Eyepiece advice

[astrogaze]

Hello,

I'm likely to be buying my first telescope soon (Skywatcher 200p). I've been having a read and would appreciate some advice on eyepieces and whether I have understood how they work.

I am most interested in viewing planets in good detail - please correct me if I've misunderstood the following:

To calculate the maximum theoretical magnification for a telescope before it starts losing quality and too much light, I multiply the aperture in inches (8") by 50 = 400x magnification.

To calculate the best theoretical eyepiece to use I divide the focal length (1200) by the eyepiece (6mm) to get 200x magnification. If I combine this with a Barlow lens I double that, to give 400x, which is the maximum advised magnification. 

However, due to atmospheric disturbance, the recommended magnification is usually around 250x.

Therefore to get best planetary views I should buy a 5mm eyepiece and no Barlow lens because 1200/5 = 240x magnification, which is only 10x below the recommended magnification.

Please can someone advise me if this is correct and therefore which 5mm eyepiece I should buy? I'm prepared to pay perhaps around the £50 mark but I am happy to pay more or less depending on what people advise.

Thanks!

[nightfisher]

Hello,

Yes you have got the sums right, and done some homework,i would advise against a 5mm as you will only get the best out of this size on nights of very good seeing.

I would be looking on astro buy and sell for a used orthoscopic in 7 or 9mm focal length

[ronin]

First forget the theoretical maximum.

It is something that manufacturers put on the box, I have seen 1.5x, 2x and recently 3x and 3.5x.

The multiplier being applied to the diameter in mm.

The maximum is whatever conditions allow and what you find acceptable.

The chances are this is less then the "maximum" but occasionally will be more.

Generally tha atmosphere will allow 200x mag but often less.

The magnification is Scope Focal Length/Eyepiece Focal Length is it not "best" is just a number.

If you are intending on planetary then look at the Celestron X-Cel eyepieces for the reason they are in 5mm, 7mm 9mm and 12mm.

When a 5mm may not work a 7mm could well deliver an image.

The other aspect is you can eventually get the remainder of the set a bit at a time and then you have eyepieces to use for a long time to come.

When it comes to "maximum" there really is no specific answer.

If one night you can use and get say 300x but just that one night in a year what does that mean?

[Sirius Starwatcher]

Hello Astrogaze if you look at my signature you can see the eye pieces I use and they match the 200p very nicely. Seeing is variable and can change significantly even within a couple of hours of observing. If you restrict yourself to a couple of eye prices you could miss out if the seeing is good and you were unable to go to a higher magnification because you have a restricted choice of EP,s. I am aware that cost is a factor and some people are not in a position to,have a large choice so in that case your understanding of magnification and sizes comes into play, allowing you to choose a spread of EP,s coupled with a Barlow lens, you are of course spot on with your understanding of the math. The BST,s are a reasonable priced EP and could be collected over a couple of months. Best of luck with your choice.

[Dannae]

Hi astrogaze,

I would agree with Jules' reply wholeheartedly; a good quality orthoscopic will remain with you even when your telescope changes.

I always recommend backing off from theoretical magnification to a smaller and more pleasant view. In my experience the lower the magnification the easier it is to begin to learn your way around a complex target like Jupiter. The moments of good and bad seeing tend to even each other out when you pull back from the max power available to you on a given night. That being said and at times of opposition or really good seeing it is worth pulling as much power as it will allow, but you will have to piece together your view as the image comes in and out of clarity much more chaotically.

It nearly always comes done to a comfortable and more stable smaller image scale versus a staccato series of fleeting images on a larger scale.

Regards

Dannae

[Gazabone]

Hi Astrogaze,

Great to see you've done the research and have understood that any telescope will have a point at which it won't deliver any more worthwhile magnification.

One point that hasn't been mentioned yet is the value of a low powered ep for two reasons; it may well help you locate your targets before popping in a more powerful ep otherwise you could find yourself making it harder for yourself than needs be. Additionally, there's a good chance that searching out the deep sky objects will take your fancy and the less powerful magnification eps are better. I've always found it a bit ironic that the wider view eps are more suitable for the more distant objects!

I concur with the view about Celestron x-cel eyepieces; they are, in my view, good value for money and you'll notice a significant improvement on the stock ep your scope will be supplied with. Personally I think the extra that the Baadar Hyperion costs is worth it but the x-cels are very usable indeed.

At the moment I'm finding I'm almost exclusively using 24mm and 8mm.

If it's any help I rarely find my x-cel 5mm on my 250P delivers a particularly good image and I end up reigning back the magnification to sharpen up whatever detail is visible.

[Fish]

Some great advice above but just to thow in the mix .

