Which Eyepiece should I buy?

[Arcturus75]

I've just bought an 8inch Dob which came with a 25mm eyepiece.  If I was going to buy 1 more eyepiece this month to accompany it what should I get?  I was thinking something like a 40mm to get a bigger FOV but not sure.  

[Buzzard75]

What is the focal length of the telescope and what sort of objects do you want to view?

[alanjgreen]

Hi,

what is your budget?

does your dob only accept 1.25 inch eyepieces or can 2 inch eyepieces be fitted into the focuser? Which dob is it?

dont be fooled by the mm rating of an individual eyepiece, this is only one factor in the equation of which eyepiece is right for you!

with a dob, the field of view FOV of an eyepiece is important especially if you have a manual push-to dob with no electronics.

i bet the 40mm you have seen only has a 50 degree field of view? This means you only see a narrow piece of sky and at 40mm the magnification is also low so you may find you get an unsatisfactory view  

magnification is calculated by taking the focal length of your scope ( check the spec) and dividing the eyepiece focal length into it. So, if your scope had 1200 mm focal length then the 25mm eyepiece is giving 1200/25 = 48 x magnification.

i recommend that you look to space eyepieces by x50 so you should look to get the following magnifications covered x50, x100, x150, x200 and then x250 later.

finally, if you wear glasses then look out for the stated “eye relief” of the eyepieces, you need at least 15mm eyerelief if you wear glasses. And less than 10mm is uncomfortable for most people. Eye relief is the distance that you need to place your eyeball away from the lens of the eyepiece.

Of course, if this is your first scope then the best advice I can give is to use it BEFORE you buy more eyepieces !! 

- you may find that you want to spent your extra budget on a better FINDER as you are struggling to actually find anything to even look at.

HTH,

Alan

[Arcturus75]

Focal Length is 1200.  For deep space I'm less interested in distant galaxies and more interested in Clusters mostly and Nebulas.  I also want to view Planets, I've seen most in a scope before but will never get bored of seeing them.  

Budget? I'd like to spend under £100 ideally and I'm happy to look at the 2nd hand market.  The Dob is a manual push dob and takes a 2" eyepeice.

 

[alanjgreen]

Clusters will be best in a nice low power, wide field of view FOV eyepiece

Nebulas the same, low power wide FOV (mostly). A few famous ones would need medium power and you will likely need to buy a UHC filter at some point to “see more” of each one.

planets are best in medium/ high power ep and fov is not particularly important either!

is your dob manual? If so then at high power planets will move swiftly across your view (due to the earths rotation plus their own movements). The planets are poorly placed for uk observers for the next few years so I wouldn’t rush to buy an ep for planets !

please do state your budget? We’re not snobs on here but there’s no point pushing you towards an ep you cannot afford.

X48 ( your 25mm EP) is pretty low magnification and will be fine for clusters.

as I say, use what you have and determine its weaknesses for yourself then you will see the benefit of whatever you decide to add next...

Alan

p.s. Add your location into your profile so we can get a view on how dark your skies are. You don’t have to be very specific but some place nearby will do?

[Arcturus75]

Yes it's manual, I've noticed the planets aren't ideally placed at the moment.  I've promised my daughter a look at Saturn now though so I'll have to think of something

Budget is less than £100 although if I really have to pay more I could.  

[alanjgreen]

Have a read of this thread

it refers to an SCT scope but ignore that, the poster is looking for a wide field low power EP in your price range.

if it were me ( I have a 20” manual dob) then I would start here.

your other option, which you should consider is to stick with the 25mm for low power and look to add a nice mid power wide EP to work alongside the 25mm

- the 40mm aero would probably make the 25mm redundant

- it’s also a 2” EP so you will be getting much more sky at the eyepiece 

1200/150 = 8 mm ( scope focal length/ desired magnification = eyepiece size needed)

so, a mid power EP would be 8mm in your scope. I will have a look and add a couple of links here in a moment. I would look for 8mm and as wide FOV that still fits within your budget.

