Eyepiece queries, f/4.8

[Louis D]

The only time I use a barlow is when I pair my 2" 2x GSO ED Barlow with a TV Panoptic Barlow Interface (PBI) to make it telecentric and then leave it in the focuser to run at higher powers all evening.  It also helps to clean up the edges in more marginal eyepieces.  The disadvantage is that I can't pair it with my GSO coma corrector because the two won't reach focus together.  However, at an effective f/12, coma isn't much of an issue.

[pete_81]

Thanks @John & @rwilkey. Yep, all views on the ES say great, nothing really bad except for the potential size/weight increase to the scope (John posted great summary image of this on 

which I think sums up the issues with "good" focal extenders/barlows!

Am I right in my assumptions that the ES_68*_24mm would essentially become a ES_68*_12mm with ER staying at the 18.4mm, affecting ONLY the focal length of the scope (not EP) => does the focal-extender change the SCOPE focal length rather than the EYEPIECE (field stop stays same dimension when solving equation for TFoV=FieldStopOfLens x 57.3 / FocalLengthOfScope.

The focal length change consequently doubles magnification, so exit pupil halves; same thing as using an EP with half the focal length and same ER... Surely a TV-PM/ES-FE should be a part of everyone's EP kit IMHO😉

[Louis D]

16 minutes ago, pete_81 said:

Am I right in my assumptions that the ES_68*_24mm would essentially become a ES_68*_12mm with ER staying at the 18.4mm, affecting ONLY the focal length of the scope (not EP) => does the focal-extender change the SCOPE focal length rather than the EYEPIECE (field stop stays same dimension when solving equation for TFoV=FieldStopOfLens x 57.3 / FocalLengthOfScope.

The focal length change consequently doubles magnification, so exit pupil halves; same thing as using an EP with half the focal length and same ER... Surely a TV-PM/ES-FE should be a part of everyone's EP kit IMHO😉

The focal extender doubles the focal length by doubling the focal ratio, slowing down the light cone (converges at a less steep angle).  This is good for lesser eyepiece designs that benefit from usage in slower scopes.

If you've got eyepieces covering pretty much every focal length from 3mm to 40mm in 1mm to 3mm steps (depending on the particular range), there's not much point in using a focal extender except as I pointed out above.  The other exception is for imagers trying to increase image size on the sensor for planetary imaging.

[jetstream]

2 hours ago, pete_81 said:

Thanks Gerry - any thoughts on the extenders (even the x3 is cheaper than a single EP and from ES, whats the drawback?!)

Not sure... I do use the Baader VIP with a 16mm T5 Nagler and it works well, sharpening the Nagler up. An extender should keep the exit pupil the same distance out, better than a barlow in this regard. If the ES extender isn't too long or expensive it could be a good option for sure. I would only go 2x on the extender as a 12mm will be great for galaxies, M42 core etc in your scope but for lunar/planetary I would choose a different set of eyepieces.

Well, maybe- how fussy are you for observing lunar planetary? 

As good as the ES 68 24mm is ( Ethos, Lunt etc too) the design will give more scatter, which drives me crazy on the moon and planets.

The 24mm ES 68 and a pocket full of used orthos is hard to beat and the extender to 12mm would work nicely I believe.

[Piero]

Congrats on the 24mm 68. Great eyepiece. 

Regarding the Barlow, I prefer these tools with zoom rather than single focal length eyepieces.

The bresser SA Barlow 2x (which, despite of its name, is a tele extender and not a barlow...) is also very good and comparable with the TV powermate. You can find these in the second hand market.

Vixen SLV. Very nice and comfortable eyepieces. I preferred their colour rendition to the TV Delos I owned. 

If you are happy with short eye relief EPs and don't mind 50deg AFOV for planetary observing, you could get the 10mm and 6mm Baader BCO, and possibly also the Baader turret Barlow 2.25x. Again, you can find these in the second hand market.

[Ricochet]

There is a second hand one showing as active on ABS: https://www.astrobuysell.com/uk/propview.php?view=149974

I use the Meade branded predecessor (same as the Bresser SA) to get my high power mono views from medium power eyepieces. That way there is only one "step" where you change from using the Telextender to not using it, whereas if you use it to try to gain intermediate powers between other eyepieces it will constantly be in and out of the focuser.

[pete_81]

Thanks @Ricochet! In the lens box now.

Thinking this kits out well with the ES68*,24mm and the next on the target will probably be the 82*6.7, which will give me TFoVs of 1.3*, 1.1*, 0.65*, 0.45*, all with the exit pupil > 1mm, then a killer with 350x, 0.2*TFoV which should have 80% of the FoV filled with M82, exit pupil drops to 0.7, but may still be OK (read somewhere to try and keep it >1 for DSOs).

Time to gaze and rest, then save some!

Thanks for the advice to all...

Edited May 23, 2019 by pete_81

[Trikeflyer]

23 hours ago, Ricochet said:

There is a second hand one showing as active on ABS: https://www.astrobuysell.com/uk/propview.php?view=149974

I use the Meade branded predecessor (same as the Bresser SA) to get my high power mono views from medium power eyepieces. That way there is only one "step" where you change from using the Telextender to not using it, whereas if you use it to try to gain intermediate powers between other eyepieces it will constantly be in and out of the focuser.

I have one of these and use it with my ES 24mm 82. I don’t find any draw back tbh. At that price it’s a steal. I’d be biting the sellers had off if I didn’t have one already

steve 

[pregulla]

The drawback is it's size and weight, at least for me. For example ES82 11mm is 272g, ES82 24mm + 2" Focal Extender is 704g+655g. That was the reason I decided against getting 2" one, even though the price was really tempting - $80 shipped.

[Piero]

You need to rebalance the scope. Said this, I agree with you.. that's a lot of rebalancing.

I own two sets of eyepieces. Each set has very similar eyepieces in terms of weight and size.