Which eyepieces ?

[RAGWORM]

Hi 

i`m new to this and only two eye pieces a super 10mm and a super 25 wide angle long eye relief 

just tried looking at planets with daughter and could just about make them out but feel i need to make them bigger 

is that possible if so what size eye piece would i need dose it work bigger number bigger image or am i being stupid

[kev100]

Hi ragworm, what scope do you have? You'll probably have noticed that the lower the focal length figure of the eyepiece, the higher the magnification. The actual mag figure that an eyepiece can deliver is related to the focal length of the scope.

Kev

[Putaendo Patrick]

As Kev says, the magnification of any given eyepiece is relative to the focal length of the particular telescope.

The formula is to divide the focal length of the telescope by the number on the eyepiece. For example if your focal length of the telescope is 900mm and the eyepiece is 10mm you will get a magnification of x90. However, if the focal length of the telescope is only 650mm, then the same 10mm eyepiece will give x65.

The formula also works in reverse. Supposing you want a magnification of x120 in a 900mm telescope, 900 divided by 120 will give 7.5. So you will need an eyepiece of 7.5mm.

[Ruud]

Hi Ragworm, welcome to the stargazers lounge.

Kev and Patrick are right. We need just one of one of the four descriptives  below to get going, but if you know more that would be helpful

1.   the diameter of the objective and the f-number of the telescope (e.g. 70 mm f/13) 
2.   the diameter of the objective and the focal length of the telescope (e.g. 70 mm,  900 mm)
3.   the brand name and type of the telescope (e.g. Celestron AstroMaster 70EQ)
4.   a link to the telescope on the internet. (e.g. https://www.firstlightoptics.com/beginner-telescopes/celestron-astromaster-70eq.html)

Have you already downloaded Stellarium? Stellarium is a free planetarium program to help you learn the night sky and discover new targets to study with your telescope. It's really pretty too. Here's a link: http://www.stellarium.org. The downloads are at the top of the page. Use the location window [F6] to set it up for your home town.

A very good target to practise on is the Moon! 

screenshot Stellarium:

[Dave In Vermont]

+10 for suggesting Stellarium! This will also allow you to set it to show you what you wish to see - like a planet, the Moon, star-cluster, etc. - will look like through your own telescope and using the eyepieces you have at present.

If you have been led to believe that planets will look like the photos in magazines, or on the boxes in cheap department-store telescopes, don't feel bad. Most of us were led astray at first. But once you do some reading and studying-up on what CAN be seen, you'll find there are some TRULY amazing things well within your reach! Allow me to suggest a wonderful book to read and/or acquire:

Turn Left at Orion - by Guy Consolmagno (Author), Dan M. Davis (Author)

https://www.amazon.co.uk/Turn-Left-Orion-Hundred-Telescope/dp/0521781906

Between Stellarium and TLAO (above), you'll be well prepared to sally forth into outer-space with your daughter!

I'll leave you a screenshot of my copy of Stellarium. Ruud showed his set-up as it will be when you first start it up. Mine is set-up for much more advanced usage. This to demonstrate how wide-ranging this fantastic - and totally FREE - truly is! I consider it to be the greatest FREE software-program ever for us astronomy-folks!

Enjoy!

Dave

 

 

Click-on image for full-size.

[RAGWORM]

thanks for the advice will go investigate 

f = 650mm on my scope 

[Ruud]

650 mm. That must be the focal length. With it you can calculate the magnification of an eyepiece.

The 10 mm eyepiece gives a magnification of 650 / 10 = 65 times.
The 25 mm eyepiece gives a magnification of 650 / 25 = 26 times.

If the telescope comes with a 2x barlow you can use it to boost the magnifications by a factor two. 
65 times, by the way, is enough to see the rings of Saturn or the moons of Jupiter

---

How far you can ultimately go with the magnification depends on the aperture of the telescope. For a refractor (lens telescope) the aperture is the diameter of the front lens, for a reflector (mirror telescope) it is the diameter of the main mirror. If you have a refractor you can easily measure the aperture yourself. Be careful not to scratch the lens.

If your telescope has am aperture of 70 mm, the maximum magnification is 140x . If the aperture is 114 mm, the maximum magnification is 228x. In general, the maximum magnification is twice the aperture.

