Solar Filters

[Ra (Rrahim)]

Hi All,

Looking for some advice regarding solar filters. I have a 8" Dob (Skywatcher) and am wanting to examine the sun for my high school science classes.

There are three main points I am interested in examining;

1. What kind of details should I be expecting to see?
2. The Scope I have has a collapsible truss is it safe for solar observing?
3. Well I forgot the 3rd point due to an interruption (even at 3am) but any other advice is welcome!

cheers

[[1ponders] (Paul)]

Ra

Check out this link. Making a white light solar filter

Good luck

[erick (Eric)]

Hi Rrahim, welcome

A white light solar filter will allow the observing of the sun's disc and sunspots. If Venus or Mercury were happening to pass in front of the sun (and such transits are well advertised in advance), such a filter would be excellent.

Nothing else can be seen. Perhaps some surface mottling, but no prominences at the sun's limb.

But sunspot activity is (finally!) slowly building up from the long solar activity minimum we have had. Maybe next year it will have plenty of spots to observe. What would be useful would be to observe over several days and see the Sun's rotation - perhaps students can draw what they see over a week and then try to estimate the rate of rotation of the Sun?

Making your own with solar film is the cheapest way.

Otherwise, buy a solar filter. Thousand Oaks' filters are good:- http://www.myastroshop.com.au/produc...ar-filters.asp

With your 8" reflector, you don't need a full aperture filter. Buy one with a small off-axis aperture - that will do.

Lots of safety issues, eg. ensure the front of your finderscope is covered. Ensure the solar filter cannot fall off (wind?) or be knocked off easily. To locate the sun, move the dob while watching its shadow on the ground behind - until it is casting the minimal shadow - then you should be close.

[vindictive666 (John)]

ill add this as well

make sure your finders scope is covered up and not tempted to look through

regards john

ps ive got 12 inch full aperture baader nice views

[astro744]

Remove or cover the finder objective.

Shroud the tube to avoid stray Sun rays when moving the 'scope toward the Sun. I would never observe the Sun with an open telescope especially with a third party and especially with a school group.

Ensure the filter cannot fall off. If using an off-axis filter ensure the remaining aperture is completely covered.

Never leave the telescope unattended.

Ensure you or the school have adequate third party insurance.

Explain the dangers of solar observing and what will happen if the filter comes off.

I find group viewing with a non-tracked telescope difficult particularly with school groups that want to put their hands on everything. May I suggest a small 80mm refractor on a mount that tracks in RA. Once aligned the image will stay in view.

A white light solar filter will show you sunspots (if there currently are any) but not much else. A special Ha solar telescope such as the PST will show you prominences and surface detail. The PST utilises a special narrow band hydrogen alpha filter. More expensive versions with even narrower filters show the surface detail even better but the flares are fainter.

A PST would be a good investment for a school science section.

[casstony]

I think with public or school solar observing you have extra duty of care to ensure nothing goes wrong. To me that means not using a home made filter and not using a truss scope. An 80mm refractor with a commercially made glass filter that is taped onto the scope would be my choice.(or a PST)

You also need to develop a procedure such as having every person look at the front of the scope to ensure the filter is in place before looking through the eyepiece.

I think you need to be over-cautious when introducing others to solar observing, for your own sake as well as theirs.

[Ra (Rrahim)]

Thanks for all the advice, I will be buying a solar filter, though making one does sound like fun, just a little time poor.

Duty of care is definitely a major concern, I like the idea for a procedure fits nicely with some of the outcomes of the curriculum too.

Anyone know about concerns for looking at the moon during the day? The brightness may be too high, but for the kids it would be a great experience.

[sheeny (Al)]

There's no issue with viewing the moon during the day - its no brighter than it is at night. The only thing you have to watch is that you keep the scope away from pointing at the sun when you move it, leave it and especially when looking through it.

Al.

[casstony]

If you're observing the moon during the day you could position a wall or a tree between the scope and the Sun so as to avoid anyone panning around and accidently having the Sun enter the view.

I don't mean my posts to sound like I'm directing Rrahim, I'm just a little paranoid about solar observing safety; I almost took my own eye out once in a forgetful moment.

By the way, welcome to IIS

[astro744]

Originally Posted by sheeny

There's no issue with viewing the moon during the day - its no brighter than it is at night. The only thing you have to watch is that you keep the scope away from pointing at the sun when you move it, leave it and especially when looking through it.

Al.

Ditto. Contrast is low though but best time is first quarter (mid-late afternoon class) or last quarter (early-mid morning class). The Moon will be 90deg from the Sun and the terminator will show plenty of craters in both sunlight and shadow.

Venus is possible too in the late afternoon even with the Sun up. It's about 45deg from the Sun so care should be excersised. Use Stellarium to find the exact time when it crosses North and note the altitude of Venus (Stellarium will tell you). Then point your telescope North (Geographic not magnetic) and point it up at the angle Stellarium says and use a low power eyepiece. (First focus on a distant tree to get close to infinity).

Also use a shroud around your truss to ensure no stray light reflects off the mirror no matter what object you are observing during the day. It's also a good idea to extend the front of the telescope with a cardboard 'dew shield' to also act as a light shield to prevent stray light reflecting off the secondary mirror.

[WilliamAPorter (William)]

Just to but in.
what does happen when an unfiltered scope is directed at the sun?
similar to a magnifing glass i presume but worse?

[Merlin66 (Ken)]

Yes! but much much worse. Anything with a clear aperture above 40-50mm is extremely dangerous.
The concentrated energy of the sun is definately enough to melt plastic fittings, heat up eyepieces so that the lenses can crack and generally do irrepairable damage to the scope.
There are no nerve endings in the eye, so if it is exposed to extreme heat (and IR) it doesn't register the damage until it's too late.

[erick (Eric)]

And, be aware that, if the scope is not pointed directly towards the Sun, the focussed light does not come out of the focusser, but is focussed elsewhere. I recently saw a photograph (Cloudy Nights?) where someone left their truss dob vaguely pointing towards the Sun for a few hours. There was a burnt line across everything at the top of the scope.

I'll see if I can find it.

Edit:- Not what I wanted, but I found this:- http://thehowzone.com/how/Sun_Photos I have a cap that I had over my focusser that looks like this. When your primary mirror has condensation over it in the morning, it is tempting to point the scope towards the Sun to encourage the condensation to rapidly lift. Don't do it!

[Ra (Rrahim)]

Again thanks all, I have ordered the filter from York and getting the shroud made. Should be a good experience for the kids especially with science week coming up.

I am also writing a safety procedure for the kids to perform prior to using the scope even though I will be supervising. No problem about being over cautious. my eyes will be glued to the scope while in use.

[Ra (Rrahim)]

Hi all,
Again thanks for all the advice, ended up being a great lesson for the kids. although had to wait for a sunspot to come around. The creation of a safety procedure was really great to.

Cheers,
Ra