How to start an Eyepiece collection

[sandspider]

Hello all,

OK, I've got the scope and have managed to see a few of the sights. Can't wait for winter now!

Anyway. I am starting to think about getting a barlow as it doubles your range of eyepieces. I have been recommended to get a good one as it becomes the limiting factor of your all the eyepieces you use with it.

My question is, is it worth paying for a 2" barlow (my scope will take it)? My thinking is that you can always fit a reducer, but you can never increase a 1.25 in, but is it a wise investment as the 2" are dearer and more limited in range?

Many thanks,

Mark

[John]

Mark,

What what eyepieces do you already have ?.

We are shooting in the dark a bit otherwise

PS: and what's your budget ?.

[sandspider]

I just have the Eyepieces that came with the scope.

9mm Celestron Plossl

32mm Celestron Plossl

[Ad Astra]

A good 25mm and a good barlow (3-element, multi-coated if you can afford it!) would do wonders for your collection... and these two would likely be less than a 2" barlow. Those babies are really pricy!

Dan

[sandspider]

I suppose I wouldn't want to spend more than €120 or £100. But if I had an idea what to look for I would watch for a secondhand.

[sandspider]

I suppose I wonder if in a few years, I will be regretting buying the 1.25 in?

[John]

If you buy a decent quality used one you will be able to recoup most of your outlay if and when you decide that you want to move on to something else.

Look out for one of the following:

- Celestron Ultima 2x barlow

- Orion Shorty Plus 2x barlow

- Tele Vue 2x barlow

You should be able to find one for around £50 on the used market. We have a buy & sell section here but you need 50+ posts plus 1 months membership to access that. Otherwise there is the much used UK Astro Buy & Sell website:

U.K. Astronomy Buy & Sell

E.bay can throw up some good items but also contains a lot to avoid !.

[AweSIM]

just my two cents worth of advice.. i think you need to determine whats your taste.. DSOs.. or planets.. i mean wide-field or narrow-field.. for DSOs youd need longer focal lengths and the other way around for planets.. since you already have a scope.. you might as well mention its FL and F# so the more experienced members can give you on-the-spot advice..

an EPs mag depends on its FL and the FL of the scope.. and then there is maximum and minimum mag that can be used with each scope too.. max being at around 250x depending on the seeing conditions.. so this restricts your shortest EP to..

min_EP_FL = scope_FL / 250

so getting an EP which results in a higher mag is probably never going to be used.. longest EP is calculated as follows..

max_EP_FL = 7 * scope_F#

this gives an exit pupil size of 7mm.. if you use an EP longer than this the light beam exiting can not be viewed fully and results in "kidney beans" effects on the resulting image..

once youve determined the longest and shortest EP FL that your scope allows you.. the idea is to fill this range evenly.. like 11mm.. 22mm.. 28mm.. 35mm.. but if your taste is DSOs.. youd want to stick closer to the higher limits.. for planets youd stick closer to the lower limits (plus a long EP since it makes finding an object easier at low mags)..

barlows are available in 1.5x, 2x, 3x, 4x and even 5x.. both 2" and 1.25".. with 2" ones you can use both 1.25" and 2" EPs with them.. right now you dont have 2" EPs but in future you might.. when you do.. those EPs wont be usable with it.. they also allow you to fill the EP FL ranges more evenly.. like my (expected) scopes stock EP is 28mm.. but im getting a 2x barlow and an 11mm EP.. this gives me 5.5mm, 11mm, 14mm and 28mm.. since a barlow + EP has more glass than an EP only.. you need to get a good (expensive) barlow..

one final note.. i had an EP fever too.. but ive learnt from the more experienced members on the forum that its better to get a few good EPs than lots of cheap ones to fill the tray.. my opinion.. get a 2x barlow only.. thatll give you a nice range of 4.5mm, 9mm, 16mm and 32mm..

ps.. this is what ive learnt from the (way) more experienced members on this forum.. im sure theyll correct me if im wrong somewhere.. ..

awesim..

[rabbithutch]

Ever the neophyte, I really appreciate the questions and answers here at SGL. I would never think to ask some of the questions (too ignorant) and always learn from the answers and opinions expressed.

I've only begun to read and absorb information about the optics and elements used in EPs. (There are Plossl's and ORs and Naglers- oh my! Sorry. Senility sets in too often.) Are there lessons to be learnt about the optical elements of Barlows as well? If so, what sort are better than others? Are they equally efficient transmitters of light? Are there some Barlows whose elements might be incompatible with the design of elements in EPs?

I have 2 Plossl's by Sirius an OR and a Super Wide Angle that I think is a Nagler design knock-off by Meade. Could I use a Barlow with all of them without ill effect?

It seems to me that this information would be as helpful as recommendations of brands to guide purchase decisions.

So much to learn; so little time.

TIA

[Ags]

Good formulae awesim, I'd like to add a couple of points. Pupil size declines with age, so a middle-aged observer should not exceed an exit pupil of perhaps 5mm. As regards the effects of excess exit pupil it has the effect of making the view - especially stars - dimmer.

Kidney beaning is a property exhibited by some eyepieces, typically widefield ones, and is not related to large exit pupil. When my hyperions are misbehaving I see a lot of kidney beaning and my exit pupils never exceed 2mm.

Another thing to be aware of, with a scope with a central obstruction, is that very low magnification can make the shadow of the central obstruction visible.

[AweSIM]

@ags.. well said.. my apologies for confusing kidney beaning.. .. about the central obstruction.. yes that too i missed accidently.. thanks for correcting me..

[AweSIM]

Are there lessons to be learnt about the optical elements of Barlows as well? If so, what sort are better than others? Are they equally efficient transmitters of light? Are there some Barlows whose elements might be incompatible with the design of elements in EPs?

I have 2 Plossl's by Sirius an OR and a Super Wide Angle that I think is a Nagler design knock-off by Meade. Could I use a Barlow with all of them without ill effect?

hi.. i dont know much myself but what ill tell what ive learnt so far..

- not all barlows are efficient transmitters of light.. cheap ones dont transmit well.. the ones that are expensive as expensive cuz they do transmit well.. but its an inverted exponential curve.. meaning that although there might be a significant difference between the optical quality of a 30GBP and a 200GBP barlow.. there wont be much difference between a 200GBP and a 250GBP barlow.. and the difference might only be noticeable by experts.. not me..

- as far as optical elements and their compatibility are concerned i cant comment on that since i dont know.. maybe others can comment on that better..

- if you mean the barrel sizes then yes.. 1.25" barlows can only accept 1.25" EPs.. but 2" ones can accept both barrel sizes.. you might be able to make a custom adapter to fit a 2" EP to a 1.25" barlow but thats degrading the EP.. to my knowledge, 2" EPs are better than 1.25" ones.. since their stop diameters are obviously larger than 1.25" ones..

for the record.. i was opting for a 1.25" barlow which was in my budget.. but i was advised not to go for that one.. for the reason that my expected stock 2" EP would be useless with it.. so why waste that EP when i can use it for two different FLs? so i went for a 2x 2" barlow.. it was expensive.. to immensely.. but enough to force me to push up my budget.. but im satisfied with the decision.. (only thing left is placing the order in a while ..)..

awesim..

[sandspider]

Hello All,

Thanks so much for all the good advice. I am in no hurry so I think I'll look out for a second hand 2" barlow. That should give me a good general setup and then I can upgrade the eyepieces as i know better whether I prefer planets or DSO's.

For the record my scope is 1200mm so F6. I am also thinking in the back of my head that a 2" barlow would be good for imaging? I know imaging will be limited on a Dob, but perhaps it is another reason to look that way?