Solar filters

JJDOBBER79 (Jas) · #1 08-12-2013, 11:48 AM

Hi, I've been looking at solar filters. It appears that they go over the entire end of the tube. Why is this necessary? Surely something could screw onto the bottom of an ep in the same way other filters do. This would have the same effect right? You still have something between the sun and your eyes. I'm just curious. There must be a reason why this wont work or they would be available

Larryp (Laurie) · #2 08-12-2013, 12:36 PM

By the time unfiltered sunlight reaches your eyepiece, the light and heat are quite concentrated-and dangerous! Filtering the light before it enters your scope stops any dangerous heat build-up.

Kunama · #3 08-12-2013, 01:21 PM

As Laurie said, the heat build up inside an unfiltered scope is immense, a filter placed near the focus point is subjected to temps as high as 600 Celcius in a matter of seconds (tested in a 6" F10 scope) , a glass filter will crack, a plastic one will melt and then your eye will suffer Photochemical retinopathy in an instant.

For refractors there is the option of a Herschel Wedge, for all others an objective filter of optical density 5.0 is required.

sheeny (Al) · #4 08-12-2013, 02:55 PM

Yep. The scope focuses the light (i.e. intensifies it) so it can do damage inside the scope before it gets to filter, or the filter or EP can be subject to extreme temperatures and fail.

If you have an old EP that you wouldn't mind destroying, you can use it in a Newtonian scope for solar projection into a screen or card, but never look though a scope without a on objective solar filter!

There are also Hershel wedges which dump 95% of the light energy out the back and only allow 5% through. These perform very well for solar imaging, but I don't think they are considered safe these days for visual work (I'll stand corrected by someone more up to speed with a Hershel).

Having seen the damage done to an uncovered finder scope from the sun as it passed over (the finderscope looked like an oxy torch had cut across the back and up the side of the finderscope and the cross hairs were of course gone!), I wouldn't recommend solar work without an objective filter without extreme care, and especially never for visual work.

Al.

OzStarGazer · #5 08-12-2013, 02:57 PM

I am a newbie of course, so my knowledge is limited, but I am sure what has been said is true.... When I unpacked my telescope a couple of days ago there were several warnings not to look at the sun without a proper filter because of the irreparable damage it can cause... First attached to the telescope itself (just in case somebody doesn't read the manual), then in the manual, then there was yet another separate warning...

Kunama · #6 08-12-2013, 03:19 PM

Just a note about the Herschel Wedge, in particular the Baader Cool Ceramic 2" Solar Prism.

The Herschel wedge can only be used with certain scopes, essentially a refractor .
They cannot be used with Maks, CATs, Newts, DKs etc.

From my research I would think that in the correct scope they are the safest way to view the Sun in whitelight as they are securely fixed in the optics train and cannot be dislodged unlike objective filters.

The prism reflects less than 5% to the eyepiece but that 4.6% passes through a factory installed Solar Continuum filter and OD 3.0 ND filter before reaching the eyepiece.

What you get at the eyepiece is the same intensity as the visual films,
the light reaching the eyepiece is also partly polarised and can be further dimmed by adding a 'single-polariser' to the eyepiece thread. Then rotating the eyepiece allows you to vary the brightness.

JJDOBBER79 (Jas) · #7 08-12-2013, 04:21 PM

Very informative as usual. Thanks guys.

Merlin66 (Ken) · #8 08-12-2013, 04:34 PM

Matt, et al,
I bought a 102/1100 achromat to use with my Baader Herschel Wedge, based on similar assumptions.
I was subsequently corrected and shown that the Herschel wedge is safe to use with multi-element refractors.
My concern was with the ol' Genesis which has a Petzval lens group close to the drawtube - this is being successfully used for white light/ wedge solar viewing in Europe!
Now the TS102 is being used for a Stage 2 PST mod!!!!

Kunama · #9 08-12-2013, 04:39 PM

Quote:

Originally Posted by sheeny

Yep. The scope focuses the light (i.e. intensifies it) so it can do damage inside the scope before it gets to filter, or the filter or EP can be subject to extreme temperatures and fail.

If you have an old EP that you wouldn't mind destroying, you can use it in a Newtonian scope for solar projection into a screen or card, but never look though a scope without a on objective solar filter!

There are also Hershel wedges which dump 95% of the light energy out the back and only allow 5% through. These perform very well for solar imaging, but I don't think they are considered safe these days for visual work (I'll stand corrected by someone more up to speed with a Hershel).

Having seen the damage done to an uncovered finder scope from the sun as it passed over (the finderscope looked like an oxy torch had cut across the back and up the side of the finderscope and the cross hairs were of course gone!), I wouldn't recommend solar work without an objective filter without extreme care, and especially never for visual work.

Al.

Good points Al. You can't be too careful with our nearest star.

I use the Baader wedge, SC+ OD3.0ND and LVW 13, 22, & 42 eps.
I initially used the wedge without the SC filter, but have reinstalled ( it gives a lime green image) & have found even in my apo the SC reveals more detail than the white image only. It also has the benefit of reducing the 'seeing' induced degradation as it passes only the green light around 540 nm.

I find its easier to remove the finderscope from the OTA altogether, no nasty surprises. My mate wraps alfoil over his finder (not a quickrelease base) and puts an elastic band around it.

I tried the LVW42mm on the Sun for the first time today ( Is it wrong to be in love with an eyepiece

)

Hi Ken, Good to know that, the Baader Manual cautions about the rear element designs but I guess the actual point of focused light is quite far from the last element.

Merlin66 (Ken) · #10 08-12-2013, 04:50 PM

Al, et al,
I can assure you, a wedge used with an ND3 filter (mandatory!) is 110% safe to use for visual and imaging. It is used on refractors up to 200mm aperture..

The addition Continuum filter enhances the contrast and gives better visual definition..

Kunama · #11 08-12-2013, 04:53 PM

Quote:

Originally Posted by Merlin66

Al, et al,
I can assure you, a wedge used with an ND3 filter (mandatory!) is 110% safe to use for visual and imaging. It is used on refractors up to 200mm aperture..

The addition Continuum filter enhances the contrast and gives better visual definition..

+1 Ken,
I find the lime green easier to view longer than the white image.

MattT · #12 11-12-2013, 07:05 PM

I use a Baader solar film on my frac. Cheap and works. Add WO binos and the view is amazing! I also put an orange filter on one eyepiece and a yellow on the other. The brain then scrambles the two together to give a colour like the orange pics of the sun. It's easy to see some details on the surface too. Better than mono.
Matt