Solar Observing: Aperture Question

[GeezerGazer]

I'll be using my Orion SkyQuest goto dob (12" - F5) for casual solar observing. I read somewhere a while back that too much aperture can be harmful to the optics. If this is indeed the case, and as I am making a filter to fit the 12", I can easily choke the aperture down to any diameter. Any recommendations on how much aperture would be safe and effective would be greatly appreciated!

[Stu]

Providing you are using decent and safe film such as Baader AstroSolar ND5.0, and the filter is firmly held onto the scope then there is no danger to optics from using the full 12”. The issues are mainly around seeing conditions in that larger apertures for solar are often affected badly by poor seeing. Somewhere around 100mm to 120mm is often the optimum giving good resolution and cutting through poor seeing. If you can fit a 100 or 120mm aperture between your secondary support vanes that would be ideal.

[GeezerGazer]

Thank you Stu.. My plan is to cut the desired aperture out of the heavy plastic 12" factory dust cap. The cap fits above the secondary (and vanes). I'll (very securely!) attach my solar film to the inside of the cap, covering the aperture. To protect the film I'll back it with plexiglass, glass, or clear acrylic. This will be behind the film to avoid direct sun exposure. I'll also paint the cap white or reflective silver for further heat reduction. Think this will work?

Thanks again!  

[GeezerGazer]

Stu, in re-reading your reply, I think I’m getting that the aperture should be cut off-center so that the vanes and the secondary are outside the aperture opening. Correct?

[Cosmic Geoff]

1 hour ago, GeezerGazer said:

To protect the film I'll back it with plexiglass, glass, or clear acrylic. This will be behind the film to avoid direct sun exposure. I'll also paint the cap white or reflective silver for further heat reduction. Think this will work?

Don't back the film with plexiglass, glass or clear acrylic.  This is not necessary, and unless the material is of optical quality, it will degrade the image.  It won't do any harm to paint the opaque part of the cap, but I doubt this is necessary.

[Stu]

2 hours ago, GeezerGazer said:

Stu, in re-reading your reply, I think I’m getting that the aperture should be cut off-center so that the vanes and the secondary are outside the aperture opening. Correct?

Yes, correct. Position the hole so it is inbetween the vanes and avoids the secondary, that will reduce any diffraction effects.

As Geoff says, don’t put anything over the film such as a clear cover as this will reduce the optical quality.

[GeezerGazer]

Got it! Thanks all!!!

[GasGiant]

One thing I might suggest also.

I dont know if this is a good safety tip, but I have been popping a cover over the finder scope too. Just so I dont get a sudden blast of sun!

If you do pop a cap on the finderscope, do it at the front, not the back. I popped one on the back first and the sun burnt right through it ! 

[Ags]

Yes, it is very important to cap your finderscope too