Eyepieces for my 8" f4 newt.

[Boothie (Sean)]

So I have been getting my new scope together and it is awesome, a major step up from the loaner scope I had been using. But I am wondering if my eyepieces that I bought to play around with are going to be okay with this very fast scope.

From some basic reading I have discovered that the very short F-ratio (F4) is going to be hard on eyepieces, or is going to show cheaper EP flaws easier (blurred edges, difficult to get sharp, distortion)

A lot of the info/forum discussions seems to be from around 10 years ago. Have eyepieces started to be redesigned to work better with the now more common faster scopes? Or is it still just the more top-shelf EP's that are suited to visual work in these scopes aimed at imaging?

Like all things budget is always a consideration and look forward to your responses.

Cheers,
Sean

[barx1963 (Malcolm)]

Sean
Only limited info in your post. What eyepieces have you got?
Secondly, my usual advice is use the ones you have. They may not be perfect, but they don't cost anything more and you can spend/waste a lot of $$$ chasing perfection that sometimes isn't actually obtainable.

An f4 newtonian is going to have coma. Coma is an unavoidable consequence of the newtonian design. I built a little dob using an f4 mirror and it works quite well. You certainly see distortions when using the widefield eyepieces but that is the price for the convenience of f4.

Malcolm

[Atmos (Colin)]

You are right, some eye pieces work a lot better in faster scopes than others, that's about as much help I have in that area

At F/4 you may want to get a coma corrector. Some learn to live with the coma and outer field distortion, others swear by correctors in fast newts

I'd suggest something along the lines of a 20mm and 10mm with a good quality 3x barlow for planetary observing.

[Kunama]

Hello Sean, the above posts are pretty spot on about an 8"f4 and its coma but depending on what you are viewing the coma may not be such a distraction.

Eyepieces, like scopes, vary in quality v price but that graph is not linear, a $1K eyepiece is likely just 5-10% better in actual image quality than a $500 eyepiece.

I have a some half decent eyepieces if you would like to try something out before buying. You might find that what you have is good enough. I'll PM my mobile number to you. I am in Garran.

[janoskiss (Steve H)]

Pentax XF 8.5mm performs brilliantly with an 8" f/4 Newt, even without a coma corrector, and it hits the sweet spot for deep sky. IMO it's the ideal DSO EP for that scope. And it's the best bang-for-buck truly premium EP around. I cannot recommend it highly enough.

[Satchmo]

Modern eyepieces are better at reducing the off axis astigmatism of a fast light cone in combination with a more traditional eyepiece. The only coma correcting eyepiece that was available for a short while in the 80's was the Pretoria

I'd recommend a coma corrector like the GSO - only $150 and the problem is sorted ( although the purists may want to add a spacer to perfect the lens to focal plane spacing ) . Coma is linear so using higher mags doesn't help much it still magnifies the coma at the edge of a smaller true field . Some eyepieces do have some ability to correct coma a bit even if its not sold as a feature .

My old 18mm Radian delivered a really sharp edge with an F4 so the Deelite which is a similar design might be worth looking at. The bad images at the edge of low power eyepieces is mainly due to astigmatism - coma is very much less of a component in the mix.

I've been restoring an old GSO 8" F4 , so I 'm going through the same considerations myself but a Coma Corrector will be a definite purchase . I will report back with some evaluation with and without the corrector with a few Televue eyepieces .

[Satchmo]

An update with the eyepieces I have at hand :

8" F4 - without coma corrector

24mm Panoptic - pretty acceptable if I focus stars 70% from centre - centre looks sharp enough and whole field looks fairly good. Much better than I expected .

16mm Nagler Type 5 - acceptably sharp at edge centre 50% very good

13nm Nagler Type 6 - improved sharpness at edge - overall very good .

I'm evaluating these optics for a binocular telescope where a coma corrector is technically too difficult . MY impression is that it is all certainly possible to live without a coma corrector for visual use with these high quality eyepieces .

[Kunama]

Originally Posted by janoskiss

Pentax XF 8.5mm performs brilliantly with an 8" f/4 Newt, even without a coma corrector, and it hits the sweet spot for deep sky. IMO it's the ideal DSO EP for that scope. And it's the best bang-for-buck truly premium EP around. I cannot recommend it highly enough.

Agree strongly ! I used the XF8.5 in my Vixen R200SS 8"f4.

(Sean, I have one you can try)

[Boothie (Sean)]

Cheers for all the replys and input, I have decided to just stick with what I have for the meanwhile, get use to the scope and enjoy what I have.

What I have at the moment is:
6.5 Plossel (ebay cheapie- most likely looking to replace first)
8-24mm zoom (not used much)
BST 27mm wide view (used alot)
Celestron 2x barlow
GSO 3x barlow

I'm just starting to get into the hobby and think I will ease into it, that way I can slowly have things turn up and not have too many discussions on the cost!