---

Alternatively, if you find the f/number of your telescope (aka its focal ratio), you can use that to find the strongest eyepiece that will work on your telescope: divide the f/number by two and you know the shortest focal length eyepiece you can use.

An example: on an f/10 telescope you can use eyepieces down to  10 / 2 = 5 mm

[Putaendo Patrick]

O.K, so you've now probably calculated that your 25mm eyepieces will give a magnification of x26 and the 10mm will give x65 in your telescope. Both are quite low magnifications and won't really allow you to see much detail on planets. Personally I generally observe the planets at between x120 and x200, depending on the atmospheric conditions.

Two factors limit the maximum magnification you can usefully achieve. The first is the telescope, beyond a certain point it will lose resolution making further magnification pointless. To an extent this depends on the type of telescope and the quality of optics and/or mirrors, but a general rule of thumb is to double the aperture in millimeters to give a rough idea of maximum magnification. Thus an 80mm telescope could theoretically handle x160 while a 200mm aperture could go up to x400.

The second factor is atmospheric conditions on the night when you are observing. In England, many people claim the maximum magnification when conditions are very good is about x200. If you ever found yourself in the high-altitude very dry Atacama Desert in Chile, you could happily double this on a good night! Remember that atmospheric conditions are least when you're looking straight up, and worst when you're looking towards the horizon. Very dark skies well away from big cities, although not so important for planets, will also help enormously for observing many types of object.

Personally I don't find a focal length of 650mm is ideal for visually observing planets, but a reasonable 5mm EP would give you a useful x130 magnification with your particular telescope. EPs vary in price from about 20 pounds to more than 500 pounds! But there are plenty of good options at the lower end of the scale. Older designs such as Plossl and Orthoscopic EPs will work well, they aren't expensive and Plossls often come up second-hand very reasonably. The only down-side is that they have limited eye relief, meaning your eyeball needs to be very close to the glass (so useless if you use spectacles!).

Another possibility is to buy a x2 Barlow which will effectively double the magnification of any EP that you have. Be aware, howwever, that the 10mm EPs supplied in the box with many telescopes are often not the best and the Barlow will also double the problems of any weakness in the eyepiece.

(Ruud has just posted above while I've been writing, and covers many of the same points, but I'll post this as well )

[k9wis]

Another bit of information about your telescope system is to determine the field of view your seeing with each eyepiece.

Set a star that is near the celestial equator in the center of your eyepiece. Measure the amount of time it takes for the star to drift to the edge. Stars on the celestial equator move at a rate of 4 degrees per minute. The time you measure is the time it takes the star to travel half of the field of view of you eyepiece. Lets say it took 2 minutes to travel across half the view of your eyepiece. That means it would have taken 4 minutes to cover the entire view..(Measured time x 2) Divide the 4 minutes by the star rate of 4 degrees per minute and you get the field of view for your eyepiece. In the example 4 minutes / 4 degrees per minute = 1 degree. Now when you use programs like Stellarium or earth centered universe or cartes du ceil, you can set the FOV of your eyepiece and use the circles to see what you would see in your eyepiece..Then you can start asking questions about star-hopping to find objects

Good Luck, your adventure is about to begin..

Brian K9WIS

[Planet Squirrel]

Hi All,

 

Reading the above it seems that for this http://www.rothervalleyoptics.co.uk/skywatcher-star-discovery-150p-goto-parabolic-reflector-telescope.html  I can go down to a 2.5mm eye piece.  I was told by the shop that 7mm was the smallest I want to go, so I'd imagine somewhere between the two, but I'm probably wrong.  But I don't want to spend £36 on something just to have a 7mm to gowith the 10 and 25mm.  I want something that is going to be worth spending the moeny on.  

Based on this am I best sticking with the http://www.rothervalleyoptics.co.uk/20277.html  7mm, or should I buy a better quality one with a different or same focal length?

Any specific suggestions on the rothervalley site would be good as I'm ordering the whole kit from there later tonight.

 

Cheers

[Planet Squirrel]

Don't worry, Faulksy has kindly answered me.  I only posted as i thought I'd put it in the wrong place.  Feel free to delete. (sorry for double posting)

[noah4x4]

10 hours ago, Planet Squirrel said:

Hi All,

 

Reading the above it seems that for this http://www.rothervalleyoptics.co.uk/skywatcher-star-discovery-150p-goto-parabolic-reflector-telescope.html  I can go down to a 2.5mm eye piece.  I was told by the shop that 7mm was the smallest I want to go, so I'd imagine somewhere between the two, but I'm probably wrong.  But I don't want to spend £36 on something just to have a 7mm to gowith the 10 and 25mm.  I want something that is going to be worth spending the moeny on.  

