Hyperion + LP filter fit with TAL barlow

[Eastridge]

Hi,

Just got a TAL barlow & as far as poorish seeing last night would allow seems to work fine with my 10mm Hyp BUT with the Baader Neo filter on the bottom it doesn't seem to want to focus as seems to 'hit' something in the TAL.

I now have a vague recollection of something like this being mentioned earlier this year but search can't find it.

Can anyone point me in the right direction as to what to do?

[Stephen]

Does the TAL have filter threads? You could put the filter on the Barlow rather than the EP.

[Eastridge]

Thanks Stephen, No it doesn't - good thought though.

Maybe I imagined the previous discussions as lack of response indicates.

[AndyH]

I've not had a Tal barlow in years, but you mention the combo 'hitting something'. Does the barlow have a depth stop inside, down the tube?

Andy.

[Eastridge]

Yep thanks, not sure what a 'depth stop' is exactly but I think it does, about halfway down the tube there is an internal ridge that the LP filter seems to hit & that stops it getting focus.

TBH I don't want a barlow I can't use the Neo filter with, so may have to sell it on and try and find a barlow that will work with the Hyp. & filter.

Thanks

[AndyH]

Right, so the combo doesn't fully sit in the barlow.

Can't you simply rack the focuser further in by the same amount that it 'sticks out' of the barlow? Or have you used up all available infocus on the focuser?

What length is the Baader's eyepiece barrel? I ask because I wonder if it's longer than standard ep's.

Cheers,

Andy.

[John]

I think you may find it difficult to find a barlow that you can fully inset an eyepiece with a filter fitted into - the optics in a barlow lens can be quite thick and come up the inside of the barlow tube futher than you think, even more so for a 3 element barlow. This limits the depth of the top part where you plase the eyepiece. You will need to find a barlow that itself is threaded for filters and put the filter on the barlow, as Stephen suggests. For what it's worth neither the Revelation 2.5x or the Celeston Ultima 2x (older versions anyway) will allow filters to be fitted, so you can discount those.

Most barlows require some additional inward focus travel to enable a given eyepiece to reach focus. If the eyepiece is not fully inserted then that eats up more inward focus travel - many scopes just don't have that much available !.

[Eastridge]

Thanks John.

OK so sounds like I take it playing around with extentsion tubes isn't going to sort this for me?

[AndyH]

Most barlows require some additional inward focus travel to enable a given eyepiece to reach focus. If the eyepiece is not fully inserted then that eats up more inward focus travel - many scopes just don't have that much available !.

Good call !

I'd still be interested in the depth in mm's to the machined 'stop'.

I've noticed the majority of my eyepieces, certainly the simple lens designed ones, have a barrel of around 30mm. Tal eyepieces are 23mm. Some eyepieces have even longer ones(possibly to hold extra glass elements at the required spacing?). Add the 10mm or so of the filter and I can see where a problem can occur.

Cheers,

Andy.

[Eastridge]

Andy, see what you are suggesting, will get the tape measure out when next at home & see what the depth is.

[AndyH]

Hi.

Most appreciated. It would be interesting to know for future reference, if you see what I mean.

It's something I wasn't aware of before, so this thread gets a wee gold star from me !!

Cheers,

Andy.

[Eastridge]

Andy,

If I stick our retractable tape measure down the inside of the barrel until the end of it hits the 'stop', the end of the barrel equates to 2.5 cm / 25mm. So, yes less than the 30mm.

The Hyps 1.25" projection is almost exactly 25mm so when I add the Neo filter which is another 5mm that makes 30mm in total.

When I next have a clear night (yes I know that is tonight for almost all of the UK but it is a cloudy 15' here right now and forecast to be cloudy for the next few days - grrr the south west's weather) I am going to try all eye peices & combinations of Hyp FTR's to check the impact on focus in each case.

[AndyH]