eyepiece choice
I have found a particular brand and style of eyepiece that is basically perfect for me. I have only the 25mm eyepiece which currently I use 100% for my visual LPV observing. Now I have seen discussions where observers talk of having a range of powers available to them useful, I assume when one is working very close to the scopes limits.
So anyway I am in a position to purchase one more eyepiece, so I am wonder what the seasoned observers would suggest. I have the choice of 18mm 15mm 12mm 8mm or 5mm. Personally I am thinking the 15mm or 12mm - but what do you say?
Thanks Glen
Yes, something I should have mentioned. I have an SCT 2000mm. And thank you for the reply
To properly select ep's you need to know the f/ratio of the telescope as well as afov of the ep. Another factor to consider is the optical quality of the telescope and your seeing conditions.
For telescopes of average optical quality, a good rule of thumb is minimum ep fl in mm = 1 to 1.5 x telescope f/ratio. So for a typical f/10 SCT a 10-15mm ep would be the limit. Superb optics can go down to 0.5 x f/ratio, but SCT's rarely fall into this category.
For most typical seeing conditions 300x or so is usually about the max magnification. Excellent seeing can allow 500x or even above 1000x on rare occaisons. An average quality 8" SCT is not going to be seeing limited, typically.
And the ep field of view makes a difference too. A 25mm plossl can be replaced by a 12.5mm Ethos or other 100deg afov for twice the power and the same true field of view. This will markedly improve visual performance, limited by optics and seeing of course.
Mike LMK
Here's a google search page with links to some calculators that may be useful ,
https://www.google.com/search?q=eyepiece+calculator
I use an 8 inch sct sometimes and the 3 eyepieces I use the most are a 45mm plossel for low power, wide fields when the star is bright , a 18mm radian for general use (and used the most) and a 9mm nagler for the faintest stars. I have a 56 2" plossel for the widest field but rarely use and 7mm that the seeing limits its use most of the time.
Richard Campbell
A lifetime of VSO has shown me that the observer can probably get by quite well with just three high quality eyepieces. The specific choice will, of course, depend on factors like the quality of the person's telescope, the size of its fully illuminated field, and prevailing local seeing conditions.
In my own case, over the past 40 years I have used 12.5" and 16" Newtonians of moderate optical quality that I designed with purposely large secondary mirrors. At my location the quality of the seeing has almost always been something of a limiting factor. I've found that my best basic low-power eyepiece is one with a very wide field and about 60x to 75x (with due consideration toward the size of the fully illuminated field). Lacking GOTO ability, this allows for quick, easy and accurate identification of the larger star patterns in the variable's field and the surrounding comp stars, as well as useful when making estimates when the star is fairly bright.
My workhorse higher-power eyepiece produces 175x-200x magnification. It darkens the sky background considerably and typically gains me an additional 1.5- 2.0 magnitudes over the low-power view. Here again a wider than average field of view is desirable so as to include the sometimes widely spaced comp stars found on certain charts.
On unusually clear and dark nights with very good seeing (the latter a rarity at my location) one additional special eyepiece should be available to press into service. This one should produce something like 300x. Mine will gain me yet another 0.5-0.75 magnitudes over my moderate power eyepiece. This modern eyepiece has an 80-degree field design and can pull out very faint stars, such as some of my faint CV's and a few of the old novae I follow.
J.Bortle (BRJ
Hi Glen,
The choice of ocular depends on the focus of your telescom too. In my home villagage I have only an 15mm Erfle for my 270/1470 Dobsonian ("The Boiler"), but an 12mm and a 9mm ocular would extend the limit of my telescope.
At Polaris Observatory I mostly use a 15mm and an 9mm ocular for the 250/1250 GSO Dobs. Sometimes I use 25mm and 6mm oculars too. These are Skywatcher LEW Goldline oculars -- not too expensive ones (~$50 or 39 EUR), but perfect for variable stars:
http://www.teleskop-austria.at/shop/index.php?lng=hun&m=2&kod=oku1-lew-0...
or in German: http://www.teleskop-austria.at/shop/index.php?lng=de&m=2&kod=oku1-lew-01...
If you use an SCT telescope with 2000-3000mm focus, a 15 or 12mm ocular would suite for you. If you have shorter focus (near 1200-1500mm) I'd recommend a 15mm ocular and a 9mm too.