Based on this am I best sticking with the http://www.rothervalleyoptics.co.uk/20277.html  7mm, or should I buy a better quality one with a different or same focal length?

Any specific suggestions on the rothervalley site would be good as I'm ordering the whole kit from there later tonight.

 

Cheers

The reason 7mm was probably recommended is that whilst a 2.5mm might offer you maximum magnification you will need amazing "seeing" conditions to benefit (rare in the UK) and also require superb vision to handle the eye relief and other limitations. Don't waste your money on cheap high magnification EP's  (the accessory kits are particularly poor).  I would stick with what you have for six months until you determine what you really want to see. After a few views of the planets (no disrespect,  but these will be limited by your modest budget telescope), you will probably start chasing DSO's where lower magnification is appropriate. 

[Charic]

Planet Squirrel.....Your  eyepiece is part of the optical train, it magnifies the telescopes  virtual image  produced at the focal plane.
Under perfect  skies,  the  telescopes capability and the eyepiece in use,  at best,  the eyepiece simply magnifies that image  produced at the focal plane ( on my Newtonian, if I remove the eyepiece, the image is still there, floating as it were, but far  too small to see any details, but its there. If I had  the same type of scope with a bigger aperture, this virtual image would be larger too )  therefore the larger and  more detailed ( more aperture ) the virtual image the scope can provide, the better the final image through the eyepiece. Its not the other way round, the eyepiece cant improve the virtual image, it only magnifies it.

How much you want to spend is down to you, the end  user, but  don't expect miracles, in the belief that an eyepiece alone will make all the difference. I have £23 Plössl's, £49 BST Starguiders and £330 Tele Vue Delos ( Retail prices ) yet they all show me the same image details due to the capability of my scope,  the only differences I sense are the wider fields of  view and better eye relief  in comparison to the  supplied eyepieces that came with the scope. 

My 5" Celestron was poor at providing the details I desired, my  8"  Skyliner  is better, yet  there are folk here who have Dobsonian's larger than 12", I can only  dream of the details their scopes provide,  I often read about it, never experienced it yet!

It turns out my eyepieces are based around the focal ratio of my present scope, and its power ability, so  my F-1200,  A-200, f/6  provides me 200x using a 6mm. More than capable. The 12mm is my Medium powered EP  Twice the focal ratio, half the power at 100x, twice the exit pupil at 2mm. My Low powered EP  is based on ( f/6) 6xEntry pupil,  the 32mm Panaview works fine for me (  30mm to stick with the formula I suggest ) If I wanted to go higher with the magnification, a 3mm would work  ( I have a 3.2) but this only gets used on the Moon,  the 5mm is a little easier providing  240x power, but with only an 8" 200mm aperture, the detail on my Planet, Jupiter, is not amazing, often only brief dependent on conditions, a larger scope would help me here! All the other eyepieces are just a affordable luxury?

You won't go wrong with matching a 5mm to an f/5 scope, but push further and the results  will diminish.

[Putaendo Patrick]

The supplied 10mm EP that comes with your telescope isn't particularly fantastic. And the Barlow, although not bad, isn't great quality. Even so, both should be enough to get you started with magnifications of x75 and x150.

Some people like Barlows, others don't, but they have two advantages: they are an economic way to increase your range of magnifications, and they maintain the eye relief of larger focal length eyepieces.

In the medium term, I might consider getting better 12mm (x62.5 magnification) and 8mm (x94) EPs together with a better x2 Barlow which would effectively add 6mm (x125) and 4mm (x187.5). You certainly don't have to do this all in one go, and depending on how happy you are with the supplied 10mm, I'd probably go with a decent 8mm to start.

EPs really can be quite a personal decision, finding the most comfortable eye relief for you and the best field of view for your observing style. So take your time. If you can go to an observing session with a local astronomical club, you may have the opportunity to try different EPs and see which you personally like best. Once you know what you want, there are good second-hand offers to watch out for so you don't necessarily have to spend a fortune.