Using offset solar filter with 10 inch dobsonian

[Callan]

Hi. I have a 10 inch Skyliner Flextube Synscan Dobsonian. I am thinking of making an offset solar filter with Baader AstroSolar Safety Film (OD 5.0). I reckon an 80mm dia. hole might be ok for the light on the primary without it catching the spider or the secondary mirror. Is this the best way to proceed or will there be more benefit from a full on filter? (A bit fragile using the Baader stuff for full on!). I realise I must follow all safety procedures for this setup. Check for pinholes, make sure it is fixed against removal by wind gusts, cover the finder optics etc. How will the image appear at the eyepiece with an offset filter? Effects of coma? I have Explore Scientific 82* 14mm and ES 68* 24mm eyepieces. I also have the Skywatcher  10 and 25mm eyepieces supplied with the scope. Anyone use a solar filter on a 10 inch Dob? I’d be very grateful for any feedback. Thanks.

Callan. x

 

[Cornelius Varley]

You will also need to wrap the open truss with a light shroud eg to prevent stray sunlight reaching the primary mirror.

[niallk]

Hi, I've a solid tube 250px, and covered the 50mm hole in the dust cover with solar film.  It's pretty cool to be able to use the scope as a white light solar setup.  Even 50mm gives interesting views!

One of these days though I will make a full aperture mask - to increase exit pupil at a given (highish) magnification on sunspots to reduce the issue of floaters.

Be careful to robustly have a shroud in place per the post above - take extra steps to ensure to unfiltered sunlight can not hit the primary; you can't afford a split second accident.  I don't know if duct tape or something at either end would be a good idea.  Hopefully others might give examples of the appropriate precautions they take.

[Ben the Ignorant]

Astro Solar is available in 50cm x 49cm (20" x 20 ") format; I cut a full-aperture filter for my GSO 300 dob, and sandwiched it between two sheets of lightweight, kinda foamy semi-rigid plastic. Then the whole assembly is attached to the four studs that protrude outside the tube.

https://www.teleskop-express.de/shop/product_info.php/info/p545_Baader-AstroSolar-Filterfolie---Visuell---50-cm-x-49-cm--fuer-Sonnenbeobachtung.html

I remove the finder, I don't cover it, so its missing mass is compensated by the extra mass of the filter, balance does not change much. Full aperture makes the grain obvious, and sunspots are like black and gray sunflowers with their patterns of filaments. I can't describe the materials, it was plastic recovered from a junk pile, but the essential is four L-shape clips with holes matching the four round studs. The filter's frame is a bit flexible to allow installation and removal.

Plus, to accomodate the light cone, the filter's real diameter is 30 or 40mm bigger than the mirror. I built a tough cardboard box with reinforced corners to store the filter.

[Callan]

Some great tips for construction. I forgot to mention I have a shroud fitted to my scope. Good idea to maybe remove finder to compensate for the weight of filter. You can be rest assured I will make sure no unfiltered light will enter the scope. I will first try an offset filter to see how I get on, and then might consider one at full aperture. Thanks again.

Callan. x

[John]

Offset / off axis aperture masks work well with newtonians so I don't see any reason why the same approach should not work, with appropriate safeguards as discussed, for solar. I occasionally use a 4" off axis aperture mask with my 12" dobsonian for nighttime viewing. It provides an unobstructed 4" F/15 optical path and the views are very similar to those that I get with my 4" apo refractors.

You might even find that using the off axis approach provides sharper solar views than using the full aperture of the scope ?

[Callan]

Thanks John. Good to know about off axis night time viewing possibilities. As you may have gathered, I’m a beginner at this astronomy pursuit. I’m doing a fair amount of reading and research on the hobby, especially SGL. I’ve sent for the Baader A4 size filter sheet, so I’ll see how it goes. Only drawback is the big solar thing is getting lower in the sky at this time of year, so more    atmosphere to look through! Hopefully still bright enough to not matter.

Callan. x