highest practical mag on pentax (1 Viewer)

i am just about to complete the purchase of a pentax 80mm off a bloke on a astro forum.

I had an email convers with him this week about ep magnification. Ofcourse astro peeps work in focal lengths not magnifications as birders do.

NOrmally most scopes for birders stop at 60x, but most 80mm scopes are capable of reaching 75x and even beyons. (nikon zooms reach 75x for 82mm scope)

HAs any one used a 80mm for birding with 80-90x ?

The above astro fellow said that the pentax could cope with 100x magnification, the problems was the size of the FOV......the higher the mag the lower the fov.
I assume that there is NO way round that?? Even if a barlow would work in the pf80.

Another factor (aside from the intensity of daylight available) is tripod shake, there comes a point where birding tripods arent as stable as astro tripods: which are far heavier and not really designed to be terribly portable.

any factors i have missed? (am assuming either ED or lanthanum glass used to minimise colour aberation)

maybe i should add....my 2 main uses for a scope:

1. seawatching, which tends to be looking invain for shearwaters passing 200m offshore.

2. wader id. This tends to be loooking through flocks on coastal ponds, so the birds are on ave 60-80m away.

SO type 1, will need wide field lower mag, and type 2, narrow field high mag.

focal length of pf80 is 480mm i believe. I already have a meade 5000 uwa (82deg) 8.8mm on order: 480/8.8= 55x

Not sure if this would be too narrow for good seawatching?

I would say that Pentax 80ED is definately mag limited to <60x. Due to the prism aperture, max FoV is at ~20x with wide angle eyepiece note exceeding 72deg AFoV.

Also, the focuser in the scope intrudes into the light path so its true unrestricted aperture goes from ~78mm at infinity to ~67mm at close focus.

I have a 5mm Televue Nagler Type 6 which gives 92x mag on my Swaro 80mm, coupled with the Swaro astro adapter.

I bought it for your 'use number 2' but in practice I hardly ever use it for the following reasons:

Eye relief is too little for me, though the image is very sharp if you can get eye placement correct. I have to push my eyeball right up to the glass to get the full 82 degree field of view, which just isn't comfortable. The exit pupil too is very small, so good light is needed to see any real advantage.

A little more detail is resolved compared to the Swaro zoom @ 60x, but this advantage is partly mitigated by the need for a sturdier tripod. Even a slight breeze renders the image almost unuseable on my manfrotto 055 CF.

The Swaro zoom gives me enough useable mag for 99% of occasions, without the inconvenience of changing eyepieces, and it is much more comfortable to use.

To summarise, there is a slight advantage in resolution if you need every last scrap of detail you can get from your scope, but for me using it was just too much of a hassle most of the time. There has been quite a bit of rubbish (IMHO) posted on here in the past about mags higher than 60x being unuseable on 80mm birding scopes, mainly due to atmospheric conditions. But for me it came down to ergonomics and convenience. By all means give it a try, if you can get the eyepiece to work with your scope.

Steve

The other use for very high magnifications is reading bird bands. There is a Portuguese poster here who does that and his web site talks a lot about using very high magnifications with scopes.

"Eye relief is too little for me, though the image is very sharp if you can get eye placement correct. I have to push my eyeball right up to the glass to get the full 82 degree field of view, which just isn't comfortable. The exit pupil too is very small, so good light is needed to see any real advantage."

There are some ep that have fixed eyerelief. SOme vixen/meade ep. Maybe they worth trying?

Yes tripod shudder is the main enemy at this lvl of mag. Its quite often stormy weather that turns up `good birds` here in uk, so that going to be a real problem!

Have a look at http://www.pt-ducks.com/cr-telescopes.htm it might help you.
A good 80mm telescope can hold a 5mm eyepiece. Eventually a 4mm might be useful but never checked if there is any gain in resolution (on 100mm telescopes it didn't).
68-70º eyepieces have better eyerelief than 82º ones so are more comfortable to use, speceally for eye-glasses users like me - I have to update the 5mm test with the data of a 5mm Nagler. Pentax XW5 it's eventually the "best" eyepiece. The Baader Hyperium 5mm it's a good and cheaper alternative.
There is a barlow that works with Pentax scopes http://www.birdforum.net/showthread.php?t=152519 I already did some resolution tests and this with the Baader zoom has similar resolution to the XW5 although this last one have more crystal clear views. I just have to take some photos to update the site and the birdforum thread.
And yes, without a good tripod and tripod-head, there is no advantage of higher powers. Most of the times the better is also to do a hide or use something else to protect you from wind.

David

woodhornbirder said:

focal length of pf80 is 480mm i believe. I already have a meade 5000 uwa (82deg) 8.8mm on order: 480/8.8= 55x

Click to expand...

The Pentax 80mm straight version has a focal length of 500mm, the angled version is 525mm.

The highest useful magnification will vary depending on the user's eyesight acuity, the quality of the individual scope sample and in this case focusing distance, since the aperture of the Pentax is not constant over its focusing range. I wouldn't expect much better than 1.8-2 arcsec resolution from a typical sample of this scope. A very rough estimate would be that all the detail the scope is capable of showing would probably be visible to a person with extremely good 20/12 vision at about 35-40X. A person with 20/20 vision would need about 60-70X to see the same sized details.

I would check out the TMB Planetary Series eyepieces. They're cheap and easily good enough to show all the detail this scope can deliver. The Hyperions should do just as well with wider fields, but for more money. You won't squeeze any extra detail from the image with a premium eyepiece because at high magnification the scope optics will be the limiting factor for resolution, not the eyepiece.

I managed to get a celestron axiom 7mm for £55. Should arrive by weekend.

By mr H link figures, thats 75x? SO Very much upper range! IF its a bit too much, then i may be able to sell it on.

"Most of the times the better is also to do a hide or use something else to protect you from wind."

true, although I find that alot of local hides are built of wood and on stilts. IF there are other peeps in the hide the floor tends to shake as they move about, and people always move about at the wrong time!
IF there is a rare bird about then normally ther is a stream of people arriving from the news on birdguides etc.

Ofcourse if i am alone, and am the person to find the rare bird in the first place then thats different!.

I personally found i didnt like the baader hyperion ep. I found the long eyerelief and lack of screw up thing to hold my eye in the right position, didnt work for me; my eye kept moving out of the spot where you see the whole field and so i got blackouts.
HAve ordered a meade 5000 UWA 9mm, as this has the screw up thingy so should be easier to use.

I may go for a meade 5000 swa 13mm when i can afford it......68deg. Cant afford nagler atmo, they rarely sell for less than £100.

Will report back if axiom is any use )

the 7mm is as expected, a bit on the dull side, with a very narrow depth of field, can only really be used on stationary birds. As its a relatively heavy ep, it not the best for use in the field; it will be rarely used and is too bulky to be worth carrying about.

I suspect a "planetary" ep will give a better image, although they have a small fov, so again only viable for stationary birds.

obviously for birds in flight, 30-40x wide is the optimum lvl of ep. 50-70wide for picking waders or ducks on inland water.