SCT and poor seeing

[Capella_Ben (Ben)]

Am I right in assuming that an SCT with 2000-2500mm focal length won't be that effective in average to poor seeing?

Am I right in assuming that you would still need to deal with light pollution the same as any other focal length scopes?

I'm just thinking about what scope I might like to move to next.

Thanks

[Merlin66 (Ken)]

Not sure what you mean about effectiveness....
The seeing disk at longer focal lengths will always be larger. This needs to be taken into account with any camera you hope to use to get best sampling.
I regularly use a C11 @f10 for spectroscopy using a 20 micron slit gap and can easily guide (NEQ6+ PHD2) for at least ten minute subs.
Light pollution is always an issue.

[Capella_Ben (Ben)]

Good point. By effective, I really mean for photography.

Just worried about being able to get sharp clear images. Yes poor seeing will inherently give poorer images, but can you get at least usable images in average seeing or are you wasting your time and it will only work in good seeing.


I don't know much about spectroscopy, but I'm guessing if you can do that then simple photos will work OK.

[Merlin66 (Ken)]

A lot of very good quality planetary images are taken with large SCT’s at even longer focal lengths - f30 or so....
The seeing will affect all instruments. The key would be to match your camera pixel size to maximize the success. Lucky imaging using fast frame cameras can give you the potential to grab those moments of better seeing.

[multiweb (Marc)]

Originally Posted by Capella_Ben

Am I right in assuming that an SCT with 2000-2500mm focal length won't be that effective in average to poor seeing?

Am I right in assuming that you would still need to deal with light pollution the same as any other focal length scopes?

I'm just thinking about what scope I might like to move to next.

Thanks

I do deep sky imaging at 3m FL occasionally and you really need to pick your night. No or very little jetstream is a must and no surface wind also goes a long way with tracking/guiding. That said at that FL light pollution and gradients are much less of an issue. My C11 has a very long dew shield and flocked all the way so I have very little stray light and good contrast both visually and imaging.

[Wavytone]

Ben, An awful lot depends on the actual scope, the magnification - and what you're looking at. Seeing is pretty much irrelevant below 150X, and on extended soft objects like galaxies or nebulae. Where it really counts is lunar & planetary and this is where the question of the actual scope also becomes important.

Recently Alex & I spent a night comparing planetary eyepieces using his SCT and my Santel, but something else was quite remarkable - at the same magnification the seeing in the SCT was very much worse than in the Santel. His Meade SCT (20cm, f/10 and one with good optics, by SCT standards) showed a remarkable vulnerability to seeing, whereas the Santel (228mm, f/13 and strehl 96.5%) showed images that shook a bit like jelly, but the details in the image remained evident.

FWIW:

a) the Santel easily showed the disks of the galilean moons - despite the seeing - whereas the SCT was really struggling to show them as better than points,
b) on Jupiter the SCT showed details when the seeing settled, but 50% of the time the details were smeared. In comparison the Santel showed details on Jupiter consistently and clearly, though wobbling like a jelly.
c) the Santel showed Saturn with the crepe ring and a retinue of little moons that were simply beyond the grasp of the SCT.

However the Santel took an hour to cool down to kill its internal thermal plume (it doesn't have a fan - yet), meanwhile the SCT was battling dew.

One aspect of a very high quality objective is that they seem to be less affected by poor seeing than a mediocre objective, and this appears to be true for refractors as well as reflectors - including Newtonians. An objective that puts ALL the photons exactly where they should be - bar none - has a distinct advantage over one that doesn't when looking at objects that are marginal - either faint stars or moons (Saturn) or on resolution.

In particular it is one of the reasons why a triplet APO performs FAR better than an achro doublet. It is also why some buy Questars - their little 90mm Maks are legendary. And why someone will shell out 12,000 euros for a 25cm Mak from Matthias Wirth - not a C11 or C14.

