Nagler 31mm eyepiece out of focus

[Granada]

Last night I tried out my TeleVue 31mm Type 5, 2" eyepiece with a Newtonian (150mm aperture, 750mm focal length), and I just couldn't get the images focused. Wondering if the two are not meant to be used together, or perhaps I'm using them incorrectly?

[gaseous]

If you rack out the focuser and the image gets closer to being focused without quite making it, you may need an extension tube for your focuser. Should be inexpensive. If for some reason it gets closer to focus the further in the focuser is racked, then you may have problems.

Or sometimes rather than an extension tube you can just have the eyepiece fitted a little further out of the focuser, but you don’t want to risk the EP falling out, particularly a 31T5.

[astro744]

See http://www.televue.com/engine/TV3b_page.asp?id=214

The 31mm Nagler effective field stop is 0.38" (9.65mm) above the reference line (base of eyepiece at top of barrel, see diagram on Tele Vue site). Refer column F. (F = -0.38 for 31mm Nagler).

Do you have any other Tele Vue eyepieces? If you have say the 17mm Nagler (F=-0.03) then you would have to rack in -0.03 - -0.38 = +0.35 or 8.89mm from where that eyepiece focusses. If you use a 1.25" eyepiece to compare with you also need to take into account the thickness of the 2"-1.25" adaptor you are using.

Find your focal plane by placing a sheet of paper over the 2" opening (no 2"-1.25 adaptor in place and no eyepiece) and focus on the Moon, (try tomorrow morning pre dawn, look at Mars/Jupiter too!). The 31mm Nagler needs to be racked in 0.38" from this position. If you don't have this distance available to you you need a lower profile focusser or you could try and loosen the collimation screws all the way and then recollimate; you may just have the 0.38" available.

[bigjoe]

Heres a good article on what can or may cause this.
www.celestron.com/blogs/knowledgebase/what-is-back-focus-what-problems-does-it-cause-for-visual-and-photographic-use-of-my-scope-what-are-related-focus-issues.

Some people actually resort to doing this , (though not recommended unless you have some skills)!
Viz: Moving the primary mirror up in its cell, the focal plane will stick out of the Eyepiece holder then.

This is a way to gain back focus . Additional spacers in the mirror cell, to bring it up closer to the focuser can do it too ! Though you would need to check your collimation again...and it could cause clipping as the secondary is no longer fully illuminated.

Or just get a lower profile focuser if backfocus is the issue here

bigjoe.

[bigjoe]

And of course if its not enough outfocus there are focusers and adaptors for that..as here for example.
www.telescopeadapters.com/varimax/318-varimax-variable-eyepiece-projection-adapter-1-125-barrel.html?search_query=Low+profil e+focusers&results=5
bigjoe.

[astro744]

I meant loosen not tighten and have corrected my post. The eyepiece has to move toward the primary mirror or the primary mirror has to move toward the eyepiece. If the latter note edge of field illumination will drop a few percent as the secondary is now undersized for the wider light cone due to the primary being closer. You wont really notice it if you only move the primary 10mm and loosen the collimation screws a fraction. You first need to determine exactly where the focal plane is so you know exactly how much to move.

A low profile focuser is your best option but you must be certain of your current and proposed low profile focuser racked in height to make sure it is low enough. I would loosen the primary mirror collimating screws first before spending any money as you may just have enough to reach focus.

[Granada]

Thanks these are all useful suggestions, I will try them.

[astro744]

Are you intending on using a Paracorr at any stage (would be nice with your 'scope and 31mm Nagler)? If so you will need extra in-travel.

I just looked at the Moon with my 6" f5.5 Newtonian and 31mm Nagler with and without a Paracorr, (last Type 1 version with tuneable top and white index marks before Type 2 introduced).

With the Paracorr I had to move focus inward an extra 6 - 7 mm approx.

Note there is a note on the Tele Vue web site stating that the 1.15x magnifying factor was introduced to push out focus but this should not be taken as to push out past the position with no Paracorr but rather push out from a reference position of a Paracorr with no magnifying factor. This is to better allow usage of the Paracorr with most telescopes without modification. The 1.15x factor also flattens the field a little.

Find where your focal plane is and then you'll know what you need to do.

[Granada]

Not at this stage, since I don't have one (and frankly, I don't know what it is).

[astro744]

See http://www.televue.com/engine/TV3b_page.asp?id=61 and click on each sub-menu on the left.

I have the previous version Type 1 version. There is a lot more info provided for this version especially in the visual use tab.

It is recommended for those that appreciate pin point stars to the edge of field since primary mirror coma is significantly reduced. As you already have the 31mm Nagler it is the perfect complement. However first determine exactly where your focal plane is before you can determine if you will have enough in-travel with the 31mm Nagler both with and without a Paracorr.