16 inch Telescope

[jetstream]

You don't need expensive eyepieces for 16" f4.5 or f5 dobs... my 18mm BCO,10mm BCO are fine in my f4.8's. A Televue Paracorr II can be used to great effect in the faster scopes if desired. Many on here have vast experience with eyepieces at these F ratios and can advise which ones work well.

[jetstream]

4 hours ago, swamp thing said:

Writing f5 like that you could easily get confused into thinking they were. Astronomers write 16" f/5 this shows that  the second figure is the focal ratio of the first figure (which is the diameter of the primary element or aperture), not the photographic speed (f stop or aperture ) of a camera lens  which could be written 2030mm f5 

Confused yet  .....???? What a riot this is eh  

 

I think Tiny Small was getting at the exit pupil issue (among others)- where any scope of any aperture illuminates the eye the same amount if the exit pupil is the same and with the image scale being the variable due to a telescopes focal length. I see where these concepts can be hard to describe for visual astronomy VS photography.

[PeterW]

On 09/10/2016 at 12:23, Timebandit said:

 

Get the 16" dobby, get some decent quality eyepieces  , get to a dark sight and you will see things that those with smaller aperture scopes and those in light affected/polluted areas can only dream about, and enjoy?

You only need a couple of good eyepieces, the new 100degree hobbies ought to work nicely or naglers. 

I have to disagree with the above though... you can see *somethings* better than I can (small scope, light pollution sufferer), but on nebulae I wouldn't count on it! Aperture is good, but too much and you have a weeny field of view, some of the best nebulae up there need bigger fields of view to see. With the right kit and filters I can do quite nicely thanks... can pop in for a cup of tea and be in bed almost as quickly... not stuck in a wet and windy tent.... ;-)!

cheers

PeterW

[Paul73]

17 minutes ago, jetstream said:

You don't need expensive eyepieces for 16" f4.5 or f5 dobs... my 18mm BCO,10mm BCO are fine in my f4.8's. A Televue Paracorr II can be used to great effect in the faster scopes if desired. Many on here have vast experience with eyepieces at these F ratios and can advise which ones work well.

If you aren't using super wide eyepieces. You can get away without a Paracor. I use the Televue Delos range (72°) with an f4 16" and find that the coma is not intrusive. With 82° or 100° it is needed. Look though a 20mm ES 100° without a Parracor and you could be forgiven for thinking that you had just engaged warp factor 8! If you do go wide, the Parracor MK 1 v2 or 3 will do you fine. The only time that it is worth re-mortgaging your soul and getting a Parracor Mk2 is if you have a case full of Ethos.....

Either way. You will find life very hard work if you stick with 50° eyepieces. Sure, the image will be crisp. But star hopping will be a complete chore. 68° is the absolute minimum that I would consider for these scopes.

Lastly, the SW 16" is a big heavy 16". Unless you can wheel it out of a shed / garage for home observing, I would look at some lighter weight options. My OOUK Solid Tube 16" is light and lives in my garage with a specially widened back door so that I can move the thing to its observing spot on a sack barrow. It just fits in a Landrover with the back seats down. My 10" is slung happily in the boot with the family luggage for a long weekend.

As already noted. 4" the 16" is a massive jump. Maybe a pause at 10" would prepare you for the jump to 16" or 20".......... If I had gone straight in at 16", I would have given up by now. Your LP will negate a fair chunk of the big scope advantage on all but the best nights. But the best nights are fantastic!

Good luck.

Paul

[N3ptune]

The 16" looks like a terrible burden for me now.. and a financial black hole.

I am out of the race

[John]

1 hour ago, N3ptune said:

The 16" looks like a terrible burden for me now.. and a financial black hole.

I am out of the race

Everbody is going to be different in this I reckon but over the 30+ years that I've been in the hobby I've found that the scopes that get used least are the ones that involve a deal of hassle to setup, move about and to cool down prior to performing well. I've found 12" in the dobsonian format the largest size that I an easily cope with and I've seen plenty of wonderful sights with it, deep sky and solar system

I have other scopes in the 4" - 5" aperture range so I've seen the difference that the 12" makes and it's not insubstantial, particularly on deep sky objects.

I have looked through larger scopes at darker sites than my back yard and really enjoyed the experience but not to the extent that I'm going to invest in something that I know will not get the use it deserves and will slowly gather dust while the easier to handle scopes get all the fun.

It's a hobby after all so a burden is the last thing I need I feel

"Your mileage may vary" as the saying goes !

