6" reflector for DSO viewing?

[Steve301]

Hi,

I have a Skywatcher ST120 that I use for widefield viewing and finding DSOs. I've been using this telescope for quite some time now and it's worked well for me but I'm thinking of getting something a bit larger. I have been looking at the Skywatcher Explorer 150P but I'm wondering if there will be much of a difference in the visual image quality over the ST120?

Any advice would be appreciated!

Thanks,

Steve

[Hals]

Have a look at Shane's report here...

http://stargazerslounge.com/topic/154252-some-test-objects-with-a-6-scope/

[Steve301]

Thanks for the link - that report certainly answered a few questions.

[umadog]

There will be a modest difference but not a "wow" difference. The rule of thumb is to at least double the light grasp if you want to see a really significant difference.

120 to 150 is: 150^2/120^2 = 1.56 times the light grasp. So there's a difference, but it's not huge. If you were to upgrade to a 200 mm aperture then you'll be pulling in about 2.8 times as much light. That'll really hit home. A 250 mm will be 4.3 times. By these apertures your scope will be seeing limited more or less the whole time.

[Tinker1947]

You could bung some Data in here and see what comes out.....

http://www.cruxis.com/scope/limitingmagnitude.htm

[LukeSkywatcher]

There will be a modest difference but not a "wow" difference. The rule of thumb is to at least double the light grasp if you want to see a really significant difference.

120 to 150 is: 150^2/120^2 = 1.56 times the light grasp. So there's a difference, but it's not huge. If you were to upgrade to a 200 mm aperture then you'll be pulling in about 2.8 times as much light. That'll really hit home. A 250 mm will be 4.3 times. By these apertures your scope will be seeing limited more or less the whole time.

I can not agree more. I recently upgraded from 130mm to 200mm and the difference really is a WOW factor.

[StarryBob]

With the 120 being a refractor and the 150 a reflector with central obstruction then does this still equate the same?

[umadog]

If the obstruction below about 20% by diameter then it won't result in any noticeable contrast loss. In other words, the contrast will be same as an unobstructed instrument (refractor) of the same aperture. Although, of course, there will be difraction spikes which some people find objectionable. It will also need to be collimated.

The central obstruction will reduce the amount of light. A 150 mm with a 20% obstruction will be losing 4% of the light because of the obstruction. You won't notice 4%. It has to reach about 15% or so before you'd really notice it.

[LukeSkywatcher]

All this talk of central obstructions baffles me. I have a Celesatron 8se. Just how much less light/performance am i getting on the scope with its central obstruction?

Can someone explain it to me, please?

Paul.

[umadog]

The light is easy:

8" unobstructed scope light gathering (arbitrary units) is 8^2=64.

Say you have a 31% obstruction by diameter: 8*0.31=2.5". So that will remove 2.5^2=6.25 arbitrary units of light gathering.

So you are missing (6.25/64)*100=9.77% of the light compared to an unobstructed telescope. You'd be hard-pushed to notice that.

The decrease in contrast is harder. Obstructions smaller than 20% are not noticeable compared to an unobstructed instrument of the same aperture. For larger sizes you start to see a decrease in contrast. BUT remember that this is a decrease in contrast compared to an unobstructed instrument of the same size. So you expect your SCT, even with its large obstruction, to have better contrast than substantially smaller refractors. To get a better understanding of the effect of the obstruction you need to get into modulation transfer functions (MDFs). Here you go:

http://www.telescope-optics.net/obstruction.htm http://www.telescope-optics.net/telescope_central_obstruction.htm

Some hard reading there... You can think of a telescope as an optical filter: passing some spatial frequencies but not others. This is what we mean by "spatial frequency": http://www.psy.vanderbilt.edu/courses/hon185/SpatialFrequency/SpatialFrequency.html The MDF tells you which frequencies a telescope will pass. A bigger aperture allows you to see finer details so that means it passes higher spatial frequencies. Adding the obstruction (or having imperfect optics) causes some spatial frequencies to be attenuated. In practice, that means that certain details will become blurred. Those MDF graphs show you where that happens. I'm afraid that's currently the limiting of my understanding, though.

[farunj]

But what causes these obstructions??

[umadog]

The secondary mirror of a Newtonian (http://en.wikipedia.org/wiki/Newtonian_telescope) or catadioptric (http://en.wikipedia.org/wiki/Catadioptric_system) telescope. A refractor is not obstructed.

[farunj]

Oh! Ok I see it now!! Thanks!!

[brantuk]

It's a bit like looking through the porthole of a ship and then sticking a beer mat in the middle lol. Except the light isn't focused and magnified in a ship of course.

[SeedyF]

And getting back to the original question - although I like where you're taking us Brantuk, I can understand you, Umadog on the other hand just baffled me......

