Dob 300P focus with my 12mm Nagler

[kirkster501]

I can reach focus with my Pan 35 and Nag 17mm fine. But with the 12mm I have to pull the EP further out of the focuser or use an extension.

Is this something to do with needing more back focus I have read about? What's the standard focus please and if I got say a 70mm one would that create other problems? I am thinking of buying a nicer and smoother focuser anyway than teh stock one. Suggestions welcome as to which one you guys have. This is 100% a visual instrument. I do not want to mess about with moving the main mirror.

Thanks, Steve

[Naemeth]

Found this, may be of some interest.

"The back focal length of the eyepiece is fixed for any eyepiece and is the minimum eyerelief possible with it unless it is modified with a negative barlow lens. This effect is what makes the Nagler type eyepiece possible which can have a backfocus of 2 or 3 times the efl of the eyepiece." (http://www.ayton.id.au/gary/Science/Astronomy/Ast_eyepieces.htm)

So you are probably looking at 36mm of backfocus that will be required to achieve focus with the 12mm Nagler. If you are going for a new focuser, because of this issue, a low-profile focuser may not be a good idea unless you want to use extension tubes often.

HTH

[John]

Looking at the specs on the TV website it looks like the T4 Naglers are not parfocal with other TV eyepieces.

You might find a short extension tube works - something that allows the 12mm to get to focus as well as leaving enough inwards travel for the other EP's.

[scarp15]

I would heartily recommend upgrading the focuser to a Moonlite CR1 or CR2. I went for a CR1 and I am aware of other forum members who have installed a CR2 (duel speed). The 50mm draw tube should work very well at accepting high to low power eyepieces. However I also have the Moonlite 1.5" extender and I have no problem in focusing, ranging from 35mm X43 up to 7mm X214. The extender is a nice feature as it also accepts 2" filters - if you wish to invest in a UHC or OIII filter, meaning of course that you can use a varied range of eps. You will also need to purchase the Newtonian installation kit which makes installing quite straight forward. Alternative focusers will almost certainly require drilling.

Speaking for the CR1, whilst only being single speed is very smooth at finely gaining focus and supports heavy eps such as the 35mm Pan very well, being a bit lighter helps with balance too.

[swamp thing]

I always had to use an extension tube when using my 12mm T4 in my old lightbridge as that didn't have enough focuser travel. After a few uses it became second nature to insert the tube first.

[kirkster501]

Cool - thanks guys for the advice. I may get a Moonlite CR1 or CR2 - they look the business! If I were to get the 70mm version this means that the focuser "comes out" more to accommodate EP's like the 12mm? Presumably, with it being 70mm, it does "go all the way in" back not the tube as it were to allow focus on the other EPs as well?

I did a few - cheap - mods and corrections on the scope and she's a beaut now..... Next up is that focuser that lets the scope down IMO.

[scarp15]

I would go with the 50mm option, which is recommended for visual - just to be sure, at the time I purchased mine, I checked this with Moonlite. If you have any particular questions, then it would be worth contacting Moonlite, they are very good at responding to enquiries.

The spacers provided with the installation kit will enable you to configure various heights, which will assist with gaining optimum focuser travel. I do not have a12mm EP, but I do use a 13mm nagler and gain sharp focus without issue.

[kirkster501]

The spacers provided with the installation kit will enable you to configure various heights, which will assist with gaining optimum focuser travel. I do not have a12mm EP, but I do use a 13mm nagler and gain sharp focus without issue.

Installation kit? That comes with the focuser I guess?

[scarp15]

Separate item unfortunately, however I quess the advantage being that you can then order one compatible with the size of your scope. One or two UK retailers, including FLO, supply moonlite products.

[kirkster501]

Is there a way to attach a power focuser to the Moonlite? I will get googling....!