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How Big a Mirror Needed?

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#1 nosmoke

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Posted 30 November 2013 - 11:34 PM

Am trying to decide how large a scope to upgrade to. The attached image is the "Leo Triplet" (M65, M66 and NGC something). Was wondering how approximately large a mirror would be required to see something of similar brightness & detail visually rather than by photography?

TIA for any help...

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#2 orlyandico

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Posted 01 December 2013 - 02:22 AM

The image you just posted is rather dim, but I think a 10" can show that visually.

I have been using an 8" newtonian for AP, and I got a chance to look through a 14" dob. Even 1-minute images from the 8" are much brighter than the 14" visually.

#3 GlennLeDrew

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Posted 01 December 2013 - 11:32 AM

As regards detail, for such fairly low surface brightness objects as galaxies and nebulae, it's safe to say that it would require an aperture some 10 times larger to begin to perceive what can be recorded in images.

The exact equivalence depends critically on surface brightness.

Consider the planets Jupiter and Saturn, whose surface brightness is roughly 6 magnitudes per square arcsecond (MPSAS).Even at small exit pupils you can see the detail an image would reveal.

The brightest extended DSOs are a good number of planetary nebulae (and a small handful of HII objects like M42), with a surface brightness of 14 MPSAS. These are comfortably above the color detection threshold of about 18 MPSAS, and so can reveal a moderate amount of detail.

The centres of many galaxies is in the range of 16-19 MPSAS, although usually in a very compact region. This is around the color detection threshold, and so the eye can sometimes perceive the core as a star-like point.

Outside the central regions, the surface brightness for most galaxies falls off rapidly to well below the sky glow level (20-21.5 at the typical amateur's 'dark' site), or there is a large outer region of near uniform but low surface brightness. And many nebulae are just dim all over. In this regime of surface brightness brighter than the sky by no more than a couple magnitudes, the eye's resolving power has become atrocious. The color sensitive cones contribute little if anything, the lower density rods shouldering the burden. Where in lunar/planetary observing our eye operates at a resolution of 1-2 arcminutes on the retina, in the dim, low contrast DSO regime it's down to no better than a degree! Which is at least 30 times worse.

If you want to appreciate just how *awful* our eye's resolving power is at the low end, examine an image of the Pelican nebula, which is easily twice the width of the Moon. On the Moon we can visually/photographically resolve details down to our scope's limit, or of order 1 arcsecond. Through that same scope visually the Pelican is a ghostly presence, with only the coarsest detail barely perceptible, to perhaps the 5 arcminute level (or some 300 times poorer.) But the same scope's prime focus image of the Pelican reveals details down to the seeing/sensor limit of perhaps a couple or few arcseconds.

Another experiment to try, if you have a solar filter: Observe the Moon through it. Yes, you can! Just take the time to dark adapt, and block out all stray light. The filter dims the Moon to a surface brightness of that for a typical DSO. For example, if the filter is rated ND= 5, it dims light by a factor of 10^5, or 100,000. This is equivalent to log(100,000) * 2.5 = 15 magnitudes of diminution. A typical lunar surface brightness around first quarter is about 5 MPSAS, and so the filter dims it to 5 + 15 = 20 MPSAS. Even at the terminator, where contrast is *really really* high, prepare to be astonished at how *little* you can see!

That will tell you in the most visceral manner possible just how much larger a scope must be to in order to reveal visually what an image of a DSO shows.

#4 nosmoke

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Posted 01 December 2013 - 12:25 PM

Thank you both for the comments. The image I posted was apparently taken with a 4" refractor, 8 minute exposure, stacked images. According to what Glen says then, a 40"er would be required so I guess that leaves me out :) OTOH, Orly's prediction is within reach!

Messier's telescopes apparently had the equivalent aperture of only about 3 or 4" at best so I wonder how much of his deep sky objects he actually saw - one might think only a dim smudge for galaxies and nebulae(?).

#5 MitchAlsup

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Posted 01 December 2013 - 12:31 PM

I can tell you that the Leo Tripple is fabulaous in a C11.
This basically confirms the 10" comment from orly above.

#6 JayinUT

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Posted 01 December 2013 - 12:43 PM

I've owned a 8", 10", 20", 14" dobs and I can tell you that I have seen the Leo Triplet look great in all of them. If your looking for that quality besides the szie of the mirror, or the magnification of the eyepiece, sky conditions also play a part in what you see. Overall, you can get great views of the Leo Triplet I feel in a 10 inch to 16 inch scope with the right eyepieces. Just depends on what you want to do and how much money you want to sink into your equipment. Can you get to someone with a 10 inch to 16 inch scope with a variety of eyepieces? That may be the best bet for you to try.

#7 orlyandico

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Posted 01 December 2013 - 02:31 PM

i only had a 10" for visual, the Whale Galaxy looked great in it from a very dark sky site. Based on that, I'd say your Leo Triplet above is doable with a 10".

#8 nosmoke

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Posted 01 December 2013 - 02:32 PM

Thank you Mitch & Jay. When you say "fabulous" and "great", do you mean as good (or better) than the image?