I doubt there would be any positive effect going from a 120 refractor to a 150 reflector. Keeping it simple I had a quick look in the Sky at Night sky guide and when they refer to the different types of scope needed to see the various objects they refer to a large scope as above 150mm reflector or a 100 refractor and a small scope as below those ranges. That suggests a 100mm refractor is comparable to a 150 reflector. Or am I being simple rather than keeping it simple

So is it potentially worse for Steve if he goes to a 150 reflector?

[Kevdan]

Hi,

Heres a link to a thread I started ages ago, which includes a chart with light gathering differences. Hope it helps

http://stargazerslounge.com/topic/133258-light-gathering-comparisons/page__hl__%2Blight+%2Bgathering+%2Bdifferences

Kev

[umadog]

I doubt there would be any positive effect going from a 120 refractor to a 150 reflector.

Yep.

Keeping it simple I had a quick look in the Sky at Night sky guide and when they refer to the different types of scope needed to see the various objects they refer to a large scope as above 150mm reflector or a 100 refractor and a small scope as below those ranges.

What's "small" and "large" depends on who you talk to. Designating telescopes into those categories is somewhat arbitrary.

That suggests a 100mm refractor is comparable to a 150 reflector. Or am I being simple rather than keeping it simple

I suppose they'd be roughly comparable. The reflector would have only a few mm more clear aperture and the coatings aren't 100% efficient. Then again, reflection also occurs from the front surfaces of lenses. Yeah, they're more or less the same thing. But other stuff matters too: do they both have equivalent optical quality? It's probably easier to make a good cheap reflector than a good cheap refractor...

[Moonshane]

It was my thread linked to above. I previously had a 120mm f8.3 Celestron XLT refractor and the 6" f11 newtonian I now have wipes the floor with it in every way, the only possible exception is that of wide field observing of open clusters where the slightly sharper pin point stars of the frac meant the image was a little neater. with a 6" newt, there is a lot more 'body' to DSOs and much better detail (due to aperture and no CA in the newt) on planets and moon. double stars are truly excellent (although a closer fight with the 120mm on doubles to be fair).

I would always choose a 6" newt over a 120mm achro especially if it's the f8 or slower version of the newt. to my eyes at least there's no comparison in the views. I'd also take a 6" f5 over a 120mm achro (even a slow one) but the fight is closer there.

an over-riding reason for me is observing comfort. I always found the observing position of a refractor uncomfortable whereas with a newt it's much better.

different people like different scopes but for me I'll never go back to refractors.

[brantuk]

For me refractors and reflectors are like chalk and cheese - hard to do precise comparisons - I imagine the ST120 is great for viewing the moon, planets, clusters, and splitting double stars. But if heading for deeper, fainter objects like nebulae, galaxies, comets etc a large aperture newt would be the tool of choice. To get a noticable difference I would want at least an 8" or 10" newtonian.

From a practical point of view (cos I ain't too good at the maths) I would recommend you get to a star party or local astro soc and have a look through a few different scopes to see for yourself how the view differs from your very capable achro. I reckon you'll find a 150P to be not significant enough a difference - but there's nothing like personal experience when it comes to making a decision.

That doesn't take anything away from what all the above are saying - they are all highly relevant points of view - just thought I'd suggest the obvious deal breaker

[Qualia]

Great thread and thank you Umadog, Moonshane and Brantuk for some great advice.

[Saleratus]

I would not bother to upgrade from a 120 to a 150. I have found that a sweet spot in reflectors is around the 7" - 8" mark. That's the point at which you really start to see some interesting stuff that's completely unavailable to you in smaller scopes.

[Steve301]

Thanks everyone for a lot of really interesting information - and some great links. I have used my ST120 for searching out many DSOs and I think it's a great telescope but it obviously lacks aperture. Afer digesting all this info, I now think I'm going to skip the 6" and go for the 8" newtonian. This won't be for another two months though, so hopefully the clouds will have gone by then!

[Naemeth]

Thanks everyone for a lot of really interesting information - and some great links. I have used my ST120 for searching out many DSOs and I think it's a great telescope but it obviously lacks aperture. Afer digesting all this info, I now think I'm going to skip the 6" and go for the 8" newtonian. This won't be for another two months though, so hopefully the clouds will have gone by then!

That sounds a great idea, it should arrive in time for much longer nights, and you will no longer have to wait hours for the sky to get dark.

[Moonshane]

undoubtedly more aperture = more satisfactory visual observing!

[RikM]

My 150mm f/5 Newtonian gives much better views than my 127mm f/9.4 acromatic refractor on all objects except planets. That said, I would still skip the 6" and go for the 8". That really is a wow jump.