Eyepiece Dilemma

[Rob Sellent]

I wonder if anyone has any insight on what to do? I've been thinking whether it is worth purchasing an eyepiece around 0.7 exit pupil, 6mm, 150x for a 4" f8.8 (902 focal length) apo?

My eyepiece collection consists of a Baader Mark IV Zoom, a complete set of Baader Genuine Orthos (5, 6, 7, 9, 12.5 & 18mm), TeleVue x2 Barlow, Delos 10mm & 14mm, Panoptics 19mm & 24mm. This set up is fine for my f5 Dob and f6.3 apo and I feel a 6mm in either scope would be redundant but not in the f8.8.

Do you think it's worth spending quite a bit of money going for a 6mm dedicated eyepiece in the f8.8, or just stop fussing and and get on with the Orthos and Barlowed Delos ?  Again, finally, there's also the option of a Nagler 3-6mm Zoom but again, is that worth it when I already have some cracking, tip-top Orthos?

Any ideas/insights would be greatly received

Edited September 19, 2019 by Rob Sellent

[LDW1]

The math says that with that scope, to maximize its performance, you are restricted to eps between 18mm - 36mm so with the 6mm ep the brightness factor should / will be practically at zero, you may just make out a bright DSO, a planet or the moon? You figure it out, is that what you want to spend quite a bit of money on when you already have it in a pretty fair set of eps already ? Clear Skies to you !

Edited September 19, 2019 by LDW1

[Rob Sellent]

52 minutes ago, LDW1 said:

The math says that with that scope, to maximize its performance, you are restricted to eps between 18mm - 36mm

Thanks for the reply. I don't get the math . The fl 4" does fine at +50x .  

[jetstream]

The Nagler 3-6mm is an indispensable tool here, that being said as aperture increases it loses out in sharpness over Delos, orthos a bit. Seeing has to be excellent to notice it.

The Delos is so much easier to use compared to orthos- and I love orthos, even in the 24".

What scope is it for exactly?

[RT65CB-SWL]

The TeleVue Nagler 3-6mm zoom is a cracking e/p for its size and excels when conditions allow for it. I use fixed length e/p's otherwise.

But as you said, quote: "...is that worth it when I already have some cracking, tip-top Orthos?..." - some have said using the Nagler brand name for the 3-6mm zoom was misleading, because they have an AFOV of 50deg., whereas the Nagler e/p has an AFOV of 82deg., (my take on that argument is that Al Nagler designed it - can you think of a better name?).

Edited September 23, 2019 by Philip R

[Rob Sellent]

Thank you

@jetstream the scope is an old Vixen Fluorite 102 f8.8. It's a cracking scope, gives gorgeous contrast, sharpness, and is a real delight to use. I made a small report here some weeks back. You're right about the Delos being so much easier on the eye compared to Orthos. 6mm Delos might be the way to go but then that "indispensable tool " of the TV zoom you mentioned does raise its saucy face...and no, I cannot buy both 😋

@Philip R that's +2 for the zoom but as you so rightly say, does it make sense when already equipped with orthos?

On a side note, I think the limiting factor of the scope will not be its optics but my aging eyes and atmospheric turbulence.

[John]

I use my Nagler zoom 2-4mm very often with my Tak 100mm F/9, much more often than I thought I would. I'm lucky to also have Pentax XW's at 5mm and 3.5mm as well though.

The instantly variable focal length is a very strong asset to have at high power.

 

 

 

[Alan White]

I have a Nagler 3-6 Zoom and really enjoy using it, nice presentation of image.
As John says and I fully agree.

1 hour ago, John said:

The instantly variable focal length is a very strong asset to have at high power.

I had some Orthos, but the very short Eye-relief did my head in.
We all have differing views on this of course.

[Rob Sellent]

@Johnthat's now +3 for the zoom and also very interesting because we have similar specifications. The Tak f9 must be a gorgeous scope to own - and I imagine as rare as hen's teeth. Do you know whether they're still manufacturing them? Moreover, I think the value of the f9 will never drop. Ever. None of that minus-one-third on resale It's the perfect mix of portability, aperture, focal ratio, weight and views in the superlative. You mention the TV 2-4 zoom. Again, I imagine these are rare to find but performance wise do you think the 3-6 zoom is of similar quality? The XW's fame is legendary. When purchasing the wider-fields, I was torn between the XW's or Delos. Both have outstanding reviews but in the end, I just felt TV have never let me down, so plumped for them.

@Alan White that's now +4 for the zoom. Not a single negative report and always mention of great performance and ergonomics. I appreciate your insight into the orthos' eye relief. I can view with them for a while without a problem, but find they do give me a 'head-eye-ache' after too much use and they're not the easiest little buddies to use when wanting to sketch at high magnifications. Maybe I'm a fool for keeping hold of them for so long, they used to get so much use but these days not so much However, we've been together for so many years and in a peculiar manner of speaking, I still consider them optically the benchmark where all other eyepieces must perform if they are to remain in the case .

