eyepieces for f/4 Newtonian

jps · #1 23-09-2005, 11:48 PM

I recently purchased an Optex OP 600. This is a relatively cheap 8" f/4 Newtonian. It came with 9mm and 25mm GSO plossls. Is it worth buying good quality eyepieces for this? Any suggestions?

janoskiss (Steve H) · #2 24-09-2005, 01:05 AM

Could you tell us first how the GSO plossls are performing in this unusually fast scope?

janoskiss (Steve H) · #3 24-09-2005, 01:37 AM

Oh, where are my manners?

jps!

ausastronomer (John Bambury) · #4 24-09-2005, 08:32 AM

jps,

Those 8"/F4 Newtonian OTA's are made by Guan Sheng as you probably guessed. You might think its cheap but they all usually have very good quality optics considering they are a fast mass produced mirror. I have seen 1 that had outstsanding optics. The quality of the optics in the scope will be way better than the GSO eyepieces are capable of supporting and you will definately benefit from using good quality eyepieces in it. Unfortunately none of the cheaper eyepieces will work well at F4. You really have 2 options with an F4 light cone :-

Option 1
Premium Televue eyepieces (read expensive)
Premium Pentax XW eyepieces (read expensive)
Premium Vixen LVW eyepieces (read expensive)

Option 2
High quality plossls like Televue or the modified 5 element plossls (Masuyama design) sold as Celestron Ultima, Orion Ultrascopic, Antares Elite and Parks Gold series (same eyepieces different print on them) will work very well in your fast scope as will the University Optics high definition orthoscopics. In fact the image quality in any of these eyepieces will be as good if not a tad better than you get in the premium Televue and Pentax eyepieces, the downside is shorter eye-relief and a narrower field of view. With plossl and orthoscopic eyepieces the eye-relief gets shorter as the eye piece focal length gets shorter. If you have problems with short eye relief a good way around this is to use longer focal length eyepieces in conjunction with a good barlow which preserves and in some cases slightly extends the eye-relief. In terms of a barlow the Televue 2.5X Powermate is the best but expensive, good mid priced alternatives are the Televue 2x barlow, Celestron Ultima 2X barlow and the Orion Shorty "plus" 2x barlow.

I would advise that the optical quality of the scope certainly justifies using good quality components but importantly these are also partly necessary due to its fast light cone. Its a very nice scope actually and you will have a lot of fun with it when you stick some decent eyepieces in it

CS-John B

mick pinner · #5 24-09-2005, 02:19 PM

l agree with John, l bought one of these scopes when l got back into astronomy a couple of years ago and they are excellent value with very good optics and certainly worth the investment of a few top end ep's. they are a very good all round scope for planetary work and the brighter DSO's, it will certainly keep a relative begineer happy for a long time, two or three good ep's and a decent barlow and you have a great scope.

Starkler (Geoff) · #6 24-09-2005, 04:19 PM

I wonder why people dont use these short tube newts to get into astrophotography. They should fit nicely on an affordable mount and collect enough light. Im sure there is a reason.

wavelandscott (Scott) · #7 24-09-2005, 05:49 PM

JPS,

I agree with the comments about good quality eyepieces...a few well chosen will last a life time...

I was wondering if a Paracor (spelling?) would be a worthwhile investment in a fast scope like that and take some of the "pressure" on the eyepieces?

If it does it might be another option...I haven't used one so I don't really know.

Not sure of your experience level with a newtonian reflector so...I'd also highly recommend some quality collimation tools...in particular a chesire...

ausastronomer (John Bambury) · #8 24-09-2005, 06:36 PM

Geoff,

You can certainly use these for photography but there "can" be issues using a newtonian that is not optimised for photography for that purpose. The main issue is that you need to push the focal plane out by an inch or two to strike the chip or film. When you see people using large aperture newts for photography these are usually optimised as photographic telescopes or at least compromised in this regard with the secondary positioned slighly closer to the primary and an oversized secondary used to push the focal plane further away from the side of the tube and to provide a larger Fully Illuminated Field than would normally be the case for visual use. That scope however should do well as an entry into astrophotography if you can get it to reach focus on the chip or CCD or whatever device you plan to use. Collimating the primary as high in the tube as possible would possibly help here.

CS-John B

jps · #9 25-09-2005, 03:47 PM

Hi all,

Thanks to everyone for your replies. Ausastronomer your detailed response was greatly appreciated and very informative.

Janoskiss asked how the GSO plossls are performing. The image I get at the moment is poor but I think this might be the collimation rather than the eyepieces. At the moment if I look at bright stars with the 9mm they have a big blob around them. When I look at Beta Scorpius the blob goes around both visable stars. An out of focus star is assymetrical. Wavelandscott suggested a cheshire collimation tool. Unfortunately before I read this advice I had already ordered Celestron's Simple Collimation Tool. Since the total price including postage was $28 I could always buy something better without feeling too wastefull. If I do buy better there is the Bintel Laser Collimator Deluxe for approx. $130 or the Orion Collimating Eyepiece for approx. $80. Any comments?

On top of this I have coma towards the edge of field(as expected in the f/4). I might need one of those paracorrs but for now I will leave it. Will the coma improve with good collimation or are they not related.

wavelandscott (Scott) · #10 25-09-2005, 05:00 PM

I have the Orion collimating eyepiece (I got it from Bintel) and I also have an EZ Collimator (Laser)...

I usually use the Orion to get "close" and then finish off with the laser...I hen re-check with the Orion and I then try and "check and fine tune" with a star test...

My scopes are F6 and F5 so not quite as fast as yours...

I am not familiar with the tool that you bought...

There are advantages and disadvantages to all of the tools available...at f4 I would think that getting it collimated right will be important...

The Orion (or other non laser tools) will work as well as any quality laser and better than the cheap ones...collimation just takes some time and patience...not fancy tools.

That said, get some well made tools and reliable tools

I would suggest talking to some folks who have similarly fast scopes and seeking their advice...

They will also I suspect have a few tips/suggestions on handling coma...

Good Luck!

janoskiss (Steve H) · #11 25-09-2005, 06:35 PM

Any collimating tool is better than none (unless its a laser that is itself miscollimated). Not sure what the Celestron thingy. Hopefully it comes with instructions. I have the "Deluxe" laser collimator. It's not the best. The long Cheshire/sighttube is better. The EZ laser there's been a thread on recently here sounds like the one to get if you are going to get a laser.

So first collimate, then see how you go with the GSO Ploessls. If these do okay, some better Ploessls might be the way to go if you want to keep costs under control: synta, televue, celestron ultima, depending on your budget. Several members are hard at work putting together comprehensive reviews of cheapest to top-shelf simple design (Ploessl/Ortho) eyepieces. First installment (from the bottom shelf) is already in the reviews section.