As for trying someone's telescope, there is a local astronomy club here (a branch of the Royal Astronomical Society) which I plan on joining so there should be lots of opportunity there. The local university also holds open houses at their observatory (they have 40 and 16"ers) so I'll also try that when the opportunity arises. Right now, I'm thinking in terms of a 14 or 16" Dob but I have weak back so don't know if that would be workable (it would have to be portable as my location (next to a large city) is not great).

Thanks again to all for your assistance...

#9 Astrojensen

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Posted 01 December 2013 - 02:48 PM

The attached image is the "Leo Triplet" (M65, M66 and NGC something). Was wondering how approximately large a mirror would be required to see something of similar brightness & detail visually rather than by photography?



Well, here's an attached drawing of the Leo Triplet, made at the eyepiece of my 72mm f/6 ED refractor:

Posted Image

It's not quite there, but the overall shapes and sizes of the galaxies are there. Also note the spiral arm in M66.

For more details in the galaxies, approaching the photo in your post, I'd say at least an 8" under very dark skies or a 12" under typical skies.


Clear skies!
Thomas, Denmark

#10 hbanich

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Posted 01 December 2013 - 06:18 PM

Hi nosmoke,

The answer to your question really depends on how dark and transparent your sky is, how experienced you are as an observer, and the sensitivity of your eyes - there's no set aperture that will show the detail in your photo example to everyone, everywhere.

#11 Mirzam

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Posted 01 December 2013 - 06:28 PM

+1 What Howard said.

A 10-inch newt is capable of giving excellent views of brighter objects like those in the Leo triplet. Dark sky conditions are essential.

JimC

#12 Bill Weir

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Posted 01 December 2013 - 06:29 PM

If you live in Calgary I suggest you check out your local RASC. Membership includes access to their telescope rental program. They actually have a wide assortment of scopes ranging from 3 inch refractors though some 6-8 inch SCTs on to 6-13.1 inch dobsonians. $2.00 per inch per month. A short drive out into the prairie away from Calgary's light will show you what can be seen. Calgary has a strong RASC Centre and many there would be able to show you what to do with a scope.

Bill

#13 nosmoke

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Posted 01 December 2013 - 07:41 PM

Once again, thanks to all for their comments. I should have mentioned something from the outset though, I already have a 10" Dob but have not used it in many, many years so can't really recall terribly well how it used to work on DSOs. My interest has been recently rekindled however and had it out the other night and looked at Andromeda - it was barely visible (just a faint smudge even with averted vision). So, was thinking I need something a lot bigger but the comments about dark skys have made me realize my current sky conditions are not nearly as good as they were 30 or so years ago because the city has grown much closer since then. My optics were also purchased decades ago and I suppose the coatings may be deteriorating to some extent. Aging eyes also probably don't help (pupils don't open as far?) so all those things combined may be what the problem is. I guess my next step then should be to take the scope to a good dark location and see what happens. Never-the-less, I still have some aperture fever which needs to be satiated :) so all comments on scope size are appreciated.

Bill, joining and renting a scope from the RASC is also part of my plans - how can you beat what they have to offer?

Thomas, that's a pretty impressive sketch for a 3" scope - I can see more detail in the sketch than in the image I posted. The Leo Triplet hasn't been available to me yet (too late and too cloudy) but I look forward to seeing it, at a dark location, ASAP. Until then, I'll wile away some time thinking about my next scope (and building one IMO is just as much fun as using it). :grin:

#14 Kevdog

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Posted 04 December 2013 - 04:19 PM

Find you home location on here. It'll give you a decent idea of how good/bad your skies are.

https://mywebspace.w...erlay/dark.html

If you're in a red or white zone, then getting out to darker skies will be a huge improvement. Orange isn't horrible, but getting out will still make a big difference.

I have both an 18" dob and an 11" SCT. I live in an orange zone. At home the 18" shows me about the same detail (but not quite) in the orange zone on DSOs as the 11" does at a truly dark site (gray/black).

I viewed the Whirlpool galaxy (M51) at the Grand Canyon with my 11". A couple weeks later I viewed it with the same conditions in the 18" and the spiral arms weren't quite as visible in the 18". They were still there, but not as defined.

I don't travel with my 18". I just have it on wheelbarrow handles and roll it out of my garage. If you're like me you could do the same... travel with your 10" and have a 16" or so at home.

#15 nosmoke

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Posted 04 December 2013 - 06:29 PM

Thanks for that interesting web site Kevdog. Looks like I'm in the purple zone (third brightest) but that was in 2006 (the apparent date of the map) so I'm guessing I am in the red zone now. Will have to get to darker skys when the opportunity arises.

Looks like a 16 or 18" might be wasted where I live so I would have to plan on travelling with it - maybe haul it on a small utility trailer and unload it via a ramp?

#16 Achernar

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Posted 04 December 2013 - 09:00 PM

If all you have is a 4-inch refractor, upgrading to a 10-inch will leave it in the dust at the same site, especially at a dark site. A 10-inch Dob is still relatively portable, but can show you object as faint as 14th magnitude. To get better views of faint object than a 10-inch offers requires at least a 14 to 16-inch telescope at the same site.