[John]

The 3-6 and 2-4 Nagler zooms have pretty much identical optical performance. Apart from the focal lengths the only difference is that the 2-4 has click stops at half mm intervals wheras the 3-6 has them at 1mm. You can position the zooms anywhere along their range though.

As I understand it, the Tak FC-100DL was made in two production runs each comprising 100 instruments. My scope was the last UK one of the 1st run available. I did wonder if Tak would do a 3rd run but with the launch of the F/8 FC-100DZ I think we won't see any more DL's made.

Congratulations on your Vixen FL102 - it's a scope that I dreamed of owning for many years

 

[Timebandit]

 

 

If you already have a set of BGO , then these are top quality and If the fov and tight eye relief does not bother you then is it really worth getting the Nag zoom?( even though I understand they are good.

If you want ortho type sharp visual performance , but more comfortable eye relief and wider fov , the go for some Pentax XW 3.5 and 5mm . like an ortho but without the draw backs.

 

 

[Rob Sellent]

@Johnthank you for going over that with me, John It's not often one sees the 2-4 zoom up for sale, so it's good to know the 3-6 is of similar quality. Your f9, the keeper of all keepers

@Timebanditthank you for your reply and you have nailed the proverbial nail. That is exactly where I'm sitting this cloudy evening. Why the zoom when you already have orthos and if you want to be fussy about eye-relief and fov, well, you know the score, Delos, Delites or XWs. Hopefully the floods, rain and clouds will clear up in the next week or so giving me time to mull this over.

[LDW1]

On 19/09/2019 at 09:08, Rob Sellent said:

Thanks for the reply. I don't get the math . The fl 4" does fine at +50x .  

Google / download the NAA Telescope Calculator, this will give you the best case parameters for any scope based on scope specs and the observers age etc., its a fill in the blanks thing and it calculates the theoreticals but it doesn’t mean a particular situation can’t go above the numbers, if you are lucky I guess ! Thats where the numbers I gave you came from so check it out, I use it all the time to check a scopes characteristics when looking at buying a particular scope to give me an idea as to whether I want it or not or might already have one that is quite similar.

Edited September 23, 2019 by LDW1

[Rob Sellent]

Got it, @LDW1 It looks like the calculator's 'optimum' power range is based on exit pupils between 4mm to 2mm correspondng to the aforementioned 36mm to 18mm eyepiece focal length. Your own millage may differ but regarding exit pupil, I've found:

• 6mm - 4mm - nice for star fields and large open clusters.
• 4mm to 3mm - nice contrast, good on star-fields.
• 2.5mm to 1.5mm - nice for most DSOs and casual lunar/planetary viewing.
• 1.6mm to 1mm - nice contrast and resolution on planets, Moon and globs.
• 1mm to 0.5mm – finer detail on planets, lunar, tight doubles - dependent on seeing conditions.
• 0.5mm to 0 – super tight doubles but really not much call for use.

Needless to say, my enquiry was mainly about that 1mm to 0.5mm range where even a 1mm difference in an eyepiece's focal length makes quite a bit a difference. It'll be interesting to hear about your own experience with exit pupil and how it compares with the little table above

Edited September 23, 2019 by Rob Sellent

[LDW1]

For the most part I don’t get too exited about exit pupil, I just point the scope and view, lol ! As I mentioned it is a good guide, a starting point if you will, a quick check when buying a scope or an eyepiece to give you an idea as to what kind of performance you are going to get as a start. Its a good reference.

[Louis D]

12 hours ago, Rob Sellent said:

It'll be interesting to hear about your own experience with exit pupil and how it compares with the little table above

I usually struggle monoviewing with exit pupils below 0.7mm due to floaters in my observing eye.  Binoviewers can help get me down to 0.5mm, but beyond that, it's pretty annoying trying to look through the myriad floaters.

[Rob Sellent]

36 minutes ago, Louis D said:

Binoviewers can help get me down to 0.5mm

It's interesting you mention binoviewers, Louis. I think when I've had a little more use of the FL102, I might look into it. The thing holding me back is not only price but of the mixed reports, some folk enjoying their use, others who simply can't get on with them. I'd take the risk in the UK with the fluid secondhand market, but here in Spain there's really very little activity. Regarding exit pupil, at the moment I'm 'comfortable' down 0.6mm/0.7mm. Below that, I just can't blink away the floaters.

[Stu]

12 hours ago, LDW1 said:

For the most part I don’t get too exited about exit pupil, I just point the scope and view, lol ! As I mentioned it is a good guide, a starting point if you will, a quick check when buying a scope or an eyepiece to give you an idea as to what kind of performance you are going to get as a start. Its a good reference.

I used to think similarly about exit pupil, until I fully understood it. Once you get your head around it, it helps explain a number of different effects, and as Rob has shown, gives a useful way of seeing which eyepieces will be effective in which scopes.

From my quite extensive experience with 4" apo refractors used for high power observing of the sun, moon and planets, I know that sticking at 1mm exit pupil or larger leaves alot of magnification on the table. I do also know that floaters are a significant issue down at 0.5mm exit pupils.

