Dobsonian Dust Cap

[Bowdie]

Just wondering.... probably a silly question but here goes

What is the purpose of the small removable cap on the main dust cap for the telescope?

[LukeSkywatcher]

Hi Bowdie

The small inner cap is basically to stop down the aperture of the scope while observing the Sun (only observe the Sun with the proper solar filter).

[Bowdie]

Cheers for the quick response!

need to get a filter now then.... star gazing in daylight!

[LukeSkywatcher]

You can pick the solar film (filter) up here:

First Light Optics - Baader AstroSolar Safety Film ND 5.0

It gives instructions on how to make your filter.

For you if you plan to use the inner cap of your scope then you just need to find a way to secure the film inside the cap so that the aperture of the smaller one is covered. Cover finderscope lens also (if you have one)

[Mr Moff]

Excellent,I was just about to ask the same question myself.

Thanks Luke.

Danny.

[Tiddles]

You can also use it when the moon is too bright, just remove the little cap to view, you do effectively reduce aperture too so resolution will not be as good but hardly noticeable for bright lunar viewing.

[jkemal]

I'm going to try this as well, Martin from FLO also recommended the best thing to do to satisfy yourself that the dust cap will prevent any further sunlight in is to try looking at a bright light indoors first to see if its completely black with the cap fitted in place. . You can use just a couple of bits of tape to secure the cap so a gust of wind cant blow it off.

[UnderTheRain]

If you're gonna look at the sun make sure your filter is secure. Once it is secure, secure it a second time. If if comes off you will irrevicably damage your eyes looking at the sun through a scope and you will do it pretty quickly and theyre not fixable. Be VERY careful looking at the sun. My advice is to use a web cam or camera and even with a filter don't look through a scope at the sun.

[wackyscot]

Just another possibly silly addition to the above, but the other "fixed" cap houses the one which you remove

David

[jkemal]

I wonder if there is anybody else who has used their scope in this way, some people seem to think this is safe others not so much?

[garethmob]

its very safe to do but alot of members do value their eyesight more than a quick view

iv had a coronado PST and the views through that was really really good but i was a bit iffy looking at the sun, even though the scope was a proper solar telescope only designed for that purpose.

a hat box could be useful for makin a solar filter out of

on the 30 year old coulter i have it came with a home made apature mask hole and a type of solar film of unknown age (i wouldnt use it) but it is professionally made and wont come off in a gust. so you could easly make one of your own out of a hat box, plastic ect.

[jkemal]

I like the idea of using the smaller apature on my dust cap, I just want to be sure that it won't let any more light in other than the light let in by the solar film. I am thinking about covering the edge with black tape.

[swamp thing]

I like the idea of using the smaller apature on my dust cap, I just want to be sure that it won't let any more light in other than the light let in by the solar film. I am thinking about covering the edge with black tape.

You can never be too careful when observing the sun. This isn't a bad idea.

Regards Steve

[R1gel]

You can never be too careful when observing the sun. This isn't a bad idea.

Regards Steve

A wise man told me to make sure that no extraneous sunlight could enter the scope especially when using a truss tube design. Tape up all holes and gaps to ensure no unfiltered light enters. It don't take long to fry the eye balls.

[jkemal]

A wise man told me to make sure that no extraneous sunlight could enter the scope especially when using a truss tube design. Tape up all holes and gaps to ensure no unfiltered light enters. It don't take long to fry the eye balls.

Good advice

[knobby]

Safest way, I think is to project sun image onto a card

[Chris Hopson]

Hi,

This is something i've been wondering....But will this work with a skywatcher flextube, and if so does the dust cap need to be moved? ie, to the front aperture?

Chris

[R1gel]

Hi,

This is something i've been wondering....But will this work with a skywatcher flextube, and if so does the dust cap need to be moved? ie, to the front aperture?

Chris

Yes and yes but please do take note of previous advice re safty in this thread.

[Chris Hopson]

Many thanks R1gel

[jkemal]

Does anyone have any photos of how they have attached the protective solar film to the smaller apeture on the dust cap of the Skywatcher Skyliner 200P Dob?

[garethmob]

heres mine its probally a little different to others as its thousand oaks glass

its very strongly built and fairly heavy too and fits very tight on the top of the scope

the other way i would make it is actually have the film under neath the whole dust cap, over the whole width of it secured with various tape and then physically tape the whole unit to the top of the scope. it might make a mess on the scope, but id rather a mess than if it blows off and i lose my sight

[Matt Scunthorpe]

Im using the small dust cap hole with the solar filter also, but i cant seem to find the sun (sounds silly i know ). It didnt help that the finder scope wasnt attached so i was lining it up by eye.

Ive got the SW 200P scope, and was using a 25mm eyepiece (lowest mag i have). Will this be too powerful to the whole disk of the sun, or will i only see fragments?

[LukeSkywatcher]

25mm EP should show the whole of the disk of the Sun. Its the same size as the full Moon. Once you have the Sun in your sights then you can increase the magnification (using a smaller EP). To focus the image, its a good idea to focus on the edge (limb) of the Sun and the sky behind. Once that is sharp...........just move the scope across onto the surface and sunspots will be in focus.

[Matt Scunthorpe]

Even the full disk of the sun only using the small hole in the cap? i thought this would reduce how much you can see?

[NGC 1502]

Even the full disk of the sun only using the small hole in the cap? i thought this would reduce how much you can see?

The field of view will not be reduced by using only a small part of the telescopes aperture. A rough guide to the amount of sky seen can be found by dividing the eyepiece apparent field by the magnification.

Example : 1000mm focal length scope, using 25mm eyepiece,

magnification is 1000/25 = 40x.

If the eyepiece has a 50 degree apparent field (most Plossls) the actual sky seen is 50/40 = 1.25 degrees.

The aperture of the scope is not used in the above.

HTH, Ed.