Questions about magnification and eyepieces

[Atomic_]

I've just recently bought my first telescope, skywatcher 8 inch dobson with 20mm and 12.5mm 70degree wide angle eyepieces, and haven't had a chance to use it due to bad weather, so I'm just trying to learn about it. 
 

From what I've read about magnification mine should be something like:

Telescope	Focal Length (mm)	Diameter (mm)	Focal/Ratio	Finder Scope	Eyepiece 1	Eyepiece 2	Minimum Magnification	Optimum Magnification	Maximum Magnification
Skywatcher 8" Dobsonian	1200	203	5.9	8 x 50	70° Wide Angle 20mm	70° Wide Angle 12.5mm	29	290	406
					Eyepiece Magnification	Eyepiece Magnification
					60	96

However I'm confused about the Minimum/Optimum and Maximum Magnification, the minimum magnification for the telescope being 171x and the eyepieces magnification being 60x and 96x for the 20mm and 12.5mm. What does it mean to have a eyepiece magnification lower than the minimum magnification of the telescope? I'm assuming it just means the minimum 'useful' magnification for details.

At the moment I'm wanting just to see the planets and moon. would the 20mm and 12.5mm eyepiece be enough or should I look at other eyepieces? from reading This Post, it suggests the 5mm, 10mm, 16mm, and 25mm eyepieces for a 8inch dobson, but what would the practical applications be for the other eyepieces be? What would they be used to see?

Thanks for the help!

 

Edited June 9 by Atomic_

[Dark Vader]

I think you can safely disregard the information in those last 3 columns. I'm not aware of a minimum magnification for the 8" Skywatcher. Theoretical maximum magnification is basically 2x your diameter, so 406x. Optimum magnification is very dependent on the seeing, and how what you're trying to observe responds to magnification.  Under UK skies I rarely get over 200 - 250x with my 8" Dob.

The magnification you've stated with the 20 and 12.5 eyepieces is correct. The 20 will most likely show the whole of the lunar disc, the 12.5 will start to show detail on Jupiter and Saturn. A 7mm should give you 171x which is a good area for planetary detail.

[Atomic_]

3 minutes ago, Dark Vader said:

 Theoretical maximum magnification is basically 2x your diameter, so 406x. Optimum magnification is very dependent on the seeing, and how what you're trying to observe responds to magnification.  Under UK skies I rarely get over 200 - 250x with my 8" Dob.

I'm thinking the math/formular I found was wrong then, from what you said I'm guessing it was meant to use Diameter instead of focal length for the calculations, which gives more reasonable answers being; 29x, 290x, and 406x for the minimum, optimum and maximum magnifications.

I'll have to have a look at some 5 or 7mm eyepieces for planetary details.

thanks

[Albir phil]

Ok if you take the mm of the eyepiece and devid it into your scope f/l that will give you it's magnification.So the smaller the eyepiece mm the higher it magnifying power.for planets and moon high power is needed however air condition will dictate how high you can go.

[Tiny Clanger]

It gets more complicated than just magnification though !

There's also exit pupil (essentially the diameter of the circle of light a 'scope/eyepiece combo presents to your eye) which affects what you can see : a mid range mag. eyepiece might seem pointless but it can help get a good contrast between the perceived brightness of the deep sky object you are looking at and the background sky around it.

Your eye also has an exit pupil range it can accommodate, which varies from person to person. There's plenty of explanation to be found about this, I'd start here :

 

 

Heather

[Albir phil]

19 minutes ago, Atomic_ said:

I'm thinking the math/formular I found was wrong then, from what you said I'm guessing it was meant to use Diameter instead of focal length for the calculations, which gives more reasonable answers being; 29x, 290x, and 406x for the minimum, optimum and maximum magnifications.

I'll have to have a look at some 5 or 7mm eyepieces for planetary details.

thanks

Best thing to do is not to get too involved with things, just try what you have see what you see, and when you're more used to the scope look into things more.

