos around bright stars. For examole I image globular clusters when the full moon is in the way, not faint nebulosity.

I generally need at least 5 nights for a target, often more. So initially you don't know what can be acheived for this target. Thus for a new target go out unless it is totally hopeless, take images and record FWHM, background, star shapes. In fact a bad night is good to start a new target, you can use the time to get the framing, settings, and see if it at all looks hopeful to try when conditions are better. And it may turn out the data was perfectly usable. After a few nights you realize this is not up to par and quit,or this is a great night, go as long as you can. And remember if you get 50% retention of good images on a bad night of 6 hours, that is equal to 100% on a good night of 3 hours. For my local conditions, 3 hours of good data is golden, so I go for the 50% for sure. But if your yield of usable images is 10%, go to bed…

Finally if there are a lot of clouds, poor seeing or wind, shorten exposures. Your yield of usable images will go up, as the clouds move quickly in your field of view, they will average out well with shorter exposures. The exposure time may not be ideal for noise, but if you don't go out and get any reasonable exposures, you will not improve your S/N. You must collect some data, even if it may not be perfect in all respects.

Put all your data together and discard what is not working–I find once you have 20 hours of good exposures, it is not difficult to discard 3 hours that are not up to par, even though at the time it was hard earned data.

In short be generous in what images you take, be critical of what images you keep.
Clear and steady skies
Rick

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