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85mm v 102mm?


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Does anyone know how much difference there'd be at the eyepiece between an 85mm scope (say a TV85) and a 102mm one (like, perhaps, a TV102)? I know it's a 20% increase in aperture, but would it make a noticeable difference on resolution and the brighter DSOs?

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It's a 44% increase in aperture, blame Pythagoras or someone, it goes by area not diameter.

The additional aperture should be useful especially on a well produced objective, what you "lose" is a little on the protability. 102mm is bigger, heavier and longer then an 85mm. Running off into the night with an 85mm under your arm to howl at the moon is easy, to run off with a 102mm under your arm means you could be out of breath and need to recover before howling at the moon commences.

Would you see more, well a little. Always find this difficult. M42, M31 would be better, as would the planets - Jupiter, Saturn maybe Mars. Left the moon out as that is bright enough that you would need a filter for it. The bunch of galaxies in the Virgo cluster then not much - they are all small and all distant, they remain small and distant. Something that is half the size of a pinhead in the 85 is not going to swamp you with feeling of awe in the 102mm (still going to be half a pinhead). What will most likely happen is that you see a few extra things that are a quarter the size of a pin head that the 85mm didn't quite present to you. So more numerically but not sure that is what people realise.

Of the 2 then the 102mm would be the weapon of choice, above 102mm you would need to be sure of the optics, they start getting to that size where aberrations need cancelling out more, especially on the modern faster scopes. Assumption is that both are ED doublets, sort of mid range lens type as not achro and not apo triplet.

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Oh yeah! That's the thing isn't it - any perceived improvement is balanced out by the increased weight of the setup. At the moment, my TV85 on a Porta Mount is easy. A TV102 on a Gibraltar (there's one available somewhere) might be a different option, although the OTA is only 2lbs more apparently.

I almost hope someone else buys it first, then I don't have to keep thinking about it!

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I moved from a 90mm to a 102mm and noticed the difference. I find 102mm the minimum I need to get satisfying views - ie: a little more than just "nice". Fortunately I managed to find a 102mm which weighs around the same as many 90mm's and is around the same size.

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Too bad that refractors get expensive so quickly. Perhaps you can find one used?

It allways fascinates me how much more the 130mm newtonian (6kg/13lbs with dob mount) shows compared to the small refractor, while the price difference is marginal. I suppose at one point it depends on how much you are willing to spend/what's still economical...

Here is a german link by the way, showing the different apertures on M13, as most simulators just show the difference in field of view, magnification, afov and such.

http://www.binoviewe...ich_deepsky.htm

translation http://translate.goo...sky.htm&act=url

Edited by Schorhr
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Indeed, but if taking in consideration what the text states (conditions, viewing distance) it's a good estimate compared to what Stellarium and on-line simulators show.

I was astonished how relatively well M57 was represented when viewing it with two different telescopes.

I have yet to get the 130mm dobson onto my mount somehow, but as it's focal length is relatively small, taking pictures even worked on the eq platform well.

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