question about EP focal lengths

[exoplanet]

Hello,

Apologies for another thread on eyepiece advice, it seems like a never ending topic....

I have a 10 inch SW Dob (1200mm focal length) which I have had for a couple of weeks. It came with 26mm and 10mm SP EPs and so far I have only used it for planetary work (planets are easy!)

The 26mm is a pretty good EP, gives a nice image, the 10mm not so good. So after a little research I have decided to get one of these:

Baader Planetarium - Baader Genuine Orthoscopic

I have read a lot of good things about them and as this EP will be used for primarily for planetary work the narrow FOV should not be an issue.

So, to my question, I am undecided on what focal length to go for, here’s the dilemma.

Take the 7mm EP. This will give me 171 x mag and 342 (with my 2x Barlow - Skywatcher - Skywatcher ED Deluxe 2x two-inch Barlow Lens)

The 6mm = 200x and 400x

The 5mm = 240x and 480x

So, the theoretical max for the scope is 500x, but I understand that is not a useable magnification whilst looking through most of the atmosphere. However, what is likely to be the highest usable magnification I can use? (assume that it is the rare occasion that the seeing conditions are as good as they ever get). Would I be able to use 400x under those conditions? If so it would seem to make sense to go for the 6mm. If 400x is never going to work under any conditions then the 7mm would make more sense. Is there any benefit to the 5mm?

The problem is that being new to owning a “proper” scope I have no experience of what will work in the real world in terms of highest magnifications.

Thanks for any advice

John

[John]

With my (decent quality) 10" newtonian I find that 250x is about the max I can usefully use usually. On really good nights, on the right object, more might be worthwhile, but rarely in my experience.

[NGC 1502]

Post #2 hits the nail on the head.

I find that under good steady conditions ( doesn't happen often for me ) that

double stars can be split at 200 - 250x, only exceptionally do I go higher )

But for planets I keep to 200x max. I much prefer a smaller sharper view than

a larger fuzzy one. A sharp view of Jupiter at 120 - 170x is a fine sight.

My favourite eyepiece for DSOs is a 13mm for 92x.

Regards, Ed.

[glowjet]

Your 5mm will give you 240x which will be near the limit of usable mag with our atmospheric conditions, remember that using a 2x barlow puts extra glass into the equation which reduces your FOV, the light throughput and to some extent degrade the image, this why if you use a barlow use one of the highest quality, all this ,of course, goes out the window on those very rare occasions when the seeing is perfect, and you will possibly be able to use much more magnification, perhaps the 9mm should be considered which will give 133x, with barlow 266x, or buy more than one orthoscopic to cover the ranges you want, but remember, orthoscopics are used for planets, star study, doubles, singles, variables etc, a good all round investment would be to consider a middle price range quality ep with a 60 to 70 deg FOV. HTH.

[exoplanet]

Thank you for the advice and very helpful insights. I will have to consider my options caefully, sounds like a 9mm might be the way to go.

Thanks

John

[Moonshane]

I agree with all the above but occasionally it's possible to get more on the moon. with the moon a bit higher than usual and the atmosphere a bit more steady I have used my 6-3mm zoom at 400-500x and still got a good image.

some targets take magnification better than others and I too have found Jupiter best at between 120x-150x.

[twotter]

Back in January I got up to 240x on Mars which was stunning (10mm and 2x barlow).

Since then I've bought a few more EPs and recently got up to 375x on the Moon (6.4mm and 2x barlow). I worked my way up to that and I confess to being surprised I got that high without the image degrading.

Apart from my 32mm, which I love using, I tend to use my 26mm as my 'search' eyepiece and then normally use the 12mm or, if conditions allow it, the 9mm. This gives me 46.2x, 100x & 133.3x.

Just my 2p, but in your shoes I would go for the 12.5mm which would give you 96x (and with a 2x barlow, 192x).

[ollypenrice]

If using a Dob you have to factor in the field of view because this determines the time you have on the object between 'nudges.' If you go for a narrow field, high power planetary EP in a Dob it will drive you nuts. Settle for less magnification and go for a more expensive (sorry!) widefield EP. Every nudge costs you concentration, serenity and detail.

Beyond 200x is not Dob territory. Forget it. I bought a TeleVue Radian on the forum recently for about a hundred pounds. For the planets that is your answer but I would suggest 180-200x max. On an EQ mount thiings are different and 250x is somtimes possible and worth having.

Olly