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Baader 35nm as ERF

Posted: Fri Jan 11, 2019 10:07 am
by AndiesHandyHandies

I have come up with a way of using this as an internal ERF with no reflected hot spot.

I have been working on my Vixen 110mm VMC110L PST conversion.

For that I had to move the negative collimator lens forward in order to luckily just reach focus.

I was having problems with a front Lunt ERF on red glass causing a red field due to dispersion in the glass illuminating the inside of the telescope.
Easy to see when I unscrewed the front part and pointed it at the Sun. A commonly reported problem. So I removed the Lunt.

So I put a 1.25" Baader 35nm H-alpha filter after the negative collimator lens. Placed here it is reflecting a parallel beam which is then going back through the negative lens and out the telescope as it came in as an expanding safe beam.

It seems to work OK. A nice black field, a slight off axis reflection from the Baader or 'lose' Beloptic KG3 now visible.

Astrograph uses a 2" Baader up to 150mm as an ERF and I calculated that the 1.25" is safe up to 110mm as well.

The F9.4 scope is stopped down to F10 by the PST negative collimating lens.

So I am going to try a 2" in my Meade 127mm F7.5.
But do I need a 1.33x Barlow to make it F10 first with the 2" Baader right after the barlow lens?
SKyBurner says using a Barlow to reach F10 works.

Mark T said to use a 2" Barlow with a negative lens in to reach focus in a refractor without sawing a bit off the back.
Stopping the lens down to 20mm. But that stops the scope down.

The Swiss chap made me a T2 to PST adapter so he has the sizes now if you want one made as well.


Re: Baader 35nm as ERF

Posted: Sat Jan 12, 2019 6:26 am
by marktownley
Hi Andrew,

You could use a MagniMax barlow from astroboot ... eedle=1.25 right infront of the negative collimating beam which would turn the f7.5 light cone towards the target f10.


Re: Baader 35nm as ERF

Posted: Sat Jan 12, 2019 9:13 am
by AndiesHandyHandies
Hi Mark,

Thanks for the link.

With a 1.25" Barlow all the energy would be on a 1.25" filter behind a collimator.

What I was thinking about the ERF being behind a collimator will not work here. And not needed as reflected focus inside the telescope.

127mm F7.5

Need a 1.33x barlow for F10.

The PST Collimator lens will be 200mm from the new focus which from graphing is 150mm inside old focus.

A 2" filter has a 45mm clear aperture so it needs to be 338mm inside the original focus to spread heat over all of the filters.
The reflected heat is expanding 70mm diameter at the objective.

A 1.25" Barlow with 24mm aperture has to be within 200mm of original focus to not vignette, 50mm back from PST Collimator lens.
A 2" Barlow can go from <318?mm to 180mm inside the original focus, allow 10mm for 2" thread. A Barlow threaded each end 2" will fit in a stack of 2" filters and extenders with 48 to 42 and 42mm to PST adaptor on the back.

Possibly need to still use the PST collimator lens seperate infront of the PST body to reach focus? Without hacksaw.