[GrumpiusMaximus]

18 hours ago, Albir phil said:

Best thing to do is not to get too involved with things, just try what you have see what you see, and when you're more used to the scope look into things more.

I agree with this notion initially and once you start branching out, it's worth knowing roughly what to expect 'on paper'.  Eyepieces with eye relief that is too long and hard to use, for instance.

20mm and 12.5mm are great starting point eyepieces.  However, if you want to buy more you need to have a rough idea what to expect.  So learning about things like exit pupil, magnification formulae, AFOV and eyepiece types will be eventually be greatly beneficial.

Edited June 10 by GrumpiusMaximus

[JOC]

20 hours ago, Atomic_ said:

However I'm confused about the Minimum/Optimum and Maximum Magnification, the minimum magnification for the telescope being 171x and the eyepieces magnification being 60x and 96x for the 20mm and 12.5mm. What does it mean to have a eyepiece magnification lower than the minimum magnification of the telescope? I'm assuming it just means the minimum 'useful' magnification for details.

The maximum magnification is easy - in theory the most magnification you can get to before things don't look right.  However, it is only a mathemetical/theoretical determination.  I've got exactly the same scope (I'm guessing you have something like a skywatcher 8" flextube) and a 4mm would give x300 - divide tube length in mm by EP size in mm = magnification.  In reality, in the UK at least, we have a battery of atmospheric conditions which consipire such that it might sometimes/rarely be possible for me to use a 5mm (x240), but practically not much more and the sweet spot is around a 7mm approx x171, for clarity on most nights.  

[AstroMuni]

22 hours ago, Atomic_ said:

At the moment I'm wanting just to see the planets and moon.

22 hours ago, Atomic_ said:

but what would the practical applications be for the other eyepieces be? What would they be used to see?

In a nutshell, having a range of focal lengths gives you the ability to zoom in and out. A practical way to reduce the number of eyepieces is using a barlow and/or buying a zoom eyepiece. Take a look at this site which gives you an indication of how the object appears in your eyepiece and has suggestions on the focal lengths to get https://www.stelvision.com/astro/telescope-simulator/

Edited June 10 by AstroMuni

[Mr Spock]

Magnification depends on seeing, aperture and target.

Assuming perfect seeing, my 4" apo is best on the moon at x211, and my 12" at x461. Jupiter on the other hand, is best at around x180 regardless of the scope I'm using. Note: with the 12" on Jupiter I use a variable polarising filter to bring the brightness down otherwise it's a 'white out'.

For an 8" f5.9, I'd recommend two Morpheus. A 6.5mm will give x185 and a 4.5mm x267. So x175 for Jupiter, and x267 for moon and other planets.

[JOC]

Mind you there are plenty of targets that aren't planets or the moon for which you actively don't want such a large amount of magnification, for example Pleiades star cluster won't give you the 'wow factor' if you are too close in.  Also, I've found that some double stars are not ideal at really high magifications and have actually split more cleanly at lower mags - that might again come down to atmospheric conditions I guess.

[pregulla]

These values from minimum/maximum/optimum magnification don't mean much.

Usually people think more in terms of exit pupil (aperture divided by magnification, or eyepiece focal length divided by focal ratio, so 30mm eyepiece in F/6 scope will give 5mm exit pupil).

There is no minimum magnification, but if exit pupil is larger than your dilated pupil the image won't become brighter as you go lower in magnification, still may be useful if it provides larger true field.

If the exit pupil is way larger than your pupil then you'll begin to see a shadow of secondary mirror as a dark area in the center of the field. I doubt you'll run into this issue unless you are trying to observe the moon with 40mm eyepiece or something like that

As rule of thumb it is not recommended using eyepieces that provide exit pupil larger than 7mm (That's 42mm eyepiece in F/6 scope). If you are observing under light polluted skies exit pupils larger than 4 or 5mm may provide too bright and washed out images. Still might be worth using it as a finder eyepiece to get larger field of view.