I took a long time, and about 6 pairs of binoviewers to finally get the hang of them. I actually think it is more a question of retraining your brain to get the best out of them. I got there eventually and now use them successfully for high power observations. The floaters are still there but much more controlled and manageable.

The only antidote to small exit pupils at high power is a bigger aperture scope, but misses the point and benefit of using a lovely 4" refractor.

[BGazing]

6mm would be very useful in your scope. 5mm T6 is my favourite Jupiter EP in 7.4 100mm Tak, and 4mm is probably the most comfortable on the Moon. If the seeing allows I would go to 3.5mm or even 3mm on the Moon, but Jupiter gets washed out. So, based on my experience, 6mm would be great for your 8.8.

One more thing. With planets so low here (and especially for you up there), I consider ZWO ADC indispensable in my Tak for planetary. Cleans it up very well and you get to enjoy the views instead of some rainbow boil near the horizon. The trick is, you would have to use some barlow before the ADC to make your f longer (I use 2.5 PM) so you get to use longer fl eyepiece as your planetary. Eg. my 13mm T6 becomes 'Jupiter EP'. 

So...in your scope I would have both 5mm and 6mm and the ADC. But that's me.  My EPs are multiplying like rabbits.

 

[Rob Sellent]

@Stuthat's excellent advice and words of wisdom, Stu It's both your own and @Johnposts about binoviewers that have kept me on the fence but I think when funds allow, I'll just have to get on with it and bite the bullet. You're right about being able to push more than 1mm and in this case it's not so much the optics which run out of puff and not always the seeing conditions which limit high powers but the eyes. The snag is although - as you so rightly point out - aperture is the antidote to tight exit pupil, I prefer viewing planets and the Moon and the star fields etc in the 4" than I do in the bigger 250mm scope. It's just a pretty picture

@BGazingthank you for your help and thoughtful post, Bgazing and I think you're right about the 6mm. It'll give a perfect 150x and isn't horrendous regarding exit pupil. The seeing here is often good but I feel anything less than 5mm will be compromised by my eyes. The floaters will become just a little too annoying. I've never heard about the ZWO ADC, so thank you for pointing that out. I'll have a read and look into it. I'm down in Spain, so I figure the planets are just a tad higher than they are for our northern European friends. Sadly, Jupiter and Saturn are dipping down now before I can get out to view, but this is also good news for it'll give me chance to work through my kit and see what we can come up with before next year and the return of Mars 😀

[RT65CB-SWL]

Below is an single frame image of Mars I took during the opposition in April 2014...

...taken with an Olympus Camedia C2040, handheld over a TeleVue 6mm Radian + neodymium filter and Celestron C6/SCT from a hotel garden near LHR.

The diameter of Mars with the Nagler 3-6mm zoom was almost the same, except the AFOV was a bit narrower, and at 3mm not much improvement of visible features, as Mars was having dust storm at the time.

[Rob]

Reading this with interest with regards to exit pupil. I've noticed my eyes have changed, and I'm finding more blackouts personally. I know I need new prescription reading glasses!.. My EP set may need adjusting then??.

Needless to say my exit pupil seems happy with the SW TMB 6mm clone

Rob

Edited September 24, 2019 by Rob

[Stu]

45 minutes ago, Rob Sellent said:

being able to push more than 1mm and in this case it's not so much the optics which run out of puff and not always the seeing conditions which limit high powers but the eyes.

Absolutely. A decent 4" apo scope under excellent conditions will keep cramming on the power beyond the capabilities of my eyes before the image breaks down.

My Tak remains the only scope I've seen the Alpine Rille with, despite trying with many others. It's the ability to cut through average seeing that I'm normally cursed with that helps I think, along with the contrast. I need to try the 8" f8 of course, sure that would do it but that's not the topic of this conversation.

[Rob Sellent]

3 hours ago, Stu said:

My Tak remains the only scope I've seen the Alpine Rille with

Ohh, this sounds like a good test for the Vixen. Clearly, I'm a very poor lunar observer for I've never seen the Rille, Stu . Do you know when the best time to do so would be? And here or elsewhere, it would be nice to hear how your 8" planet killer gets on and how you think it compares to your fluorite

[Stu]

13 minutes ago, Rob Sellent said:

Ohh, this sounds like a good test for the Vixen. Clearly, I'm a very poor lunar observer for I've never seen the Rille, Stu . Do you know when the best time to do so would be? And here or elsewhere, it would be nice to hear how your 8" planet killer gets on and how you think it compares to your fluorite

If you have a look in the Lunar Observing section Rob, there is a great sticky from @Doc about his Observe the Lunar 100 project, and one from me with a number of resources which might be useful in your own attempts. There is a .pdf document called OTL100.pdf which lists the best day in the Lunar cycle to observe each feature. The Alpine Valley is number 19 and listed as best on day 7. I have not been able to verify these days, but they should give a good indication of when to try in the lunar cycle.