Focal ratio and eyepieces

[ApprenticeAstronomer]

Hi all,

Ive been trying to work out my scopes optimal eye piece size. It has a focal length of 900mm, aperture 130mm and a focal ratio of 6.92. My calculations give me a high power eyepiece at 5.19mm (173X), medium to high power at 8.65mm (104X) and a medium to low power eyepiece at 13.84mm (65X). Do these eyepiece sizes seem a little oddball? I am not very confident with even the most basic of mathematics Could someone check that my calculations are correct? Thanks in advance.

[John]

Broadly, that sounds about right. I'd also want a low power, wide field eyepiece - something like 30x so a 30mm eyepiece - some of the deep sky objects are rather large !

I guess if you were to get a set comprising 5mm, 9mm, 13mm and 30mm that would cover most observing.

[ApprenticeAstronomer]

Thanks for the quick reply John Could you recommend a reliable and safe website for purchasing eyepieces?

[nckjsmitj]

First light optics will do you proud.... They also sponsor this forum!!!

N.

[John]

I tend to buy mine used from the UK Astro Buy & Sell website - I could not afford the quality ones I have otherwise. First Light Optics are very good for new purchases but Telescope House and Harrison Telescopes have good reputations as well. Many folks on here swear by (never at !) the BST Explorer eyepieces sold by the dealer called Skies the Limit.

[AWR]

You could also go for a 10mm and 30mm EP and a barlow ,giving 5mm and 15mm, and covering the same range.

I don't use my barlow much but it is a way of getting startted and avoiding short eye relief problems with your 5mm EP.

Andrew

PS: Vixen plossls are also excellent quality for the price.

[ApprenticeAstronomer]

Thanks for the help guys. Is there much difference between the different brands of eyepiece? They seem very expensive for a beginner. Any recommendations for a beginner with a slightly tight budget?

[AWR]

I'd recommend plossls as the design to go for if you're on a budget. Go for one one of the recognised brands. (I like Vixen - others have recommended Revelation. Televue are great if you can get them second hand)

Also use the EPs that came with your scope - don't feel that you have to replace them all at once. Just buy one at a time and see how you like it.

Andrew

[ApprenticeAstronomer]

I have been looking at these eyepieces. The page says that these eye pieces are suitable for a telescope with a f/6 or below ratio. My scopes ratio f/6.92. Am guessing these EPs are of no use to me? First Light Optics - Skywatcher SP Plossl eyepieces

[tjr]

Vixen or GSO plossls get good write-ups and are in the £30-£40 range. I would regard these as superior to the skywatcher SPs for only a small rise in price but others may disagree...

[John]

Vixen or GSO plossls get good write-ups and are in the £30-£40 range. I would regard these as superior to the skywatcher SPs for only a small rise in price but others may disagree...

I'd agree with that

[Ags]

In an F7 scope I think the TS Planetary HR eyepieces I use would work well in the 5 to 9 mm range. BST Explorers are also widely recommended. These EPs are ergonomically nicer than plossls, have better eye relief, have a wider fied of view, are very sharp, and cost only a little more....

[nckjsmitj]

Do your homework and understand how eyepieces (Oculars) are made and that will allow you to come to the right decision. Try this website: OCULAR (EYEPIECE) BASICS.

N.

[ApprenticeAstronomer]

Thanks for that. The link will come in handy.

[LukeSkywatcher]

Thanks for the help guys. Is there much difference between the different brands of eyepiece? They seem very expensive for a beginner. Any recommendations for a beginner with a slightly tight budget?

Vixen NPL. Top quality at a low price.

GSO are also top quality for about the same cost but the Vixen has the edge.

IMHO.

Paul

[AWR]

I have been looking at these eyepieces. The page says that these eye pieces are suitable for a telescope with a f/6 or below ratio. My scopes ratio f/6.92. Am guessing these EPs are of no use to me? First Light Optics - Skywatcher SP Plossl eyepieces

f/6 or slower means f-ratios above f/6 (such as yours). Faster f-ratios (such as f/5) require the EP to bend the light much more and some EPs are not upto the job.

Plossls will be great in your scope. (But the Vixens are much better than the SW). The only problem would be the 5mm. This will have a very short eye relief (the distance you have to place your eye from the lens). Two ways round this: 1) Barlow a 10mm EP instead, 2) Pay a bit more and buy a different design EP. If it was me, I'd do (1) for now using your existing EPs and save up for (2).

HTH

Andrew

[Uhwaz]

I got the Baadger Hyperion Zoom EP 8-24mm.

I know some ppl love them some ppl hate them but since i got it i pretty much dont use any other

So if you dont like exchanging EPs look into it