2mm Exit Pupil for DSO = Perfection?

[Littleguy80]

47 minutes ago, Pixies said:

Occasionally I've been using a 4mm ortho in an F6 8" (x300 magnification). It's not very often the seeing is good enough - but I've found it useful when trying to split something very tight.

I must admit, it's not relaxing.

When I got the Pentax XW 5mm, I spent quite awhile comparing it to my BGO 5mm. I concluded that the BGO had a small edge in performance but also felt a lot harder work. The XW is my favourite for Lunar. The extra FOV just gives you that little bit more time to pick out the details. The BGO only tends to come out when I’m going for something really faint. 

[Stardaze]

On 07/05/2021 at 22:20, John said:

I used to have a 40mm Aero ED - very nice eyepiece. I just didn't find it effective for me though because the background sky was just too bright with it. The 31mm Nagler is better in this respect but even that suffers a little so the 21mm Ethos and more recently the 17mm ES / 92 and the 13mm Ethos are my most used DSO eyepieces with my F/5.3 12 inch dob.

The 31mm Nagler works better with my F/6.5 102mm refractor where the exit pupil is under 5mm.

Well that's what I've found anyway

YMMV as they say  

Just reading through again and assuming your 12” is 1600mm, the comparable to your 21/13/8 core is 17/10/7 for me John. 

[John]

38 minutes ago, Stardaze said:

Just reading through again and assuming your 12” is 1600mm, the comparable to your 21/13/8 core is 17/10/7 for me John. 

Yes - it's F/5.3.

I do often find shorter focal length eyepieces useful with the scope as well. Down to 4mm. Rarely anything shorter although I do have options down to 2mm if I want to be really silly

 

 

[Stardaze]

13 hours ago, Littleguy80 said:

Mmmm that’s a tricky one. The first two 100 degree eyepieces I got was the 20mm and 9mm. For awhile, I wondered if I needed the 13mm at all. In the end, I got the 13mm as there were occasions when it was useful. At the moment, despite the 12.5mm and 10mm being too close on paper, I find what have works well for me. 

Maybe the answer is the Nikon Nav HW 12.5mm. I think that comes with a focal extender so you can use it as a 10mm too. They’re supposed to be tier performers. Pricey but if it’s two eyepieces in one….

Wow, I knew the Nikon was expensive but not £1100 expensive! I'm not in a rush at this time of year. Think I'll get the XW 7 and see if I can pick up a 10E secondhand, I shouldn't be out of pocket that way. Use that alongside the APM 13 and see how I get on. Whether 20 to 10mm is too large a jump, most of the time, I guess I need to work out. 

[Littleguy80]

11 minutes ago, Stardaze said:

Wow, I knew the Nikon was expensive but not £1100 expensive! I'm not in a rush at this time of year. Think I'll get the XW 7 and see if I can pick up a 10E secondhand, I shouldn't be out of pocket that way. Use that alongside the APM 13 and see how I get on. Whether 20 to 10mm is too large a jump, most of the time, I guess I need to work out. 

Good plan. My original plan had been to keep the APM 13mm with the Delos 10mm and then get an XW 7mm. That only changed when I got the chance to get the Noblex. 

£1100??? I didn’t realise it was as much as that.

[John]

1 minute ago, Littleguy80 said:

.... That only changed when I got the chance to get the Noblex....

 

I'm the same - a number of my best laid plans have changed because something interesting "came up"

The Noblex was worth changing things around for though, I'm sure. Out of production now as well.

 

[Stardaze]

13 minutes ago, Littleguy80 said:

Good plan. My original plan had been to keep the APM 13mm with the Delos 10mm and then get an XW 7mm. That only changed when I got the chance to get the Noblex. 

£1100??? I didn’t realise it was as much as that.

The Noblex is that good then. Sometimes I feel that the APM 13 needs a bit more contrast for sure, normally seems to be that the sky is still too bright. I haven't anything else to reference that against, it could just be the exit pupil rather than the contrast? The jump down to the 8mm has been too much normally there, so really am wondering whether 10mm is the remedy. 

[Littleguy80]

1 hour ago, Stardaze said:

The Noblex is that good then. Sometimes I feel that the APM 13 needs a bit more contrast for sure, normally seems to be that the sky is still too bright. I haven't anything else to reference that against, it could just be the exit pupil rather than the contrast? The jump down to the 8mm has been too much normally there, so really am wondering whether 10mm is the remedy. 

It's been a big improvement for me and has had lead to a definite drop in my use of the 10mm Delos. Although as I said earlier, the 10mm Delos offers advantages on fainter targets with the increased mag. For example, Saturday night I was looking for C/2020 R4 ATLAS. I couldn't see it with the Noblex though the nearby galaxy NGC 4314 was showing well. I initially mistook the galaxy for the comet. Moving to the 10mm Delos, I was getting a hint of the comet but not total confidence. I then switched to the Noblex with the Baader VIP barlow which I have set to give an approximate equivalent of a 7mm eyepiece. The comet became clearer at this magnification and I had the confidence to log an observation. Each step brought improvements.

