2nd eyepiece choices

[icytailmark]

im looking at buying my 2nd eyepiece for High power planet viewing and im tossing up between the 8mm ethos, pentax 7mm XW or the pentax 5mm XW. The 5mm pentax is the cheapest of them all at $450 from star optics, the 7mm is $749 from star optics as well and the 8mm ethos i can get for $689. I have no idea which one to get.

I currently have a 13mm t6 nag.

im using a 12" SW dob with an argo navis.

[dpastern]

I'd be going a Radian myself.

Dave

[wavelandscott]

In an undriven Dob I think that the Ethos wide field would be would be great either the 6 or 8. I've not used either in real life but have looked through them "in the show room" they look good.

I have the 7 mm Pentax XW and have used the 5 mm XW...they are fabulous eyepieces.

I don't think you could go wrong either way...

Do you wear glasses or need lots of eye relief?

[icytailmark]

no i dont wear glasses. I plan of getting a servo cat jr next year too so i can make my dob a goto and hopefully learn how to do some astrophotography.

[icytailmark]

would the ethos eyepieces keep the planet in fov alot longer then a pentax XV eyepiece?

[bobson]

This is from Starry Night FOV demonstration. It gives you basic understanding of the difference between 10 mm Ethos and 10 mm Pentax XW.

[icytailmark]

thanks for that!! Im definately going for the ethos.

[ausastronomer]

Hi Mark,

There are a lot of people that observe planets with a dobsonian using the "drift method". ie. let the target drift from one edge to the other before moving the scope. IMO this is self defeating. A newtonain has a fairly small "diffraction limited" field. In other words the view is better in the centre of the FOV than at the edge. It isn't hard to see whan observing a planet like Jupiter or Saturn that the image is slightly sharper when the target is centred. IMO the right way to observe with a dob is to continually "nudge" the scope keeping the target as close to centred as possible. When you observe like this, the 100 deg AFOV of the ETHOS is superfluos and a 60 deg AFOV is plenty.

All the eyepieces you are considering are outstanding so you will end up happy. The 7mm Pentax XW gives ~200X in your scope which is about ideal if you normally get reasonable seeing in your locality. If you don't get good seeing consider the 8.5mm Pentax XF which would give about ~160X in your scope. It only costs a bit over $200 and is an outstanding performer. It only has a 60 deg AFOV but performs very well. It is a better eyepiece than the 8mm TV Radian for a good bit less money. If you want to go with the 7mm Pentax XW consider buying direct from Japan. This is an excellent company to deal with, only problem is no local warranty, you would need to return to Japan is something went amiss, which is very unlikely as these are so well made.

http://www.kkohki.com/English/kkohkiparts.html#Pentax XW

Buy direct from here and the 7mm Pentax XW will cost you about $AUS 400 landed.

Cheers,
John B

[JethroB76]

Agree with John, I would be looking at the Pentaxes

[Greenswale]

I use a 7mm XW Pentax in my F6 8" dob, with truly excellent results. I often use the wide field of view to let objects drift accross the field of view, so that I am more able to concentrate on the object itself, rather than on moving the scope. Depends on the object, though.

The future will see a 5mm XW added to my eyepiece collection, currently 20mm XW, 10mm XW and the 7mm XW. Great eyepieces!

[ausastronomer]

Hi,

Using the drift method depends more on the scope, than the target. Your 8"/F6 scope has an infinitely larger diffraction limited field than the original posters 12"/F4.7 scope. The size of the diffraction limited field of a newtonian decreases logarithmically with a decrease in the F-Ratio of the scope.

This article by Bob Royce explains it all.

http://www.rfroyce.com/mirror_performance.htm

My experience at the eyepiece confirms exactly what Royce is saying in this article.

IME you can use the drift method very successfully with a 6"/F8 scope, with a fair degree of success with an 8"/F6 scope and with little success in any scope of F5 or faster, although a paracorr helps a lot with these faster scopes.

Cheers,
John B

[Starkler]

The 7mm pentax makes an ideal high power eyepiece in your scope giving just over 200x.
For mine if the seeing doesnt support 200x it's not worth bothering to view planets in any case.

[GrahamL]

I had a 7 mm nagler and found I didn't get as much use out of it in my 12"..the 8.5 xf I replaced it with I use a fair bit more.

with regard to the price of the 7 xw, I'm happy to support local vendors
and realise there may be a small price differance, but $750 ?.. but this
f/l dosn't seem to carry this price jump elswhere.

[Greenswale]

John (Ausastronomer), Ta for yet another learner in this delightful science!

Wren