oo UK vx10 or 10l?

[sunshine185]

Just doing a bit of steaming and was checking out the above dobs, one has a 50mm secondary and the other 50mm , the f ratios have a big difference too. Which would be the most versatile scope? Would one be better for planets, the other deep sky?

[dweller25]

The VX10 F/4.8 has a 63mm secondary giving an obstruction of 25% - excellent for deep sky observing.

The VX10L F/6.3 has a 50mm secondary giving a 20% obstruction - just as good as the VX10 for deep sky and slightly better for the planets.

It comes down to which tube length suits you - 1200mm or 1575mm.

[sunshine185]

so the 10l would be around 16"longer? that would be one long tube! i guess sitting while viewing would be a problem with the 10L.

[John]

I have a 10" F/4.8 Orion Optics but I'd prefer a F/6.3. Easier to collimate (much larger "sweet spot") and the smaller secondary will enhance contrast on both the planets and deep sky objects. F/6.3 is much less fussy on eyepieces too.

[sunshine185]

i did a rough check, it'd cost me about 1200 quid with the upgrades, i better start saving.

[cotterless45]

f7 or near to it is ideal for Dobs.

Nick.

[ollypenrice]

Unless you want it for DS imaging or physical size is a big issue I'd say the slower scope would really hold all the high cards. You'd have a less wide FOV but still good. Once you get to huge sizes a fast F ratio keeps the size under control but here both scopes are reasonably compact.

Olly

[sunshine185]

Not gonna go on the slippery road that is AP.

[cs1cjc]

For visual use, the longer model would be virtually free of coma, collimation would be much less critical and it would be kind to eyepieces. Its length would really not be a problem. It would however be limited to a field of view of perhaps 1.5 degrees, but your other scopes can do wider stuff. It would be wonderful for all objects of less than a degree including planets, galaxies and planetary nebulae, etc. One of these might even be my next scope, but only after I find the size of the 14" (of the same focal length) to be too much of an issue.

[cs1cjc]

But see: http://www.astrobuysell.com/uk/propview.php?view=57362

[a10ken]

i would agree with john, exelent scope and not to high in a dob mount

[Theninjagecko]

I got a 1200mm f4.8 250 spx with hilux and 1/8th mirror. I think the quality of this scope allows you to push high magnification with no problem, i used a 5mm xcel lx on a poor night with clouds and a warm scope and it gave me the best view ive ever had of saturn.

having the longer length scope limits you I think.

[Moonshane]

I have a 6" f11 which is the same focal length and it's not a problem sitting to observe although I use my home made observing stool which is infinitely adjustable. it's amazing on planets and I suspect the 10" f6 would be great too - even better I'd expect. one issue with these tall slim OOUK dobs though is the COG is a little high sometimes and they have a tendency to topple. I did some mods on mine but mainly use it on an EQP now which practically bolts the thing to the ground.

[sunshine185]

Your mods are awesome, its such an advantage being able bodied and good at diy, seeing as I am neither I have to make do with a " stock" scope. I was hoping to use the ppi claim to get myself a nice top of the range dob, but when the 250px, fully flocked came up for £250 quid, I thought it was an offer that was too good to turn down. I am managing to do the simple dob mods and, as this scope is a keeper,I have lowered the ota down the rocker box by about 3.5-4 inches, this puts the focusser at the same height as the 200p I had which is ideal for me to view. Do you think the 6" is a better planetary scope than the monster dobs you have? If so, is this due to the long FL or the fact it has better coatings?

[Moonshane]

you made a great choice. given the choice of my 16" and my 6" I'd choose my 16" so for visual aperture always wins for me. I am lucky as my 16" can be masked to 170mm to give the best planetary views I have ever had. on average though the unmasked large aperture is not as good as the 6" on planets/doubles/moon. the masked aperture is best for me but the 6" is a lot easier to set up. the 6" is a better solar scope. luckily I don't have to choose between the two.

[alan potts]

f7 or near to it is ideal for Dobs.

Nick.

I'm looking at getting a 16 inch, better get a set of ladders too!

Alan.

[Moonshane]

I'm not sure where the f7 being good for dobs comes from as like any 'rule' it depends on circumstances. an f7 16" dob is impractical but a 6" - 8" f7 is great. maybe it's about the relative ease of mirror making at f7 and the higher collimation tolerance allowing for flex etc?

if you make a dob well then f4-5 is perfectly fine and produces much wider fields of view.

[John]

I'm not sure where the f7 being good for dobs comes from as like any 'rule' it depends on circumstances. an f7 16" dob is impractical but a 6" - 8" f7 is great. maybe it's about the relative ease of mirror making at f7 and the higher collimation tolerance allowing for flex etc?

if you make a dob well then f4-5 is perfectly fine and produces much wider fields of view.

It may well have come from John Dobson, the inventor of the dobsonian design. I believe he envisaged that F/7 would be an optimum focal ratio. It may also have been influenced by the availability and cost of fast mirror sets back then (early 1980's ?) as Dobson was aiming to create an affordable larger aperture scope. The availability of low cost, decent quality, fast mirrors, is a comparatively recent thing as I understand it.

[Moonshane]

I suppose he knew what he was talking about ! I think it's an out-dated view though now as you say.

[sunshine185]

Here's a bit I don't understand, the 250px is f4.9 (I think) and the 200p I think is f6, moonshane you say a lower f ratio gives a wider field of view than a higher f ratio, but I was told that the 8" and 10" sw dobs both have same fov. This confuses me.

[Moonshane]

focal length dictates (or is a factor of) field of view as FOV (approximately anyway) = field of view of the eyepiece / magnification.

magnification = focal length of scope / focal length of eyepiece

focal ratio = focal length of scope / aperture

so two scopes with the same aperture and same focal length will always have the same FOV with the same eyepiece

two scopes with different apertures but equal focal length will have the same FOV but different focal ratios