[Stardaze]

1 hour ago, Littleguy80 said:

It's been a big improvement for me and has had lead to a definite drop in my use of the 10mm Delos. Although as I said earlier, the 10mm Delos offers advantages on fainter targets with the increased mag. For example, Saturday night I was looking for C/2020 R4 ATLAS. I couldn't see it with the Noblex though the nearby galaxy NGC 4314 was showing well. I initially mistook the galaxy for the comet. Moving to the 10mm Delos, I was getting a hint of the comet but not total confidence. I then switched to the Noblex with the Baader VIP barlow which I have set to give an approximate equivalent of a 7mm eyepiece. The comet became clearer at this magnification and I had the confidence to log an observation. Each step brought improvements.

I'll have to debate whether 100 deg is necessary at 10mm too. At 8mm I'll admit that it was nice to have, but at that mag, you are starting to drill into a target and so not totally necessary. But where's the line? 

I think that ultimately, if the 10mm is to replace the 13mm, then I'd need it. You have it nicely balanced with the 12.5 for ultra FOV and then drilling down from 10mm. 

Edited May 10, 2021 by Stardaze

[Littleguy80]

20 minutes ago, Stardaze said:

I'll have to debate whether 100 deg is necessary at 10mm too. At 8mm I'll admit that it was nice to have, but at that mag, you are starting to drill into a target and so not totally necessary. But where's the line? 

I think that ultimately, if the 10mm is to replace the 13mm, then I'd need it. You have it nicely balanced with the 12.5 for ultra FOV and then drilling down from 10mm. 

When I was considering swapping in the 100 degrees eyepieces for something narrow, I set up the FOV circles in SkySafari for current and planned eyepiece and then went searching through a bunch of targets that I like to look at. It gave me a good idea of what would work. 100 degree eyepieces give that slightly more immersive experience of loosing the edge of the FOV. I found that there's not too many targets that really need a big FOV like that. At least, not in my repertoire! 

Edited May 10, 2021 by Littleguy80

[Don Pensack]

On 09/05/2021 at 12:16, Xilman said:

I don't know about you guys but I find magnifications > x200 or so essentially unusable with a Dob. The speed at which objects traverse the field of view (not to mention having to focus during that short interval) and the difficulty finding them again afterwards makes calculations based on exit pupil rather academic.

Perhaps it's just old age and incompetence but I restrict myself to x150 or lower.

Only if you have a decent drive should you start worrying about diffraction-limited magnification or excessively small exit pupils. Again, IMO.

 

My 3.7mm eyepiece yields 495x in my dob.  It's a 110° eyepiece.  If I push a small object out to 3/4 of the way from center to edge on one side of center, and let it drift until it is 3/4 of the way from center to edge on the other side,

I get 40 seconds of viewing at that magnification before I have to move the scope again.

Try holding your breath for 40 seconds to see how long that is.

At only 230x, I get over 2 minutes between pushes.

So it is obvious the apparent fields of your high power eyepieces are too small.  In an undriven dob, high powers should be really wide field eyepieces to give you more observing time in between pushes.

As your magnification goes down, you can tolerate narrower fields because the movement through the field is slower.  I personally go from 70° at low power to 110° at high power in a smooth manner so the field size doesn't drop rapidly.

Here is what I mean: I've added every eyepiece I use (One great flaw to this presentation is that the object in the center doesn't have its size changed with magnification.  At the lowest power, the galaxy would be a point.  And it would not be as large as in the image until the magnification got to about much higher powers.:

Edited May 10, 2021 by Don Pensack

[Deadlake]

20 hours ago, Littleguy80 said:

£1100??? I didn’t realise it was as much as that.

Which one, APM has a sale, €332 + vat for 10 mm

https://www.apm-telescopes.de/en/optical-accessories/eyepieces/58-74-wide-angle/other/nikon-nav-sw-10mm-eyepiece

Looking around has the Nikon SW been discontinued ?

Edited May 11, 2021 by Deadlake

[Stardaze]

1 hour ago, Deadlake said:

Which one, APM has a sale, €332 + vat for 10 mm

https://www.apm-telescopes.de/en/optical-accessories/eyepieces/58-74-wide-angle/other/nikon-nav-sw-10mm-eyepiece

Looking around has the Nikon SW been discontinued ?

It’s this version, though they don’t seem to have the 12.5mm in stock. They’re well out of my budget.

http://apm-telescopes-englisch.shopgate.com/item/32373639

[Deadlake]

16 minutes ago, Stardaze said:

It’s this version, though they don’t seem to have the 12.5mm in stock. They’re well out of my budget.

http://apm-telescopes-englisch.shopgate.com/item/32373639

They're cheaper if you go to Japan:

https://www.kyoei-osaka.jp/SHOP/nikon-nav-125hw.html

[Xilman]

2 hours ago, Deadlake said:

They're cheaper if you go to Japan:

https://www.kyoei-osaka.jp/SHOP/nikon-nav-125hw.html

Still twice what I paid for the entire telescope, and that is without shipping and import duty.

Think I will pass on that one.

 

[jetstream]

14 hours ago, Deadlake said:

They're cheaper if you go to Japan:

https://www.kyoei-osaka.jp/SHOP/nikon-nav-125hw.html