Recommendation on ERF filter required

[Philip Benson]

I have a 2" Lunt white light wedge that I want to use with a newly purchased 6" refractor.   

The wedge of course already has the built in ND3 filter and I supplement that with a double polarizer to cut down the light level to an acceptable level. I also need to use an ERF filter to screw into the nosepiece of the wedge on the telescope side.   Any solar imagers/observers with recommendations on which one I should get?

Thanks in advance.

[Merlin66]

Philip,

It's usually not necessary to fit an ERF before the wedge.

The wedge itself can handle the input energy. The issue is the output to the camera/ eyepiece after the wedge.

The ND3 etc. handles this well.

Why do you think you need an ERF???

[Moonshane]

I agree. I have read reports of one of the lunt wedge designers using a 1.25" wedge on a 10" apo with no issues. I use a 1.25" on a 120mm with no issues.

[Philip Benson]

My question really just stems from an earlier post I placed on the solarastronomy.org forum where I enquired about the safety of using a white light wedge with a medium aperture refractor.   The Lunt website does indeed mention that the 2" version can be used with refractors up to 6" with the ND3 filter in place (plus a polarizer if needed) so I heed your comments and give it a try.

[Moonshane]

The only possible question mark with herschel wedges is heat. The units are designed to take and dissipate heat effectively. With my 1.25" there is an incremental increase in the warmth of the disc but it remains warm not hot. It cools very quickly too. Even with my 120mm it gets only warm not hot.

I observe visually only though on a non tracking mount.

[Philip Benson]

Before I upgraded I used a TMB130 apo with the wedge and it just seemed a bit unnatural to be aiming such an expensive scope towards the Sun unfiltered at the front.    The word for word reply I got from my post on the solarchat forum was...

"Your 6" works fine  with the WL wedge, but it is of advantage to use a UV/IR block filter 2" up front the wedge.    E.g. Beloptic"

[spaceboy]

From what I gather pointing your expensive apo at the sun is not the worst thing you could do. Fungus loves exotic coatings and is partial to a bit of the cheaper stuff also. With the constant condensation and dew issues brought upon from using optics in a cold damp conditions then warm environment it is a breeding place for fungus to start attacking the coatings which can often be mistaken at first for a small dust spot on your lens and before you know it the fungus is a squatter you can't get rid of. Regular exposure to UV is said to reduce the likely hood of fungus forming on your lens.

There is obviously a risk to your scope as anything sat in direct sunlight for extended period of time either suffer fading/ yellowing or different parts can become weaken over time.

As for using a 1.25" wedge in a larger than recommended scope, I have tried using the 1.25" in a 5" scope and while there was no obvious problems other than the back of the wedge was notably warmer (ambient temperature was only 9°c) than in my 4" scope. The views were rather breath taking to say the least but I think I would always be cautious using this configuration in the dead of summer as the heat sink may get to hot for comfort? As you will see in this vid https://www.youtube.com/watch?v=Kft2yxQ85sY the heat sink is held in place by I guess some sort of silicone ?? I'm pretty sure this is going to be heat resistant to some degree but none the less a possible weak link in the chain when using above the recommended apertures?? I have a Baader UV/IR filter that Daystar recommend as a ERF when using their Quark so I assume this would also work with the HW just as well ???