If you are getting the 200p with the stock eyepieces then consider just getting to know the scope with the eyepieces that come with the scope , you will get hours of good viewing with them.

Save up a bit more cash and get a Baader Hyperion Mark III Zoom 8 - 24mm , then look for a 32mm TV plossl . Some will say that the Baader has too much distortion but when I use my 200p these two eyepieces are the only two I need for a nights viewing and they give fantastic views of everything. Those two eyepieces will reside in your eyepiece case for quite some time and when the time comes to upgrade you will get a good return on them.

Best of luck with your choice :-)

[damnut]

All the advice you need is above, but have a quick look at the links below.

http://www.12dstring.me.uk/fov.htm

http://www.stargazing.net/naa/scopemath.htm

[astrogaze]

Thanks for all your advice everyone it's really helpful, I will have a think!

[style121]

Sorry Astrogaze if I hijack your thread, but instead of starting a new one with a similar query, I just have one or two questions. I've just recently purchased my Skywatcher scope and mount, which came with a 10mm and 25mm eyepiece.

Now i'm very interested in buying a new set of eyepieces, but the sites i'm looking at are starting to confuse the issue. You can buy a set of eyepieces listed as 32mm, but in the set I can see various sizes ranging from 6.3mm up to 40mm. So what gives? How do I know which ones to buy? Are they for a particular strength of eye, or does the 32mm simbolize the diamter of the eyeholes? Really confused. Sorry, complete amateur who knows absolutely nothing, and the astronomy club is too far away from where I stay.

[damnut]

Hi Style121,

In essence you need a low power eyepiece, a mid range and high power. To  calculate the magnification of each eyepiece by dividing the focal length of the telescope by the focal length of the eyepiece. So not wanting to re-invent  the wheel or change its colour there is a great guide to eyepieces on SGL by The Warthog and another guide written by R Wilkey to be found here :- http://www.swindonstargazers.com/beginners/eyepieces.htm

If you tell us what scope you have it will allow folk to advise on what might be the best for you. The key is not to go mad with acieving a high magnification, there is always a limit to what your scope can meet and the conditions under which you view.

[sungrazer]

Hi Astrogaze & style121,

As a relative newcomer to astronomy myself I will offer my sympathies - choosing the right eyepieces is a minefield.  I will, however, go with the general consensus that magnification is not the be-all and end-all, as there is a point of diminishing returns, usually coupled with less-than-ideal seeing conditions.  

I can certainly vouch for the Celestron X-Cel range, I started off buying the 9mm and 25mm, which coupled with a 2x barlow and my 'scopes focal length of 1500mm gives me a good range of magnifications ranging from 60x to 333x.  I also added a 40mm omni plossl, which gives a much lower magnification, but a wider field of view (the amount of sky you can see through the eyepiece), which is great for viewing open clusters such as M45.

The X-Cel eyepieces also offer good eye-relief (the amount of distance between the eyepiece lens and your eye), more is better as it's more comfortable (especially if you wear glasses).

If you want the full range of magnification, Baader do a "zoom" eyepiece, which has clickstops at various focal lengths, I can't vouch for it's performance though.  Maybe someone on here has used one.

Enjoy your new scope!

SG

[bingevader]

Astrogaze, as Fish has said, relax and get to know your 'scope a bit first.

Having said that, I have the 5mm and 7mm X-Cel LX eyepieces and think they're rather good, but as others have said, the 5mm rarely gets used on planets.

Also, though, I have the 6mm Williams Optics SPL which is splendid and an 8mm plossls for when I need to drop back even further.

The reason for the 4 eyepieces at only 1mm apart is because the difference in magnification is greater at these short focal lengths.

[style121]

Hi Style121,

In essence you need a low power eyepiece, a mid range and high power. To  calculate the magnification of each eyepiece by dividing the focal length of the telescope by the focal length of the eyepiece. So not wanting to re-invent  the wheel or change its colour there is a great guide to eyepieces on SGL by The Warthog and another guide written by R Wilkey to be found here :- http://www.swindonstargazers.com/beginners/eyepieces.htm

If you tell us what scope you have it will allow folk to advise on what might be the best for you. The key is not to go mad with acieving a high magnification, there is always a limit to what your scope can meet and the conditions under which you view.

Thanks for the link, will go through it now. I have a skywatcher 150 x 750 refractor. Which came with a 10mm and a 25mm eyepiece. I've ordered what looks like a set of eyepieces ranging from 6,5mm to 40mm, except it's listed as a 32mm set. I don't know what the rules are regarding the site and posting links to online stores, or else I would show you a picture of the set to get some clarity. I now understand the magnification limitations of the scope