https://www.firstlightoptics.com/bst-starguider-eyepieces/bst-starguider-60-8mm-ed-eyepiece.html

8mm eyepiece (60 degree FOV at 49 pounds)

https://www.firstlightoptics.com/explore-scientific-eyepieces/explore-scientific-82-degree-series-eyepieces.html

8.8mm eyepiece (82 degree FOV at 127 pounds)

FOV comes at cost! Use the sky calculator found in the linked thread to see what difference the extra fov would make in your scope ( you will be surprised)

https://astronomy.tools/calculators/field_of_view/?fov[]=52|134|||1||&fov[]=14|124|||1||&messier=31

good luck.

[Arcturus75]

Thanks for the links.  What reason would I pay £30 extra for the Aero over the Panaview?

 

[Ricochet]

When I bought my 8" dob I started by adding 8 and 12mm Starguiders at about £50 each. Of these the 12mm is more useful in an 8" f6 dob because it results in a 2mm exit pupil.

I would not buy a 40mm eyepiece unless you will be doing all your observing from a dark site. I would not go above a 5mm exit pupil (30mm eyepiece) from suburbia. The 40mm will be so washed out under light polluted skies that I doubt it would get much use.

[alanjgreen]

54 minutes ago, Arcturus75 said:

Thanks for the links.  What reason would I pay £30 extra for the Aero over the Panaview?

 

Aero performs better in faster scopes (yours is F/6)

Aero is lighter - less likely to cause Balance issues when in your scope

 

fyi, exit pupil is eyepiece focal length / scope speed, so 40/6 = 6.5mm

the width of light coming out of the eyepiece will be 6.5mm, if your pupil has not dilated to this width then light is lost. Of course, the 30 & 35mm aero EPs will produce smaller exit pupils. You can calculate them...

As my scope is very fast (f3.6, f4 with coma corrector) my low power EP is a 21mm 100 degree FOV. 21/4 = 5mm exit pupil (with coma corrector) and its EXCELLENT for nebulas

- 21mm 100 degree shows same amount of sky as a 30mm 82 degree which shows same amount of sky as a 40mm 70 degree FOV !!

- The extra degrees FOV mean that I have time to let the object drift across the view BUT FOV comes at a cost However, its lovely to see maximum amount of sky combined with greatest possible magnification. Best of both worlds. Wide View and Object of reasonable size to view

The problem you will face is keeping the object in the FOV while your daughter gets to the eyepiece to view it. Objects drift away pretty quick and if your daughter is too small to manhandle the dob then you will be always popping back to recentre the object. Wider FOV eyepiece will give you a bit more time before the object drifts away...

See how you get on with what you have and determine what specific issues you are having is the best advice. Then buy from that point to solve the specific issues YOU (and/or your daughter) are having.

[Saganite]

7 minutes ago, Ricochet said:

When I bought my 8" dob I started by adding 8 and 12mm Starguiders at about £50 each. Of these the 12mm is more useful in an 8" f6 dob because it results in a 2mm exit pupil.

I would not buy a 40mm eyepiece unless you will be doing all your observing from a dark site. I would not go above a 5mm exit pupil (30mm eyepiece) from suburbia. The 40mm will be so washed out under light polluted skies that I doubt it would get much use.

Have to agree. The 25mm that came with the scope is quite reasonable and will give you 48X for low power. The 8mm and 12mm BST's will give 150X and 100X, the former being ok for giving your daughter a decent view of Saturn. They are very good value for money, with a 60 degree FOV, and you could get both within your budget.They would be a fair start to keep you happy while you consider other options in the future. 

[Arcturus75]

Hi all, I went for the 8mm and 12mm BST Starguiders as recommended by a couple of people and got them both from FLO for £90.  I can't argue with that.  Thanks everyone for the help.

 

[ronin]

Nice choice, next month, or whenever, get the 25mm BST. It will be worth swapping out the supplied eyepiece even if the focal lengths are the same.

Be interesting to find out which one of the pair you have you will find the most used. Really suspect it is not going to be the higher magnification one.