Matching equipment to local conditions

[Hans Tucker (Hans)]

Stupid question, short notice but how many people select their scope aperture based on their local seeing conditions. IMO there is no point trying to use a 10" in conditions that only support a 5". How do you evaluate the conditions and make your selection.

[Steffen]

IMO the Rayleigh resolution limit is only one aspect of aperture, the other (just as important) one is light gathering. Even if the seeing is poor large scopes will show fainter objects than small ones. Under light polluted skies larger scopes also make the use of filters like UHC or OIII more practical, since they pull in more light to start with.

Cheers
Steffen.

[AG Hybrid (Adrian)]

If the seeing is bad. I just use less magnification. For reasons mentioned by Steffen I will almost always prefer my 12" over my 4" regardless of the seeing.

Besides, I've seen so many arguments on CloudyNights about seeing supporting certain aperture scopes more or less that it would seem that there is no conclusive proof that a certain scope size is better in bad seeing.
There are those 'murican fractor nuts who swear that their 80mm APO cuts through the seeing like a knife through butter, and those with large reflectors rebutting that regardless of seeing they still see more.

[Wavytone]

There are many unknown variables and unless you're going to do side-by-side comparisons between two scopes at the same magnification on the same object at the same site at the same time, who cares. And even if you did what's the point ? Sounds suspiciously like a pissing contest to me.

Anyway why agonise about this stuff - if you've got a scope go use it.

[JB80 (Jarrod)]

This is something I have been thinking about lately but I have to admit I really don't know much about. Here conditions are so bad all the time anyway so I have never wondered a great deal about it.

However we will be moving come July and I'm just starting to research what conditions may be like in terms of seeing, while it will still be a vast improvement it would seem that the region is in part of the summer jet stream which is not going to help with planetary imaging much.

It's one of those things that everyone seems to have a varied opinion on but I guess my main question would be ... Is there going to be a noticeble difference between a 9.25 and an 11" scope to warrant one over the other in average seeing?

Anyway why agonise about this stuff - if you've got a scope go use it.

This makes sense to me but surely a lot of the agonising has to do with the size of ones wallet.

[Wavytone]

Originally Posted by JB80

the region is in part of the summer jet stream which is not going to help with planetary imaging much ... Is there going to be a noticeble difference between a 9.25 and an 11" scope

Nope. The jetstream will limit the seeing. A good 100mm refractor will show all there is to be had most nights, though the extra aperture will help so go with the lowest cost option, unless you have the chance to visit a site with excellent seeing.

[Steffen]

Originally Posted by JB80

This makes sense to me but surely a lot of the agonising has to do with the size of ones wallet.

Larger wallets have greater problem resolving power and are capable of pulling in more elusive objects than smaller wallets

Cheers
Steffen.

[JB80 (Jarrod)]

This is most certainly true.

though the extra aperture will help so go with the lowest cost option, unless you have the chance to visit a site with excellent seeing.

To me that says why not go for the larger option if you can. I think the seeing is pretty reasonable at times, at least according to meteoblue but I don't know how reliable that is as a source.
And where we are going only gets 37 days of rain a year, of course that is about to change but it's better than the 200+ days we get here.

[mental4astro (Alexander)]

My main planetary scope for many years was a C5. As you allude, it's resolution is to the typical limits of Sydney at sea level.

I picked up a C8 about a couple of years ago. This allowed me to compare the two scopes side by side. In the end I let go of my precious C5 as the opportunity the added aperture gave on those occasions of great seeing made the C8 the better option. Yes, on most occasions I can't go beyond the magnification the C5 gave (250X), yet enough opportunities do happen that make 400X a practicality. Aperture here won out.

With my 17.5", seeing conditions also are the limiting factor to magnification. Then what matters here is the light gathering grunt that AG mentions. At the same magnification between a C8 and the 17.5" (easy to do as they have the same focal length), the difference in image is so significant that making a decision of limiting scope size to prevailing conditions would have been a very poor choice. Aperture always wins, regardless of conditions. So, conditions won't allow for much magnification, big deal really. Magnification really isn't what it is all about, is it. Tell me that there is no difference in the image quality between a 4" aperture and a 17.5" at the same magnification, and you really haven't experienced the difference.

Wanna see the difference? The two sketches below were done from my home in Sydney's eastern suburbs. The one on the right was done with an 8" f/4 Newtonian (not the C8). The one on the right a 17.5". Yes, there is a magnification difference - same EP was used with both scopes. But the difference in the brightness and visible detail of each depiction of M42 shows the advantage of aperture, regardless of prevailing conditions.

[mental4astro (Alexander)]

Edit: re-read your post Hans. How do I make my selection of weapon of choice for given conditions? I don't consider conditions first. I have some 6 different scope, each for a particular task. I first consider the task and select the best weapon for the job. Only then do I make allowance for the conditions once the scope is set up for the night. I'm just not organised enough to plan ahead for the prevailing conditions. If the intended target requires high power and it can't be had, plan B is created in a hurry. This is mainly because grabbing a scope is a matter of chance, and planning for conditions is not feasible. Change on the run is my only option.

Wavytone recently showed me a BOM feature for pilots where the conditions of the upper layers of the atmosphere are displayed, in particular the inversion layer that may be in play and is most responsible for crappie conditions.

I've added a weather balloon gathered chart that shows the current altitude/temperature/windspeed/wind direction values over Sydney. The greater the amplitude (how jagged it is) in the wind value (red line) the more turbulence there is. Right now there is a nasty inversion layer over Sydney at low level (upto 2.5km), with another higher up. This means terrible conditions for high magnification.

Here's a link to the BOM page where you can download the Aerological Diagram for your local area. Just click on the city/town best suited to your location.

Having this information on tap I guess now I have an opportunity to actually better plan my session! BUT, this is only for the intended target.

[multiweb (Marc)]

Originally Posted by Hans Tucker

Stupid question, short notice but how many people select their scope aperture based on their local seeing conditions. IMO there is no point trying to use a 10" in conditions that only support a 5". How do you evaluate the conditions and make your selection.

Right now the only equipment matching my local conditions is a big umbrella.

[astroboy]

The seeing here is rarely great but I would never say there is no point going over 4"
On a run of the mill night A C9.25 that star testested very well I was using on the moon showing some really nice detail and hinting at some really fine detail I then went to a 10" F6 newt with a small secondary and the differance was amazing , the fine detail hinted at by the 9.25 was vivid in the 10.
So I think for visual for here an 8" or 10" optimized scope is ideal for planets , although on those steady nights the views of jupiter through the 24" have been mind blowing , something I'll never forget.

[el_draco (Rom)]

The astrophotography question is also complicated by seeing stability over time. I am about to delve into Hyperstar to adress narrow imaging window issues.... Let you know how it goes...