There is a tradeoff between image brightness and magnification. As magnification goes up image brightness reduces. General rule is that optimum is somewhere in the middle around 2mm exit pupil. It's a very rough guideline, larger exit pupil for dimmer objects, smaller for brighter objects, some objects are best viewed at much higher powers than that, some need much lower powers to fit in the filed of view. But very roughly "optimal magnification" eyepiece would be something in the 10-14mm range.

Theoretical maximum magnification is usually considered around 0.5mm exit pupil, that's 400x for your scope, but seeing (atmospheric stability) will become the limiting factor way before that. 

I would get 2" eyepiece in the 30mm range. Something like 30mm UFF or 28mm UWA is a good budget option, but even something as simple as 30mm Superview will provide enjoyable viewing experience even if it's going to be a mess at the edges. 

You can also get a zoom eyepiece + barlow, that will cover all other magnifications fairly cheaply and you can play with it to find out what magnifications work for what and what fixed focal lengths would you like to get. 

I use 30-18-11-7 for DSO and zoom+ barlow for moon and planets.

[Gonariu]

A rule of thumb says that the maximum magnification for a given telescope is 20-25 times the diameter of the objective in cm; then this limit is higher for refractors (in particular for apochromats) and lower for reflectors. In the user manual of my Ziel Gem 60 I found this table which I report by translating the terms from Italian.

Objective diameter  (mm)                                 maximum useful magnification

                                                                        reflector                           refractor

            60                                                           120                                    150

          100                                                           210                                    264

          120                                                           232                                    300

          150                                                           262                                    346

          200                                                           305                                    412

          250                                                           343                                    469

          280                                                           364                                    500

          300                                                           377                                    520

          350                                                           408                                    566

          400                                                           437                                    608

Edited June 20 by Gonariu

[Louis D]

On 20/06/2024 at 06:17, Gonariu said:

A rule of thumb says that the maximum magnification for a given telescope is 20-25 times the diameter of the objective in cm; then this limit is higher for refractors (in particular for apochromats) and lower for reflectors. In the user manual of my Ziel Gem 60 I found this table which I report by translating the terms from Italian.

Objective diameter  (mm)                                 maximum useful magnification

                                                                        reflector                           refractor

            60                                                           120                                    150

          100                                                           210                                    264

          120                                                           232                                    300

          150                                                           262                                    346

          200                                                           305                                    412

          250                                                           343                                    469

          280                                                           364                                    500

          300                                                           377                                    520

          350                                                           408                                    566

          400                                                           437                                    608

I haven't seen too many consumer refractors with 10 to 16 inch objectives, though. 😉

There is this 20 inch objective if you have the money:

According to the table, you should be able to get up to about 775x.  I'd hate to wrangle that OTA into and out of a trailer, though.  It would be about 21 feet long and who knows how heavy.  I'm trying to picture the mount for it. 🤪 Perhaps a folded design would be best.

[Gonariu]

Hi, I can tell you that in the Turin Astronomical Observatory there is the double Morais refractor, one of 38 cm (14.96") which was used for photography and the other, the 42 cm (16.54") which was used for observations visual and as a guide of 38 cm. In a very old astronomy magazine in Italian ("Nuovo Orione" n. 3 of July-September 1980) a great amateur astronomer of the 80s, E. Moltisanti, writes that he used the 42 cm to observe Ganymede and that he saw it easily a dark spot at X667 (certainly it was Regio Galileo). I would add that the director of the Turin Observatory, W. Ferreri, would have said that he had sometimes used X1000 on doubles, always with 42 cm. The problem is that, given any optics, we then need to see if the seeing allows us to use a high magnification.
The problem with the 20" that you mentioned is essentially this, being able to buy it (and clearly the matter doesn't end there, you also have to buy a suitable tripod and mount not to mention the rest of the equipment......): one must transform astronomy from a hobby to a reason for living, that is, into a real job (and here P. Lowell teaches!); if one also has this vocation beyond money then.......