[N3ptune]

11 minutes ago, John said:

I've found 12" in the dobsonian format the largest size that I an easily cope with and I've seen plenty of wonderful sights with it, deep sky and solar system

Ok, could be a good advice for the OP also.  and I keep that in mind.  Does the quality of the EP apply less for a 12 inches then a 16 inches? Or is it less important?

[John]

19 minutes ago, N3ptune said:

Ok, could be a good advice for the OP also.  and I keep that in mind.  Does the quality of the EP apply less for a 12 inches then a 16 inches? Or is it less important?

It's the focal ratio of the scope, rather than it's aperture that provides the challenge for the eyepiece I feel. The problem is that large aperture scopes are bound to be reasonably "fast" (and therefore challenging for eyepieces) to ensure that they are practical to make, ship and use. An F/6 16" dob would be relatively kind to eyepieces and be pretty much coma free but the tube would be 8 foot long and the observer would need to be on a small set of steps when observing much above 45 degrees.

 

 

 

[N3ptune]

14 minutes ago, John said:

It's the focal ratio of the scope, rather than it's aperture that provides the challenge for the eyepiece I feel.

 

Yeah that's the problem I have with my 1000 x 200 But it's not really a problem in another way (Some inexpensive EPs are giving great results).

So The situation is only relative to the low F ratio, bellow F5 whatever the aperture it's going to require sophisticated EPs, that's the bottom line.

 

[Paul73]

6 hours ago, N3ptune said:

 

So The situation is only relative to the low F ratio, bellow F5 whatever the aperture it's going to require sophisticated EPs, that's the bottom line.

 

There doesn't seem to be a single answer to this one. But, we can assume that someone shelling out for a 16" scope will have deep enough pockets to spend minimum £50 each on eyepieces. 

You choose the Fov that you can afford, given your quality of view and comfort of use thresholds. The faster the scope, the tougher it gets to find a combination that stacks up.

The trick seems to be. Never ever look through a decent frac. After that your quality threashold rockets and there isn't enough money in the world! i.e. A case full of Green & Black with coma correction is your only hope....

Saddly, the more that you observe, the fussier you get. I was happy with a set of humble MaxVision 68°s at f 4.7.......

Paul

[spaceboy]

7 hours ago, John said:

It's the focal ratio of the scope, rather than it's aperture that provides the challenge for the eyepiece I feel.

I'm half and half with you on this one John. I agree totally with what you say but I also think a 16" is going to reveal a fair few more stars than say an 8" would. The problem then being the outer edges are going to be full of stars showing coma and so could become more distracting in doing so. Also as has been mention the image scale is much larger so you have more of what you want to look at filling the FOV which is going to be effected by coma and the inherent higher magnifications in larger dobs means your going to struggle to back off the magnification enough to frame the object with out the exit pupil becoming a problem. Hope my logic makes sense ?

I know many of us agree that a scope that is too much hassle to use just isn't going to get used. Going from a relatively grab and go 4" scope that can be set up in minutes to a 16" scope that if you didn't brake your back first setting it up, could take up to an 1 hr to cool,  requires a very motivated person IMHO.

[Chris]

8 hours ago, N3ptune said:

 

So The situation is only relative to the low F ratio, bellow F5 whatever the aperture it's going to require sophisticated EPs, that's the bottom line.

 

To quote Tiny Small from earlier "Just get the best eyepieces you can afford". I used to have a set of ES100 degree EP's but with leaner times I've been using a set of simple Plossls for the last year, they still did the business for me in my f/5.5 rich field refractor the other night.

Everythings a compromise, you could of course get a cheaper more manageable 12" f/5 Dob and spend the money saved on Hyperwide EP's. Just do what you can with what money you have  As much as I like to support astronomy retailers the second hand market can help you get the most for your money so might be worth considering.

It doesn't have to be TV EP's when it comes to Hyperwides either, the ES100's work well enough in fast scopes, I know someone who used these in a 16" dob and really liked them.

[Tiny Small]

18 hours ago, swamp thing said:

Astronomers write 16" f/5 this shows that  the second figure is the focal ratio of the first figure (which is the diameter of the primary element or aperture), not the photographic speed (f stop or aperture ) of a camera lens  which could be written 2030mm f5 

 

 

Not to resurrect the dead, but this is the important distinction. Whether people believe me or not, it is a fact that f5 is f5, no matter what scope or camera its on. Whichever scope you have, f5 will collect light at the same rate per unit area! A 16" scope has a lot more area so physically catches more photons but only at the same rate as any other scope of the same aperture. 