Taras

#17 kfiscus

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Posted 04 December 2013 - 09:42 PM

NoSmoke, get that 10" dob collimated and enjoy! It will show you great things if you don't have too much light pollution. I would agree with several of the previous posters that the photo of the Leo Trio is quite close to what a 10" shows.

#18 orlyandico

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Posted 05 December 2013 - 01:05 AM

one other option.. not available to us outside the US.. Collins i3piece.

Granted it's not cheap, but it's probably cheaper than a 16" dob. Much lighter too.

That or a Mallincam of some sort..

#19 csrlice12

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Posted 05 December 2013 - 08:31 AM

One Collins was recently in the classifieds....$1500 (US), and that was used........not knocking them, they're fantastic at what they do....their facility is just a stone's throw from my house....would love to have one, just that the cost is prohibitive.

#20 FirstSight

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Posted 05 December 2013 - 10:26 AM

As big a mirror as:
1) You can afford that has a high-quality figure;
2) Will still allow you to observe at all altitudes with your feet still on the ground (depends on f-ratio of mirror);
3) Is within feasible size to transport (both in terms of available vehicle and available muscle power).
4) Is within a price range your wife will knowingly approve you purchasing without initiating divorce proceedings.

#21 Jon Isaacs

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Posted 05 December 2013 - 02:09 PM

As big a mirror as:
1) You can afford that has a high-quality figure;
2) Will still allow you to observe at all altitudes with your feet still on the ground (depends on f-ratio of mirror);
3) Is within feasible size to transport (both in terms of available vehicle and available muscle power).
4) Is within a price range your wife will knowingly approve you purchasing without initiating divorce proceedings.


My thinking:

No photographer would have just one lens or just one camera. Likewise, amateur astronomers need more than one telescope, more than one eyepiece.

Different situations, different objects, different telescopes... Mirror quality is critical for planetary viewing, not so critical for hunting down tiny faint objects. Some scopes are more portable, more transportable some are less...

As far as the Leo triplet.. I think Glenn's comments are of fundamental importance but the photo in question was not representative of what is possible with a 80mm scope. Here is a photo taken with an 80mm apo.

Leo Triplet ED-80 (Eon)
My experiences:

- Larger apertures are a help when skies are light polluted but aperture cannot overcome the loss of contrast caused by the bright sky, it can only provide greater magnification or a brighter overall image but both the galaxy/nebula and the sky be equally brighter.. A small scope under dark skies will show things that cannot be seen with a large scope from a light polluted backyard.

- Observing skills are probably more important than aperture.. Thomas's drawing of the Leo Triplet, lots of us would not see that through a 6 or 8 inch scope.

Jon

#22 nosmoke

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Posted 05 December 2013 - 02:39 PM

Wow, if I could see that thru my future scope, I would be v happy. :)

Still no chance to try things out under dark sky BTW- temp of -25C at night here. :bawling: Have to become a snow-bird I guess...

The Collins looks interesting but I think not at $4K (even if I could purchase it being non-USAian).

#23 acochran

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Posted 05 December 2013 - 04:57 PM

Wow, if I could see that thru my future scope, I would be v happy. :)

Still no chance to try things out under dark sky BTW- temp of -25C at night here. :bawling: Have to become a snow-bird I guess...

The Collins looks interesting but I think not at $4K (even if I could purchase it being non-USAian).

Here in the northern latitudes, Leo Triplet can be seen nicely in Spring/Summer evening skies, don't forget the Virgo cluster/right next to it, plus all those great galaxies by the Big Dipper...
Andy

#24 Kevdog

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Posted 05 December 2013 - 06:11 PM

Thanks for that interesting web site Kevdog. Looks like I'm in the purple zone (third brightest) but that was in 2006 (the apparent date of the map) so I'm guessing I am in the red zone now. Will have to get to darker skys when the opportunity arises.

Looks like a 16 or 18" might be wasted where I live so I would have to plan on travelling with it - maybe haul it on a small utility trailer and unload it via a ramp?


Purple is the 3rd darkest area. If you're in purple then you have good skies.
From darkest to brightest
Black
Grey
Dark Purple
Light Purple
Blue
Dark Green
Light Green
Dark Yellow
Light Yellow
Dark Orange
Light Orange
Dark Red
Light Red
White

There further away from the white areas, the better.

Try this one as well:
http://darksitefinde...th-america.html

You can also use this outside to get a feeling of your Bortle Scale for your skies:
http://darkskydiary....g/bortle-scale/

Scroll down to the flow chart and follow along.

If you're in Orange or better, you can do some decent viewing at home (I know I do). But there's something magical about truly dark skies. But I'm not giving up the nights viewing at home either!

#25 MitchAlsup

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Posted 05 December 2013 - 06:23 PM

Thank you Mitch & Jay. When you say "fabulous" and "great", do you mean as good (or better) than the image?


Brighter than the image with somewhat smaller stars.






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