Matt it would be great to catch up and get an idea of the difference in the EP's and how they perform in my set up.

[janoskiss (Steve H)]

Sounds like you're sorted for barlows but IMHO you need at least one more EP. On a budget a decent 15-17mm plossl would be a good idea: combined with your barlows, you'd get three useful focal lengths. But if you don't want to buy any new EPs yet, your zoom might also fill that role (depending on what it is).

In any case, be sure to learn how to collimate the scope and definitely get a collimating tool if you haven't got one yet. A fast Newtonian's performance is particularly sensitive to collimation.

[75BC (Brendon)]

From my experience, and I have nowhere near that of others on this forum, Steve’s suggestion of maybe 1 more plossl is a good one going by what you already have.

Meaning no disrespect at all to others but I’m not sure why some have suggested eyepieces worth 100’s of dollars when your OP said “budget is always a consideration”.

Don’t be put off by reading about flaws in eyepieces showing in fast scopes. It’s not going to ruin the view when starting out. I had a 10” f4.8 dob for 10 years and barely noticed the flaws experienced people talk about. But that’s just me.

[Kunama]

I don't think anyone suggests that the OP should spend 100s on eyepieces. In fact I would suggest not buying anything until he has had quite some time at the eyepiece.
In reality most recent eyepieces give a reasonable view. The OP's 8-24 would probably cover most situations.

[75BC (Brendon)]

Matt on re-reading the posts I see your point of maybe not suggesting to buy, but a lot of premium items mentioned. Like I said, no offence intended. I have utmost respect for you experienced folk. I’ve learnt so much on here since I joined the forum. Just didn’t want Sean to think those eyepieces were needed to get a good view.

And Sean you should take note of Matts comment of using the zoom as it will give you a good idea of the focal lengths that will work well with your scope when you are ready to buy.

[Satchmo]

Seans original question was :

"Have eyepieces started to be redesigned to work better with the now more common faster scopes? Or is it still just the more top-shelf EP's that are suited to visual work in these scopes aimed at imaging?"

So the short answer is no - cheap simple design eyepieces will only be sharp in the central region - there has been no breakthrough there . Premium eyepieces are mentioned because its the only way to answer the question effectively . Your sensitivity to off axis aberrations in the view is something that only you can determine

If you want the cheap ones to all look somewhat better then the purchase of a $150 com corrector is money well spent . As my examination last night with a 200mm showed , better quality eyepieces - they can work quite well at F4 without a coma corrector.

[dimithri86 (Dimithri)]

For some people the distortions make little difference. I am not that fussed with having sharpness to the edges. Given I only have 1 wide lenses, most of the time an object wont take up the whole field so its ok to have the sharpness only in the middle.

[janoskiss (Steve H)]

@75BC A few $100 on good EPs for a fast Newt is not much at all. And around $200 for a premium widefield EP like the Pentax XF 8.5mm that's well matched to the scope is an absolute bargain; fits very well with the OP's "budget is a consideration" requirement.

Mark is spot on that options for an f/4 Newt are limited. Budget widefields are out. Basically, the only viable options are:

1. Budget 4-element Plossls ~$50 (cheap, so-so performance but passable if cannot afford to spend more).
2. Premium 4-element Plossls ~$100-$140 (better but not ideal for f/4).
3. Orthoscopics or 5-element modified "plossls" (Masuyama style, e.g. Parks Gold Series) ~$80-$150 (nice, great value for money).
4. More complex designs, premium widefields ~$300 an up.
5. The XF 8.5mm is the one exception: outstanding value for ~< $200, but would rarely be useful with the 3x barlow.

Of these #3 and #5 offer the best performance per dollar for the telescope in question. Combined with the two barlows the OP already has, 15mm Parks Gold Series @$110 from AEC would be my top budget pick. It's really worth spending the extra money over cheaper plossls. Other good options would be 18mm and/or 12.5mm Fujiyama HD orthos (or equivalent).

But it's been a while since I've used these combos. I'll go out next clear night and test my 15mm Antares Elite (same EP as the Parks GS) and 12.5mm UO HD ortho in the 8" f/4 Dob to refresh my memory and will report back.

[75BC (Brendon)]

Steve Thanks for taking the time to explain the suggestions made by yourself and others as it not only will help the OP (Sean) understand why certain things are suggested which of course is the main consideration, but I’ am always learning from you guys also.

Sean sorry if I hijacked your thread a bit but I think this is all still on topic and the guys have given you some great well explained info.

[doug mc]

A coma corrector for low power and use your Barlows for high power.