I'd be interested to see someone compare two 20cm dobs side by side at around 200X magnification in average seeing in planetary targets - one with superlative optics and one with average optics. My guess is the good one will be less affected by seeing than the average one.

[Capella_Ben (Ben)]

Thanks guys. Very useful info.

[Merlin66 (Ken)]

Seeing is seeing...
I do agree that superior optics have the potential to give improved resolutions but the bottom line is better seeing gives better results.

[Paul Haese]

I can tell you seeing makes or breaks images with an SCT. I only use my C14 for planetary imaging and you have to be patient and wait for the right night. Especially for planetary imaging. For DSO work it is still a concern but less so. With good guiding and a well tuned imaging system you can produce good results in average seeing conditions. There will be some blurring but the results will be streets apart.

Light pollution always needs to be dealt with the same way with each imaging system you use. You will be limited by how quickly your system shows the light pollution. You might use a light pollution filter to help. Using an SCT will not change any thing you need to do to limit the effects of light pollution.

There is also the problem of dew on the corrector. This can be problematic in that you need to keep heat on the corrector and that can effective create tube currents and limit your systems ability to produce sharp images.

[Hisanori (Hisanori)]

I recently started few attempts of planetary imaging in my backyard in Sydney suburb using 10-in f10 SCT. I'm truely recognised temperature assimilation is critical (often leave there 3-6 hours) also using a long dew shield & heater. Also sensitive to jet streams with 100 knots or above and surface wind. However I didn't think much sensitive to LP since I'm rather surprised what I could image in Sydney.. obviously my standard is not very high.

[andyc (Andy)]

I do a lot of planetary imaging from Sydney, and have in the last year switched from Newtonian to 14in SCT, imaging near 10m f.l. Light pollution doesn't matter for the planets, obviously it's a serious issue for deep sky, I'd imagine that doesn't really change much with scope type, but will with focal length (?).

While seeing is crucially important for getting the best planetary shots, there's no point in only waiting for the perfect nights, as you'd miss some really good nights. Possibly my sharpest Jupiters (example) were on a night with a 50kt jetstream! I've imaged about once a week this year, in a range of conditions. Scope cooling and internal fans are definitely important to circulate the air and even out temperature gradients, I leave mine for about 2hr with fans on. I have no issue running the dew control on the corrector, it causes no adverse issues for me (I think because of the air circulation), and that plus the shield stop dew even on the worst winter nights. It's on every night I'm imaging just now.

My general experience seeing-wise, having used a Newt last year and an SCT this year is that I do wonder how much people's consideration of differences comes down to how well the air is circulated in their SCT? After all, seeing is external to the scope, surely?

[Wavytone]

The worst effects of seeing are due to turbulence and thermals in the densest part of the airmass above you - ie the first 1000-2000 metres.

Second comes wind shear - while you may think it’s calm on the ground yet your scope shows poor seeing, the cause can be wind at 30+ knots just 50 or 100m above you.

Lastly inversion layers, where a layer of cold air sits above a warmer layer under - and often the wind direction in each layer is at 90 degrees. The warm air tries to rise into the colder air in big bubbles but the cross-wind knocks the top of these and turbulent “rotor” is the result - rather like an invisible breaking wave rolling over as a tube. The effect on swing is terrible.

FWIW inversion layers are often formed at cloud base, typically 1500-2000 metres above sea level.

The significance of the jet stream is overrated IMHO - the air up there is too thin and well mixed.

[raymo]

You image once a week Andy!!!!! I haven't had one usable night for any
kind of imaging in the last five weeks, and don't expect any in the near
future.
raymo

[Wavytone]

Raymo you might consider choosing another location. The significance of the local microclimate has been pointed out before, here on IIS.

Moving 100 metres can make a huge difference.

[raymo]

Wavytone, I would have to move several hundred kilometres to get out from
under this area's almost constant blanket of cloud at this time of year,
and am unable to afford to do so.
raymo