White light solar observation, questions.

[pluton]

Hello,
I have some important questions for me in relation to white light solar observing..:
- Using a Hershell prism to observe the sun in white light, is it better to use it in a refractor with ED or APO lens or is it the same if an achromatic refractor is used? Better with long focal length F8 or F10 or with an F6 also worth it? For example, in visual is a 4 "F / 9 or a 5" F6 better?
- From what aperture can we begin to have a detailed image with good resolution of details?
- And one last question, if you allow me, for visual observation to part of the filter ND3 and Top Pol, it is necessary to use a uv / IR filter in the eyepueces?
Regarding the Baader Continuum filter, does it really improve the contrast in visual, if so, from what aperture?
thank you very much in advance for your advice
regards
Paul

[vlaiv]

Since we are talking about white light solar - all wavelengths of light will be present (unless using Baader Continuum or similar) and achromatic refractor will have lower contrast due to CA - same as with Moon or planets.

You want your scope to be long focal length scope, although I would not mind a good APO at F/6 in this role. Just take care you don't use any scopes with rear glass elements - like flat field scopes or some "advanced achromats". Remember, if trying to reach high magnifications - long focal length scopes do it easier - no need for barlows and such, and are generally easier on the eyepieces. You should choose scope you would otherwise use for planets (best avoid short tube achromats). So I would choose 4" F/9 over 5" F/6, unless planing to use Baader Continuum or similar filter most of the time, or 5" is very good APO .

If you plan to observe using Baader Continuum filter, you can use achromatic refractor and still have sharp view - this is because this filter is almost narrow band filter - so it will remove any unfocused light and refractor is capable of focusing on single wavelength (actually two wavelengths at the same time). I have used Baader Continuum filter with F/5 102mm fast achromat, and still had very decent white light views (although not at very high mag). Yes filter does enhance view quite a bit. By the way, this filter is also very good for such achromat on the moon - it will sharpen up view considerably and even help with seeing effects.

As for resolution, I guess you will be seeing detail (sun spots) with any scope, but to really go zooming in to see granulation (from x100 upwards) you will need at least 3-4" scope.

IR/UV cut filter is not necessary - Herschel wedge with ND filter (should be installed inside, but please make sure it is!) and single polarizing filter (wedge does polarization itself, so one polarizing filter is enough to tune amount of light) - is all you need.

Some people used IR/UV cut in front of the wedge to cut down amount of heat dissipated on the wedge - but you should only consider that on large scopes and use only interference filters for that (they reflect unwanted light back thru objective lens).

[Merlin66]

Vlaiv,

Your comment re rear glass elements""""Just take care you don't use any scopes with rear glass elements - like flat field scopes or some "advanced achromats" """" is not correct.

All the manufactures of such Petzval designs have gone into print confirming their products are 100% safe to use with Herschel wedges.

 

[vlaiv]

Just now, Merlin66 said:

Vlaiv,

Your comment re rear glass elements""""Just take care you don't use any scopes with rear glass elements - like flat field scopes or some "advanced achromats" """" is not correct.

All the manufactures of such Petzval designs have gone into print confirming their products are 100% safe to use with Herschel wedges.

 

Good to know! I have not myself used such a scope, neither on its own nor with Herschel wedge, but I have come across such a warning online before, and as with any warning related to Solar observing, I took it pretty seriously. My understanding was that due to relative proximity of rear glass to focal point it might fracture due to heat induced stress (although it is far fetched because of such distance from focal point - one has Herschel wedge that is made out of glass further out - and its doing fine ).

 

[Merlin66]

Yeah, an "urban Myth" which doesn't seem to ever go away.......

[RT65CB-SWL]

Which Herschel/solar wedge are you thinking of purchasing?

What I am about to say below is based upon the Lunt 1.25"...

1. The Lunt has the ND3.0 pre-installed and is non-removable. Other brands of Herschel/solar wedge, the ND3.0 is not fitted, (i.e. screw into nosepiece of eyepiece or other accessory), and will need to be purchased before use.
2. The Sun's disc in "white light" when no other filter installed it is white.
3. The Sun's disc is green when you add the Baader Solar Contiuum filter. 
4. Adding a Polarising filter reduces the brightness, (same as a variable Polarising filter for the Moon).
5. No need for a IR/UV filter.
6. DO NOT use with a catadioptric (ie Mak or SCT) or any other type of reflector OTA.

 

The photo above showing my TeleVue Ranger (480mm/70mm @ f6.8) with the Lunt 1.25" Herschel wedge and TeleVue 15mm Plossl. It was taken last week (30.08.2018) and no sunspots were visible.

Another option to viewing the Sun in "white light" is to purchase and or a DIY full aperture solar filter and are OK for use with a catadioptric or other reflector OTA. They also need to be checked before every use against wear & tear, (i.e. pin-holes, scratches etc). I do have a Thousand Oaks Type 2+ glass Solar filter and have noticed a few pin-holes, so no longer being used. Below is the types of material that are currently available...

1. glass with metallic coatings.
2. silver-black polymer, (ready made or DIY).
3. Solar Safety Film, (ready made or DIY).