Eyepiece choice?

[Dbroz (Daniel)]

Hi everyone

I have built a 18” F/5 truss Dobsonian. I’m just seeing if buying a expensive eyepiece will make much difference to the views. I am more interested in DSO’s, and what brand and size will be best? Any suggestions would be much appreciated, I have the Skywatcher Panorama set but I feel like they are not the best, they are good but not great. Also when looking at bright objects like the moon I get a lot of rainbow colours around the edge of view, is this normal? or is it the eyepiece itself. Again any suggestions will be much appreciated

Thank you
Daniel

[Wavytone]

Anything Televue... Plossls, Naglers, Delos etc will be fine.

[clive milne]

Explore Scientific, (Especially the 90 degree LER series)
APM aka) Lunt. Hyper wides & soon to be released LER.
Baader Morpheus
TV Nagler, Etho$ & Delos.
Takahashi LE & Orthos (narrow field of view)
Pentax XW (especially the 10mm)
Docter UWA (if you have money to burn)
Nikon NAV HW (as above)
ZAO (Ziess ortho's) (When you get bored of spending money like a normal person)

One thing to consider with respect to minimal glass eyepieces such as Plossls, Ortho's, etc) is that irrespective of how much money they cost or what brand label is printed on the barrel, they will not work well at F5.

Another thing to consider is that the dominant aberration that will limit off axis sharpness is coma. Every eyepiece will perform significantly better with the addition of a coma corrector. Best on the market is the Paracorr 2. Explore Scientific a close second.

[Dbroz (Daniel)]

Thanks guys for your help

I will look into those brands and also look into getting a paracorr, I appreciate your input. Thanks again

Daniel

[wavelandscott (Scott)]

You might consider attending a viewing night (or organizing one) and then see if anyone would let you try out their eyepieces (of brands previously mentioned) before you buy them.

[astro744]

Originally Posted by wavelandscott

You might consider attending a viewing night (or organizing one) and then see if anyone would let you try out their eyepieces (of brands previously mentioned) before you buy them.

This is the best advice. Get out to a star party and look through some different eyepieces.

First question. Do you require eyeglasses when observing? If yes then consider eyepieces with 20mm eye relief.

Otherwise one could easily recommend Tele Vue 21mm, 13mm & 8mm Ethos eyepieces for your telescope for a first and final set maybe complementing with 6mm later, but if you don't like or have difficulty with 100 deg. fields then this is money wasted.

One could also easily recommend 31mm, 22mm, 13mm & 9mm Nagler eyepieces. This combination was (and still is) very popular before the Ethos came on the scene.

Its nice (but not essential) to have eyepieces with the same apparent field of view for then the true field of view changes in proportion to the magnification.

As for the Paracorr, this is a very personal choice. I can see coma at f6.4 and have used a Paracorr for a very nice 'clean' image but does the coma bother me and do I use the Paracorr each observing session; no in fact usually not at f6.4. Same applies to my f5.5 telescope. At f5 too you will likely find coma not to be bothersome but again it is a personal decision.

I'm not familiar with the eyepieces you mention but I doubt they are responsible for the rainbow of colours and all of the obove mentioned eyepieces will produce a similar effect since I believe other factors are at play.

Is your telescope collimated?
Is your telescope acclimatised to ambient temperature (usually cooled).
What was the altitude of the object you were observing? If low in the sky atmospheric refraction will give you colour fringing on bright objects.

Tele Vue eyepieces are designed and tested for fast telescopes and the only aberration you will see is primary mirror coma which is linear from the centre outward and which the Paracorr will negate. However you will find the coma seen is small or faint depending on magnification and unless it really bothers you can put the money towards another eyepiece. Note some planetary observers will still use a Paracorr to give the largest possible coma free image on axis.

See http://televue.com/engine/TV3b_page.asp?ID=2
and http://televue.com/engine/TV3b_page.asp?id=154
and http://televue.com/engine/TV3b_page.asp?id=214
and http://televue.com/engine/TV3b_page....212&plain=TRUE
and http://www.televue.com/engine/TV3b_p...id=61&Tab=_con

See if you can get to a star party and have a look. You will find some eyepiece you like and some that you don't for whatever reason.

Whatever you choose, enjoy!

[Dbroz (Daniel)]

Thanks again everyone. I am actually going to the snake valley star party next month, I may hold off buying any new equipment until then, I’m sure I will meet some nice people there that will let me test their eyepieces in my scopes.
I collimate my telescope every time I use it, as for temperatures I was only out for about an hour, the mirror is 18” and 2” thick so I guess it needs a lot longer to cool. I’ve used this scope about 6 times, 2 of the 6 nights I had sharp crisp images on all objects I viewed. The other 4 nights was still good views but a bit more on the blurry side and noticed more seagul looking stars. I guess this is coma? I am fairly new to astronomy, I bought my first 12” skywatcher only 8 months ago, and I built my own 18” which I have finished only 4 weeks ago. I still have a lot to learn. I have a 3.1” Antares 1/20 secondary mirror on the way, I am using a 3.5” GSO secondary at the moment, I did originally buy a 3.1” GSO to start with which I felt gave clearer views than the 3.5. The only reason I got the 3.5 was only temporary as the 3.1” I originally bought fell face down on my concrete driveway and got chipped and heaps of scratches all over it. But with the scratches I still felt it gave better views. It could be because the 3.1 is the correct size for my telescope. Anyway I’m just blabbering on now. Thanks again for everyone’s help. I’m looking forward to the star party. I should get lots of tips there.

Thanks again
Daniel

[astro744]

"...and noticed more seagull looking stars. I guess this is coma?"

No this is a combination of aberrations mainly though astigmatism from the eyepiece.

Stars with coma (from the primary mirror) exhibit a small 'V' shape flaring outward to the edge of field.

Note a Paracorr will not correct for astigmatism although it has a small improvement because of its 1.15x Barlow effect on the light cone. A Paracorr will clean up primary mirror coma so your seagulls will take on a different appearance.

A fully corrected eyepiece (say Nagler) wont have the astigmatism so when combined with a Paracorr you end up with pin point starts to the edge, (varies with focal ratio, see Tele Vue chart on their web site for the Paracorr).

As I said in my earlier post, coma or aversion to it is a personal choice. When I first looked through a Nagler I was please to see only pure coma and nothing else at the edge of field (coma is linear from centre to edge) and it was small and not objectionable, this through 6" f5.5 and 10.1" f6.4 and a friend's 16" f5. I did eventually invest in a Paracorr and yes it takes the view up a notch but I don't use it all the time. Even on my kids' 10" f4.7 the coma seen is not objectional and I would like to see what a Paracorr does to it but the design of the focuser is such that the Paracorr cannot be fully inserted, but that is another story.

I say look through as many different eyepieces as you can and decide which feel best for you. Later when your viewing becomes more refined, perhaps then invest in a Paracorr to make the stars at the edge pinpoint too. My problem is that I also have a TV-101 and it has nothing but pinpoint stars and I try and emulate this on my Newtonians (sometimes) so if you don't want to see pinpoint stars across the field don't look through one.

Whatever you choose, enjoy!