which mm to get for 130mm apature

[Bungielad]

I want to get myself a new BST eyepiece but am still a little confused as to which one to buy in regards to mm.

I have the skywatcher 130p which has a 130mm apature and focal length of 650. I already have a 6mm HR Planetary which when barlowed x2 gives me 216x mag, if I put my 2.5x Barlow in with the 6mm it is to much for the scope at 270x mag.

If I get the 8mm BST im assuming when barlowed with the 2.5x Barlow it will give me a 203x mag.

If I get a 7mm BST and barlowed with a 2.5x it will give me 232x mag....what I want to know is. ...obviously eye peices are better than others but is the slight difference in mag with my 6mm Hr planetary barlowed at x2 going to be much different to a BST 7mm at barlowed at x2.5.

Also a big question that I need to know. ....am I getting a little confused on what the mag should be for certain objects or is it always best to try for the highest mag(highest being lowest) poss when observing DSO or planets??

Bungielad.

[Andy_E]

Its very rare you'll ever be wanting to go over 200x in the UK with a 130p. As for what magnification, it entirely depends on the object you're after. a good spread of magnifications is what you'll be wanting... You'll be wanting a nice spread from 30x - 150x for the vast majority DSOs in your telescope. 150x will also give you nice views of some of the larger planets too. It's easy to forget just how big DSOs are... the Andromeda galaxy is over four times the size of the full moon, it's just a lot fainter. I'd look at getting the set of say 25mm, 18mm, 12mm and 8mm (you can always barlow the 8mm if you want to push things). This will see you well for most DSO observing.

[TwoPi]

Planets generally benefit from high mag, DSOs generally don't. Not just because of size, but also because the higher the mag, the fainter the object appears (the same amount of light is spread over a larger area - for planets you generally have plenty of light, for DSOs you generally don't).

I tend to use somewhere between x50 and x100 for most DSOs. For planets x200 on a reasonable night, and (although I've never experienced this myself) x250 for very good nights.

Hope this helps.

[inedible_hulk]

I found this really useful when selecting mine

http://stargazerslounge.com/topic/43171-eyepieces-the-very-least-you-need/

[AlexB67]

For your scope and DSOs I'd go for the 12 or 15mm, it is all about balance in contrast and FOV and striking the right amount of power per inch in aperture, not darkening the image too much yet seeing enough, magnifying as much as possible is not a good thing to go by for DSOs, that being said it can help suppressing background sky brightness. In the end of the day it is a balancing act of a more than one factor. In a bigger scope different rule apply as to what focal length is suitable. What I can tell you having the same aperture and focal length as you is from experience in such a scope, though not extensive experience I should add. The stock 25mm eyepiece is not bad, but the 10mm is weaker. The 15mm is the first BST I got anyway.

If planets is what you are after or moon get the 5mm or 8mm, but for DSOs the 5mm would not be good apart from a really bright object like M13 perhaps I had some good views in the 6mm UWA I have, but it is rare, and the 8mm you would use in some cases more often.

Correction. It is the 8mm I got first, because when taking FOV into account with the BST it was the closest to the stock 10mm eyepiece, still I would stick by what I said

[Qualia]

Bungielad, my own journal and a number I've seen on the web have a number of observations on exit pupil (EP's focal length / telescope’s f/ratio) which effectively conclude:

• 6mm - 4mm - nice wide field, handy for star hopping, really nice for star fields and large open clusters
  
• 4mm to 3mm - nice contrast, good on star-fields.
  
• 2.5mm to 1.5mm - nice for just about any DSO and general planetary viewing.
  
• 1.6mm to 1mm - nice contrast and resolution on planets, Moon and globs.
  
• 1mm to 0.5mm – possible finer detail on planets tweaked, Moon work, tight doubles.
  
• 0.4mm – not any occasion to justify an assessment.
  

So, in terms of your 5" f/5, it's quite easy to work out a general range of focal length EPs. Just take the exit pupil range and multiply it by your telescope's focal ratio. For example, to get the 6mm to 4mm exit pupil, you're looking at 30mm to 20mm EPs etc. (Hope I got my math right ).

In practical, everyday terms, on most evenings I'm not pushing 200x. I may - on occasion - use 250x for the Moon, some doubles, but not on anything else. I’m not saying it isn’t possible, it’s just that I haven’t had cause to do so.

My own opinion is that your first purchased EP should be a moderate-power eyepiece that can be Barlowed to provide a high-power eyepiece. This will be your work-horse. For a 5" f/5 scope, I'd recommend an eyepiece that provides something around 60x to 90x and if I wasn't going to purchase for a while I'd probably go for that 90x. If you barlow the EP, you're effectively doubling your EP case and reaching maximum useful magnification not only for your scope but also on general atmospheric conditions. For your second eyepiece, buy one on the other end of the range just given, that is, the 60x and Barlowed will give 120x.

So with just 2 EPs you've got: 60x, 90x, 120x, 180x. That should cover most of your nightly viewing on just about any object you decide to hit upon.

If I were you, I'd also think about converting the telescope itself into its own finder. A 25mm plossl, for example, in your own f/5 will give about 24x and a whopping 2º true field of view . That's quite a serious amount of space and should be a wonderful scope on wide field views.

But it is at this junction that the 'bad' news kicks in .

A 5" f/5 is not an ideal high-mag, or planetary scope. Being of such a short focal length, it relies on very short focal length (FL) eyepieces to gain significant magnification. A longer FL telescope allows one to use longer FL eyepieces, which in turn would give one more eye relief making viewing a more comfortable possibility. To get the 90x, for example, you're going to need a 7mm to 8mm EP and unless it has decent eye-relief, you may find it a little uncomfortable to use.

As you'll appreciate, magnification is equal to the telescope's FL divided by the EP's FL. So, if I wanted to enjoy Jupiter, for example, at around an average night's viewing of 140x to 160x, I'd need an eyepiece of around 4mm to 4.5mm. I often view Saturn around 160x to 200x, so if this were the same for you, we'd need EP's around 4mm to 3.3mm.

So if planets were your thing and you wanted to keep with your scope, I'd recommend either

• a zoom eyepiece of decent quality of between 3-6mm (a TV could be picked up for around £250)
  
• a decent Barlow which in effect helps double your EP collection whilst giving you the choice of buying a little longer FL eyepiece (say an 8mm giving you a hand in viewing DSOs and lunar work) and retaining its longer eye relief.
  
• a 60º field EP of around 4mm which due to its wider field will be easier to track your object and finally,
  
• an ortho of around the same focal length.
  

Another important point is to set your expectations a priori.

Just looking at the numbers, I figure that if you want to do some serious high magnification viewing, you will be pushing your system. This is not to dump on your scope in any fashion, you have a beautifully crafted wide-field scope, it's going to be amazing on so many gorgeous objects and these telescopes can look at planets all the time but whatever our telescope, I think it is always helpful to the soul and mind to have reasonable expectations of what is realistically possible.

I'm not sure if this ramble helps in any fashion but I feel the numbers offered do help getting a general gist of what is possible and some kind of map to work through and around.

Good luck on what you decide and hope this post was of some tentative assistance

[m37]

Did you get the standard 10 and 20 mm eyepieces with your scope? They are not the best but are perfectly usable and would give you an idea of what to choose. Also Google 'sky at night field of view calculator'. This simulates object size for various equipment combinations and let's you change eyepieces etc. Watch the magnification it tells you for brightness, lower power eyepieces give brighter views.

Sent from my Nexus 7 using Tapatalk 4

[Bungielad]

Thanks for your help everyone, lots of very useful information as always, I'll wrap my head around all those figures and let you know what I decided on.

Bungielad.