Tweedledee Posted May 17, 2015 Share Posted May 17, 2015 Well not quite Just making an off axis mask for the 14" dob using the big plastic end cap. This will allow me to experiment with using it as a 120mm aperture F13.6 scope or a 90mm aperture F18.2 scope. This mask will eliminate the loss of definition caused by the secondary obstruction and spider vanes. Although the views will be dimmer with the smaller aperture, this mask may improve the views of planets, the moon and double/multiple stars. We shall see. Link to comment Share on other sites More sharing options...
BAZ Posted May 17, 2015 Share Posted May 17, 2015 If I remember right, Doc did something like this when he had the 16" lightbridge. it was as you said, to improve the view rather than for the light gathering capacity. I don't know how he got on with it. Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 If I remember right, Doc did something like this when he had the 16" lightbridge. it was as you said, to improve the view rather than for the light gathering capacity. I don't know how he got on with it. Hopefully, Doc will soon let us know his thoughts on this. Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 I hope to make the mask in such a way that I can securely fit the sun filter from the ST120 on it. It will let me see the sun at substantially higher magnification than in the ST120, as the dob has 3x the focal length. Link to comment Share on other sites More sharing options...
tuckstar Posted May 17, 2015 Share Posted May 17, 2015 I got some good views of the sun spot on eclipse day with my dob stoped down to 93mm, F12.9, although I had to use a 32 mm to get the full disk in view. https://flic.kr/p/t5EMJ4 Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 I got some good views of the sun spot on eclipse day with my dob stoped down to 93mm, F12.9, although I had to use a 32 mm to get the full disk in view.https://flic.kr/p/t5EMJ4 Very nice looking setup Andy. Is your filter homemade? I will get a full disc view in a 9mm ep at 180x if the seeing is good enough to use that power Link to comment Share on other sites More sharing options...
Doc Posted May 17, 2015 Share Posted May 17, 2015 To be honest I cannot remember to much about it. According to the thread on SGL it turned out very good. http://stargazerslounge.com/topic/78301-aperture-masks-really-work-on-jupiter/?hl=%2Boff+%2Baxis+%2Bmask#entry761987 I made three holes the following sizes: 80mm F22.8110mm F16.6145mm F12.6 I do remember that the edges of these holes were rough and this does effect the views. But generally I obtained sharp stars as there was no secondary to take into account and I lost the glare from a large 16" of aperture so to my eyes Jupiter appeared more detailed. Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 (edited) Thanks Mick, You obviously wanted to try more than one focal ratio like me, but went one further and made three sizes. As far as I have read, the improved views (on certain objects) from using an off axis mask come about as follows... 1. Eliminating the secondary obstruction and diffraction spikes from the spider vanes creates better, tighter star images. 2. The smaller aperture reduces the image brightness and can in some cases allow more detail to be seen, since the image is not too bright and washed out. 3. This smaller aperture increases the darkness of the sky background giving better contrast and less glare on bright objects. 4. In times of poorer seeing a smaller aperture will show less of the bubbling and boiling effects in the atmosphere. 5. Substantially reducing the area of the mirror that is actually used (in my case, by a factor of 9 at 120mm or 16 at 90mm) should encompass fewer image degrading surface defects thus allowing more precise focus and definition. If we all had pristine dark skies with excellent seeing and top quality optics, then better views and more detail should be obtained at full aperture. Obviously there would always be a large negative effect on the views of nebulae and faint objects. An off axis mask should usually give improved quality views of certain brighter objects like planets, the moon and double/multiple stars. However, it is important to pick the right targets and conditions to get the best out of an off axis mask. At least in theory, an off axis mask should produce images approaching the quality of a similar aperture apo refractor Edited May 17, 2015 by Tweedledee Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 Hoping Philjay might add a comment or tell me I'm talking a load of <insert expletive>, as Mr. Pugh would say Link to comment Share on other sites More sharing options...
tuckstar Posted May 17, 2015 Share Posted May 17, 2015 It was home made Pete. Some camping mat like material a couple of discs with apropiate size holes and baader film sandwiched between. Then rolled one to diameter of tube. Then cut in half, and another wrapped around that to original size. Then taped up and one of the half inserted inside then the disc with filter then the other cut half to keep disc in place. Then a 4" wide plastic strap around it all to hold it all together. Heres a quick pic from the eclipse taken with my note https://flic.kr/p/s9dJYj Tweedledee 1 Link to comment Share on other sites More sharing options...
Tweedledee Posted May 17, 2015 Author Share Posted May 17, 2015 Looks like a nice secure job Andy Did you use any glue on the foam as well as tape? If so, what glue? Link to comment Share on other sites More sharing options...
tuckstar Posted May 17, 2015 Share Posted May 17, 2015 (edited) Just duct tape along the seam of the outer foam, and then plastic banding. The duct tape could do with being reviewed but held in place all day no problems, just started to come lose when I'd packed up and had been messing with it. You can't really see the banding unfortunately. The foam is a nice snug fit over the end of the tube but I still wrap some parcel tape round it so it definitely won't come off. Edited May 17, 2015 by tuckstar Link to comment Share on other sites More sharing options...
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