 

21 hours ago, Timebandit said:

 

Good job you did not go and buy a 3" f5 instead of a 16" f5 for a cure for aperture fever?

 Mock all you like, the simple fact is that you misunderstood what I was saying.

[John]

If I was going to get a really big dob and didn't have a big eyepiece budget I'd probably get some good quality "normal" FoV eyepieces such as Tele Vue plossls to use with it.

The 50 degree FoV should keep coma to reasonable limits because you are not seeing as much off axis as wide / ultra wides do and the TV plossls, like their other eyepieces, are tested to work well down to F/4. I have faith in the glass types, coatings and baffling that they use as well so at least I'd have the comfort of knowing that the view across the 50 degree AFoV was going to be a decent one.

On the used market you can still pick up TV plossls for around £50 a throw.

 

[Timebandit]

10 minutes ago, Tiny Small said:

Not to resurrect the dead, but this is the important distinction. Whether people believe me or not, it is a fact that f5 is f5, no matter what scope or camera its on. Whichever scope you have, f5 will collect light at the same rate per unit area! A 16" scope has a lot more area so physically catches more photons but only at the same rate as any other scope of the same aperture. 

 

 Mock all you like, the simple fact is that you misunderstood what I was saying.

I was not mocking Tiny and I am sorry you feel this way. I am afraid everything you were saying and implying was incorrect to what I have seen and read by persons much more intelligent and knowledge than both me and you. I would have the decency and integrity to admit if I had been wrong on the matter, it's a shame you cannot do the same and again carry on raising this. Thanks 

[Timebandit]

46 minutes ago, John said:

If I was going to get a really big dob and didn't have a big eyepiece budget I'd probably get some good quality "normal" FoV eyepieces such as Tele Vue plossls to use with it.

The 50 degree FoV should keep coma to reasonable limits because you are not seeing as much off axis as wide / ultra wides do and the TV plossls, like their other eyepieces, are tested to work well down to F/4. I have faith in the glass types, coatings and baffling that they use as well so at least I'd have the comfort of knowing that the view across the 50 degree AFoV was going to be a decent one.

On the used market you can still pick up TV plossls for around £50 a throw.

 

 

This is what I initially did when getting the fast f/ 4.6  14" dob. As televues test down to f/4 then I was confident that the plossl eyepieces would work well in my Dob to which they did. And in the lower magnification eyepieces in the televues plossl range the eye relief is not to bad also. When you get up to higher magnification the 8mm it does start getting a bit tight. Therefore if you are thinking of taking the televues plossl route it may be worth getting the lower power magnification ones, and barlow or powermate them so they are more comfortable.                                                                                                        The only trouble is with me I saw a second hand William Optics uwan 28mm with 82d fov (in all fairness this eyepiece has worked well with my ,f/ 4.6)and once I purchased that it gave me a taste for 2" and wide angle eyepieces  and drove me into Naglers ect ect, it has been a slippery slope since and now I am a bit of a collectionist?

[Tiny Small]

21 minutes ago, Timebandit said:

I was not mocking Tiny and I am sorry you feel this way. I am afraid everything you were saying and implying was incorrect to what I have seen and read by persons much more intelligent and knowledge than both me and you. I would have the decency and integrity to admit if I had been wrong on the matter, it's a shame you cannot do the same and again carry on raising this. Thanks 

I do have the integrity to admit when I'm wrong. But I'm not wrong. The physics and maths behind why a focal ratio collects light at a specific rate per unit area are well founded. Both science and maths work whether you choose to believe them or not and because light is collected at the same rate, the image will either be the same brightness for varying scales or will be the same scale with varying brightness, depending on the unit area of the mirror. This means that taking the eye out of the equation (as everyone's eyes see differently anyway), theoretically, there is no object that a 10" scope could see that a smaller scope couldn't providing they have the same f ratio. The only question that then remains, is whether or not the native magnification on the smaller scope is enough to detect a small object.

[N3ptune]

If a person wants to pay 4000$ for a 16 inches to gather lights to watch the nebulae, I don't assume they have a few thousands more to get 100 degrees Ethos or less like Delos or Nagler or even ES100.

