Smallest Viable Aperture?

[Naemeth]

In a scenario when you need your scope to either be in a backpack, or a hold-all (for instance when you are going on holiday without a car, or walking to a dark site), what is the smallest viable aperture to make taking a scope worth it as opposed to binoculars? Where you can still see DSO's with smaller apertures because the skies are so dark.

[alan potts]

I would say about 80mm for visual. If you look at the Telescope Services site you will see they are selling 50mm APO now, I am sure at the price they are not finders. Some people must use these as grab and go imaging set ups. The other question is of course how far you are thinking of carrying it and how strong you are. I would have no problem with an 80mm and mount, if you set it up right you can carry the whole lot over you shoulder, just ensure nothing is knocking together.

Alan.

[acey]

Having two objective lenses rather than one gives a larger light-gathering area, the factor increase being the square root of two, i.e. a 50mm binocular gathers about as much light as a 70mm refractor (50 x root2 = 70.7 approx). The smallest feasible aperture for a scope is anything larger than the pupil of your eye: even a 1-inch telescope will show stars not visible with the naked eye. The crucial thing is that the sky should be dark enough to make the scope worth using. For travel the issue is how much you want to carry: 50mm binoculars will potentially show all the Messiers at a dark site, an 80mm scope will show a bit more than the binoculars. A tripod will enable you to see more than you would with a hand-held instrument, but is more luggage to carry. I have travel scopes of 80mm and 100mm aperture, the latter being a folded refractor spotting scope which introduces extra reflections, hence light loss. I get no more gain with the 100mm but it's highly portable since it has a built-in zoom eyepiece. With the 80mm scope on a photo tripod I've viewed lots of Messiers and Caldwells during trips abroad. I've also used 15x70 binoculars which give equally pleasing views but reveal far less detail when hand-held. With a scope you can use different eyepieces, filters etc, all of which broadens your options. It's just more stuff to take though.

[DirkSteele]

My view is 85-90mm is the minimum to really start bringing more objects into play and justify carrying the additonal components such as mount and tripod. My smallest scope which I use for travel has a 105mm objective.

[Stu]

This is my smallest setup which I take abroad with me.

http://stargazerslounge.com/index.php?/topic/161700-WO66-Grab-and-Go-Setup#entry1636206

From dark sites you still get to see plenty, and have the added benefit of higher magnifications for planets and smaller dso's. It is more compact than larger binoculars which would need a tripod to stabilise them any way.

That said, I probably do agree that a 70 to 80 mm scope would give just that little bit more, but for me it is about being able to take the smallest scope which still gives m e much enjoyment looking through.

Stu

[S.A.M]

I would say the minimum for a refractor is about 80mm and 130mm for a reflector, all depends on what you would like to view, amount of money you can spend and how much weight you can carry safely.

Peter

[Capricorn]

I suppose that 60-70mm is the smallest, I am working on the basis of smallest not the largest small scope you could get away with.

The WO ZS70 fits, the older 66mm being another good option.

50mm is too close to many binoculars and 60mm exact is probably a bit on the small size. Although TV do a nice 60mm.

[John]

I'd go for an 80mm as the minimum I think. I've seen the Veil nebula with a low cost ST80 F/5 achromat and a UHC filter so they can go quite deep under dark skies. I've also seen the Veil with 70mm binoculars but the view with the little refractor was more pleasing and the binoculars are more bulky and more awkward to mount steadily than the ST80.

[Moonshane]

personally I don't like to observe with less than 6 inches of aperture. I made a travel dob to accomodate this. it all fits into the bottom box (15x15x10 inches) and 10kg.

[John]

Thats real neat Shane

A 6" scope under really dark skies can be very impressive.

[acey]

First scope I owned was 40mm aperture (1.6 inches) and I saw M31 and the Orion nebula with it. I much prefer the view through larger scopes but we tend to over-estimate aperture requirements these days, partly because we're spoiled by what's available, partly because of light pollution and the mistaken belief that aperture is the cure for it. Lacaille discovered M55 and M83 using a half-inch aperture (12.7mm), Bevis discovered the Crab Nebula with a 3-inch (76.2mm). Certainly good to take the biggest aperture you can manage but from a dark site you don't need much to be able to see something. If people regard 60/70/80mm as smallest feasible then I think that says more about commercial availability and consumer expectation rather than what's optically possible. A 50mm finder would be a perfectly useable travel scope for low-power viewing - though the inverted view would make a mount essential.

