Re: Is the focus on the mirror or on the star
From: Greg Rudzinski
Date: 2017 May 26, 18:43 -0700
A couple more things to keep an eye on would be fungus, mold, or foreign substance on the internal surfaces of the scope optics. Lens coatings can degrade unevenly also. Watch out for lens/mirror fogging. Early morning would be the worst time for fogging. One last thing...try using the lightest filter on bright stars. This gives me a dimmer point of light rather than a bright fuzzy asterisk.
From: Randall Morrow
Date: 2017 May 26, 07:50 -0700
Thanks to all for your observations. The mirror is a first surface type, purchased from "First Surface Mirrors" who make these for scientific purposes so I suppose it is optically flat. there is no hint of aberration even with low angle sights standing 20 feet or more back from the mirror. It is made of glass and is 1/4 inch or 6mm thick and is attached to the base by foam back tape so it is not being pinched. The base is 1/2 in thick aluminum plate that was machined to be perfectly flat.
Perhaps my vision is an issue. I am over 60 with the normal presbyopia and have always been nearsighted at 20/200. My vision is good with glasses and I wear these when taking most sights. I have installed cut-out "peeps" in both of my telecopes. Lately all my sights have been stars and planets but doing doing sun and moon there are still focus problems. It seems that the direct and reflected images cannot both be perfectly focussed at the same time, which makes no sense to me. I have to use an average focus between the two images. The difference in the lenght of the ligh pathway between the horizon image and that in the index mirror is only inches and should not matter. My sight runs are always taken in the dark of night rather than twilight.
Could it be that a 1 dimensional image in a black background with no frame of reference around it, is "resolved" by the brain as being on the surface of the glass? If there is any background at all the brain my react differently to what it "sees". In this way it is not the same as a photograph. Over the weekend I will try the suggesed comparisons of things seen in the mirror at terrestrial distances to the reflected ones. The suggestion of a liqud horizon will not work though as I do not have mercury. No other liquids will reflect the dim light of stars in my experience. The science is clear on this but my description of my experiences is accurate I assure you. I'll get back to you. And thanks again.