 

[Moonshane]

I think that people are basically agreeing. The problem with Tiny Small's comment (which is quite correct in my view) is that it is akin to saying that all objects in the sky being possible with naked eye (in other words, both are true but counter-intuitive). You can, assuming a very dark sky, 'see' all objects in the night sky with your miniscule eye aperture of 6(ish) mm. The problem is that our eye does not have the resolution (or the magnification / image scale) due to very short 'focal length' to separate the objects from the stars or they are so small that they do not show up as individual stars or objects in the same way that (within reason) galaxies never show themselves as individual stars, just a fuzz of light. This compares directly with a 3" f5 scope with a focal length of 375mm and a 20" f5 scope with a focal length of 2500mm which shows things at the same brightness as at a scale larger by a factor of 6.7 thereby allowing more separation/distinction.

As for cameras, whilst I don't really understand how the light path works, the aperture is as far as I know, the size of the hole in the diaphragm. Therefore cameras (assuming there's a lens with adjustable f stops attached to it) has a variable aperture unlike scopes which are fixed unless an aperture mask is used.

[Moonshane]

That said, let's stick to the issue of guiding the OP towards an informed choice. I don't think anyone would really dispute that, whatever the reason, the views through a larger aperture at a darker site will always be generally more impressive than in a smaller scope in the same circumstances.

[Timebandit]

Ok Tiny your right with aperture and everything else . I give in. ?

[Chris]

Amen to that Shane, this is one of the bigger off topic tangents I've seen guys, lets agree to disagree and help the OP

[Paul73]

1 hour ago, N3ptune said:

If a person wants to pay 4000$ for a 16 inches to gather lights to watch the nebulae, I don't assume they have a few thousands more to get 100 degrees Ethos or less like Delos or Nagler or even ES100.

 

The point that I was making, was that you could spend £50 to get quality views. And, I'll stick with the assumption that anyone willing to spend £1,800 on a nice new 16" Dob, wouldn't be averse spending an additional £100 for a couple of eyepieces, even if the purchase may have to wait a month or two due to the scope purchase clearing out the Astro coffers. I've been there.

A lot of people decide that the additional expenditure to buy the wide angle exotics just isn't worth it. We each have our own budgets & priorities. I choose not to buy 100° for anything below 20mm and stick to 50° for the really hi mags. I'd like to buy the whole Ethos range, but I choose feeding the family over my hobby.

Paul

[Tiny Small]

57 minutes ago, Timebandit said:

Ok Tiny your right with aperture and everything else . I give in. ?

I don't want people to give in, I want them to understand.

What follows is a rudimentary mathematical proof but using rounded figures, so please keep this in mind. If my maths is wrong, someone please highlight where this is the case.

On a 130 PDS, the mirror has a radius of 0.065m.

Using  πr², this gives us an area of π x 0.065² = 0.0133m².

The 130 PDS has a focal length of 650mm (0.65m)

A 10” newt at f5 (the same as the 130 pds, not the more common f4.9) has a 250mm mirror and using the same formula, this gives an area of 0.049m² with a focal length of 1250mm.

Lets now assume that light can be collected at a billion photons per square metre per second. This means that every second, the 130 PDS will collect 10⁹ x 0.0133 = 13,300,000 photons.

The 250 will collect 10⁹ x 0.049 = 49,000,000 photons.

The inverse square law dictates that (intensity₁/intensity₂) = (distance₂²/distance₁²). If we take the focal lengths, with the 130 PDS as distance 1, we get:

1.250²m/0.65²m =3.698

This means fo objects viewed at the same distance (read same image scale), an object in the 10” scope will be 3.698 times brighter than in the 130 PDS.

Now lets take eyepieces into the equation. A 20mm EP will give 32.5 x mag on the 130 PDS whereas the same EP will give 62.5 x mag in the 10” scope.

Magnifying has the same effect as increasing the focal distance so 62.5/32.5 = 1.923. Using the same EP, you have increased the effective distance by a factor of 1.923 times. Again using the basic principle of the inverse square law, the intensity of light is the reciprocal of the distance squared so 1/1.923² = 0.270. Take our initial photon count of 49,000,000 and multiply it by this factor and what do you get? Allowing for rounding errors, you get about 13.3 million photons… exactly the same as the 130PDS.

So there it is… a basic mathematical proof that an object viewed at the same scale will have and increased brightness, or if viewed at a larger scale, will have the same brighness. From this we can conclude that any object capable of being seen in terms of brightness in an f5 scope, can be seen in any f5 scope.

[Tiny Small]

This is only rudimentary though and takes no account of transmission or reflection values, pupil size, eyesight or any other variables. But it serves to demonstrate the point and is simple enough that everyone should be able to follow it.

It also assumes that the 650mm focal length of the 130PDS has a distance of 1 unit. You can set any arbitrary number to unit one as long as it is subsequently used as the base unit of measure.