[gustavo_sanchez]

In my case, a 90mm maksutov is a perfectly travel capable scope and can use a sturdy photography tripod. It's very compact, and has also a long focal ratio for viewing the planets and the moon too. If you want wide field views, you can use a long focal length eyepiece. I got mine on eBay (new, latest model) for $150.

[Naemeth]

I've done some research, and the limiting magnitude of an 80mm F/5 refractor is the same as my 130mm F/5 reflector at my current location if the skies are 5.1 mag or darker, so as long as I could get skies darker than 5.1 mag, I think I could probably outdo my current location with a small refractor.

There is no budget as such yet, because it's more speculative browsing at the minute, but budget would probably be around £100 or so (up to £130). Weight limit would probably be about 8 - 10kg, and I think the ST80 + Table-Top EQ (with counterweights) is about that. Bulk is one problem, and my current scope seems difficult to transport in one piece without some kind of trolley. I was thinking that it may be possible if I detached the OTA, and use an EQ Mount which in combination may be able to fit in a rucksack, but I don't know how feasible that is.

Shane, that 6" is a beauty, I've seen it before and of course something like that would be perfect if I had the money (or the tools and time to make one myself ).

I'm on the fence with filters (not having tried any) because often I've thought my scope has an aperture too small to gain from them, and my skies are too bright (3.95 mag un-adapted).

I'd much prefer wide-field views (over 2 degrees if possible... I can currently get 2.08 with my 130mm) to be honest over the Planets and the Moon, because both aren't effected at all by light pollution. So I think Maks may be unsuitable, as they are often slow and therefore have a smaller FOV.

[Moonshane]

I can certainly confirm that an Oiii makes a massive difference at a dark site on e.g. the Veil.

[John]

I've done some research, and the limiting magnitude of an 80mm F/5 refractor is the same as my 130mm F/5 reflector at my current location if the skies are 5.1 mag or darker, so as long as I could get skies darker than 5.1 mag, I think I could probably outdo my current location with a small refractor......

I took a 6" scope to one of the SGL star parties and under the dark skies there it showed DSO's with the same clarity my 10" does from my back garden. A couple of us spent a lovely night with 6" scopes picking up faint galaxy after faint galaxy - objects that we would have really struggled to see, if at all, from our back yards.

[Naemeth]

I took a 6" scope to one of the SGL star parties and under the dark skies there it showed DSO's with the same clarity my 10" does from my back garden. A couple of us spent a lovely night with 6" scopes picking up faint galaxy after faint galaxy - objects that we would have really struggled to see, if at all, from our back yards.

Which is why I'm trying to find a portable enough set up to at least get to slightly darker skies with few obstructions. In this regard, would a better option be to find another mount (either EQ or Vixen Porta) for the Heritage that would give it a nice height off the ground? The mount would at least be collapsible so it would be able to be carried a few miles in some sort of bag. I know the Heritage OTA is very light, probably 2 kg or something (it's the Dobsonian mount it has that's the heavy part of it!), so would something like an EQ3-2 be enough, or would this be too much mount for the OTA?

[John]

We were using an AZ-4 mount and a Skywatcher Skytee I to carry our 6" scopes on the night I described above. The Heritage would sit well on a Vixen Porta mount I would think - it has a dovetail bar fitted I believe so it would bolt straight on.

[Naemeth]

We were using an AZ-4 mount and a Skywatcher Skytee I to carry our 6" scopes on the night I described above. The Heritage would sit well on a Vixen Porta mount I would think - it has a dovetail bar fitted I believe so it would bolt straight on.

It does have a bar instead of the standard place for rings so the Vixen should work, but I suppose I could screw the standard bar off if needs be. Do you mean this mount:

http://www.firstlightoptics.com/vixen-mounts/vixen-mini-porta.html

It says maximum OTA external diameter to be 120mm, which is 10mm too small, but the Heritage OTA is very light and short (only about 2kg, I checked ), so it may work.