Eye relief with a Barlow

[Albenn]

If I have an ep that has say 4mm eye relief, do I maintain that eye relief when I use a 2x Barlow with it?

[jnc71106]

No the ER will increase by how much I'm not sure

[Naemeth]

Yes you do I think, the Barlow goes in before the eyepiece, so you get exactly the same eye relief as you did without the Barlow (of course, Barlowing a 4mm on most nights wouldn't be a great idea as you would probably go above what conditions would allow).

EDIT: See Above/Below

[jnc71106]

No the ER increases. If you were using a powermate it would stay the same but a normal Barlow will increase the ER

[John]

A barlow increases the eye relief and also tends to move the focus position inwards a few mm. The amount of eye relief increase depends on the focal length of the barlow - the shorty barlows increase it more than the longer ones.

[Albenn]

So, using a Barlow gives longer eye relief. That would explain why my Vixen NPL 8mm used with a 2x Tal Barlow is more comfortable to use than my 4mm.

Thanks for the info guys

[John]

Yep - it's a positive feature when barlowing EP's with tight eye relief but can be a pain at the other end of the scale when eye relief gets over long and it's difficult to find the correct eye position - you end up "hovering" over the eyepiece which can be rather hit and miss.

[LukeSkywatcher]

So, using a Barlow gives longer eye relief. That would explain why my Vixen NPL 8mm used with a 2x Tal Barlow is more comfortable to use than my 4mm.

Thanks for the info guys

This is because the exit pupil stays the same.

[Albenn]

I can see how that would also help. Thanks Paul

Next chance I get, I'm going to Barlow the 4mm for a laugh and see how I get on!

[LukeSkywatcher]

If memory serves me, i think i managed to barlow a 4mm once. I was observing the Moon and remember that the views were pretty amazing even if the Moon appeared to be boiling. It was a Celestron Omni 4mm with a TAL 2x and the scope was the Heritage. The planets must have been in perfect alignment that night to allow me to use that much magnification.

[Albenn]

I was also thinking of trying the Barlowed 4mm on the moon first rather than Saturn which races across the Barlowed 8mm at a great rate of knots.

That 8mm and Barlow gives good views of Saturn even though I'm nudging the scope every few seconds.

[LukeSkywatcher]

Yep, Saturn moves pretty fast.

[Naemeth]

I was also thinking of trying the Barlowed 4mm on the moon first rather than Saturn which races across the Barlowed 8mm at a great rate of knots.

That 8mm and Barlow gives good views of Saturn even though I'm nudging the scope every few seconds.

Funny you say that actually, Saturn at 4mm for me didn't move that quickly, I could quite easily keep it in focus. It might be due to the Vixen having a smaller FOV.

[John]

I've tried barlowing a 3.5mm Nagler a few times which is fun. The barlow is a Revelation 2.5x so I get an 82 degree FoV 1.4mm eyepiece !. It's much too much power in all my scopes but on a good night I could pretend I was Neil Armstrong bringing the lunar module in to land - the lunar surface looked that close at 857x

[LukeSkywatcher]

I've tried barlowing a 3.5mm Nagler a few times which is fun. The barlow is a Revelation 2.5x so I get an 82 degree FoV 1.4mm eyepiece !. It's much too much power in all my scopes but on a good night I could pretend I was Neil Armstrong bringing the lunar module in to land - the lunar surface looked that close at 857x

Thats the same feeling i got when i barlowed my 4mm Omni. Stupid magnification i admit (on my part), but that night it worked.

I now have a Revelation 2x and to be honest i cant get it to work with my 8SE. No worries though because ive given up the idea of barlowing now.

[Albenn]

I bought the 4mm NPL because I was getting fed up Barlowing the 8mm with all the messing about it entails but now I'm fed up having to have my eyeball glued to the 4mm to get the same fov and nudging the scope every 20 secs or so! That combo definitely teaches you how to manipulate the scope though.

It is definitely easier on my eye using the barlowed 8mm.

[LukeSkywatcher]

I stupidly thought that by buying a 4mm EP (Celestron Omni) that i would be able to view the planets at a reasonable size. Up til then i used an 8mm EP kit Plossl. I didnt understand eye relief and exit pupil stuff at that time. Trying to see a planet with a 4mm EP is impossible enough, let alone track it manually in the Heritage.

I have not introduced my 4mm EP to my 8SE yet. I'm sure that it will be easier to observe planets with given the 8SE is Go-To. Then again, we simply do not get the weather conditions here in this part of the world to allow us to use 4mm very often.

Not my most sound investment ever (4mm Omni), but at least i know i can see something through it with a manual scope, so with an automated scope (given half the chance), it should be a bit easier.

[LukeSkywatcher]

"I stupidly thought that by buying a 4mm EP (Celestron Omni) that i would be able to view the planets at a reasonable size. Up til then i used an 8mm EP kit Plossl"

I have since found out that all i had to do was to increase the aperture of my scope to be able to see planets at a reasonable size. I now see planets (Jupiter,Saturn and Mars) at a size that i am more then happy with. Mind you the 4mm Omni was a BIT cheaper then my 8SE.

Its amazing the difference 70mm makes (Heritage v 8SE). I know there is a lot more to it then just the aperture difference.

[Albenn]

The view I get of Saturn with the 4mm is as good as the Barlowed 8mm but it takes a little more work with both eye positioning and tracking.

I don't know if everyone does this - but I point the scope so that the planet tracks from one side of the ep to the other. I find that with the NPL ep's they are quite sharp across the FOV so this gives me a little more time to view before having to slew the scope again.

I really like my little Heritage. In fact a couple of weeks back one of the local astro club members invited me along on a viewing session and she couldn't believe just how good the views were through the little dob. Another of the members who was using the clubs 6" TAL Newt was pretty impressed also and said he was thinking of getting one himself.

I'm getting a little OT here so better stop.....

I now know that the eye relief increases but what about the FOV. Or should I post that as a new question?

[John]

....I don't know if everyone does this - but I point the scope so that the planet tracks from one side of the ep to the other. I find that with the NPL ep's they are quite sharp across the FOV so this gives me a little more time to view before having to slew the scope again.....

It's a good technique with undriven scopes. Even better with eyepieces with a very wide field of view of course because you get that much more time before you need to move the scope. Once of the reasons that some folks love those 82 and 100 degree field of view eyepieces.

[LukeSkywatcher]

I'd LOVE to try an EP with 82+FOV.

[Albenn]

Sounds like a good idea but I have a hunch they would be quite expensive

[LukeSkywatcher]

My SCT can handle both formats but i have too much cash invested in 1.25 EP's to change to 2"

[Albenn]

Paul, I assume from your reply that they aren't available in 1.25" then anyway?

[LukeSkywatcher]

I dont really know. The 8SE can happily handle both from what i can tell.