White light mod

[eric1138]

From what I've seen here, virtually all the mods are for h-alpha. So I thought I would stir things up a bit and discuss a couple of white light mods.

1) Let's take a Herschel wedge, eg, the Lunt solar wedge. You go through the objective, the wedge, the ND 3.0 filter, and then the eyepiece. Let's do it a different way - put the ND 3.0 in front of the objective, and then carry on through the wedge, and then the eyepiece. Either way, the light reduction is the same. And in the latter case, you could do this on a Mak or SCT without frying the secondary. Of course the ND 3.0 that comes with the solar wedge is 2", so you end up with a 2" objective, but the original Questar solar filter was 40mm IIRC.

2) Forget the wedge. Just get a pair of ND 3.0 filters and stack them in front of the objective. Now you have the equivalent of ND 6.0 (and still 50mm objective) and you don't need anything special to hold the wedge at the back of the SCT or Mak.

Can anyone see any safety issues with any of this ?

Cheers,

Eric.

[marktownley]

The issue I see is finding a sufficiently large ND filter(s) that are optically flat enough to go over the objective. Best bet in this situation IMHO is the baader film.

[peter drew]

+1 for Mark's comment. If you're going to compromise then the Baader film is the best option, it still retains the full aperture resolution.

[eric1138]

Hi Mark,

I have some Baader film that I use in my backyard obs. I take my ETX all over the place and I really like the idea of having a thread-on filter that is (near) bullet proof. In my observatory, I have control over the situation - it's a small 10x10 room, I can see who is in there. And the far end of the telescope is up high (away from kids). When I use the ETX, there's usually many people around and lots of kids, so I want something that can't be punctured, and can't be removed easily.

Baader has 50mm ND 3.0 glass filters for ~$60 each. And I just noticed that IDAS has a 52mm and 82mm glass filters. Kenko has a glass ND filter but I'm not sure that one is safe. The IDAS filter appears to be coloured glass so pinholes would not be an issue as with other glass filters.

E.

[Merlin66]

Eric,
I understand your concern.....but in ten years, I've never seen the Baader Solar film punctured by any "normal" or sometimes even "abnormal" handling.
Properly mounted in a suitable cell and the cell secured by a couple of wraps of gaffa tape; it will never move!
Just my 0.02 euro

[peter drew]

I have a screw on glass white light filter that came with my seconhand ETX 90 so they are available. The reason Queestar used only 1" aperture was because that was the largest off axis unobstructed diameter that would fit between the primary diameter and that of the secondary.

[eric1138]

I was more concerned with optical safety - stacking filters, reversing the order of the filters, etc. With all the PST mods going on around here, I figured this is the best place to go.

I haven't had anyone poke a finger through the film in ~15 years. But I have had crinkles, pinholes in the Baader films that I have used after maybe 2 years. Maybe I need to be more careful with the film. I know the stuff the lest expensive thing out there so it doesn't cost much to replace.

I think I've spent ~75 on Baader film over the past 15 years. $120 for a pair of 50mm ND 3.0 filters to use with an ETX that goes everywhere might be worth it - never have to worry about pinholes, wrinkles, fingers, pointed sticks.

Has anyone looked at the transmission of the ND 3.0 filters or the Herschel wedge ?

[marktownley]

If you rear mount ND filters on a ETX90 2 things will happen; firstly the secondary spot is glued onto the corrector lens and the full intensity of the focused sunlight will likely melt the glue on the spot and could crack the corrrector plate. Secondly ND filters are absorption filters, they WILL absorb the heat and crack. What you propose is not a safe solution.

[eric1138]

Mark, I intend to put the ND filters in front of the objective on the ETX - just like any other glass filter so there should be no damage to the telescope internals.

E.

[peter drew]

Sorry to labour this one but I would not expect ND filters placed before an objective to offer the same degree of optical quality as the Baader solar film.

[marktownley]

Sorry to labour this one but I would not expect ND filters placed before an objective to offer the same degree of optical quality as the Baader solar film.

Completely agree Peter...

may be of interest http://solarchat.solarastronomy.org/ind ... sale#29867

[papamidnight]

Hi-new here and new to astronomy. I am using a white 3.5 mil Nd on the front of my 90mm MAK and have had good luck. http://youtu.be/OZp7dJU-QMM
Any suggestions on a good eyepiece that wont cause flaring as seen in this video I took today?

[solarchat]

flaring? That looks more like an out of focus dust bunny. maybe Im missing it. Im in the driveway taking images and can barely see the screen.

[DavidG]

The saftest and one of best optically is a white-light solar newtonian. Here is a link to the one I built http://www.considine.net/dgroski/wlnewt/
There are no filters to come loose. If by any chance an optical element would fail no light can get to the observer's eye and the optical surfaces are of the highest quality. Since I'm using cross polarization I can also easily tune the brightness to match the observing conditions. You can also make it inexpensively.

- Dave

[poita]

That is brilliant.
How does one go about making the wedge though?

[Merlin66]

Peter,
There's a thread on my DIY Herschel Wedge - used a 30degree 60mm x 60mm prism ex-Surplus Shed. 2" diagonal, remove the mirror and mount the prism on the rear. Worked 100%.

[poita]

I'll find the other thread. Thanks.

I have a lunt Wedge, but would like to build one of these white-light scopes to take to the schools, it sounds safer with no filters for kids to stick their fingers into.
I think I'd prefer to build a wedge that could stay with the scope.
Now to find some f10 mirrors.

(By the way, what is the correct syntax for 'f' F10, F/10, F:10, f10....?)

[Merlin66]

Usually small f, then the number.

[DavidG]

I'll find the other thread. Thanks.

I have a lunt Wedge, but would like to build one of these white-light scopes to take to the schools, it sounds safer with no filters for kids to stick their fingers into.
I think I'd prefer to build a wedge that could stay with the scope.
Now to find some f10 mirrors.

(By the way, what is the correct syntax for 'f' F10, F/10, F:10, f10....?)

You can get the a 4" f/10 mirror here http://www.cloudynights.com/classifieds ... =26&page=2
There is another way to make the scope. Use a 1" uncoated penta prism for the diagonal. The penta will give you two reflections but you still need a ND filter in the eyepiece or a polarizer

- Dave

[sullij1]

Hi-new here and new to astronomy. I am using a white 3.5 mil Nd on the front of my 90mm MAK and have had good luck. http://youtu.be/OZp7dJU-QMM
Any suggestions on a good eyepiece that wont cause flaring as seen in this video I took today?

Second dust bunny. Time for a flat or cam/EP cleaning. B)

Had another thought, I take it this is an EP projection image? If so, did you use the same EP to center with before attaching the cam? If so, could be eye crud on the EP. :sick: If not, is the mak design such that it is one of the maks with the silvery center obstruction that may be reflecting back against the solar film causing the corruption?