(ATM) Ferguson's mirror 1

[Mel Bartels <mbartels@efn.org>]

>
>
>Before starting my first project, I spent many hours in the archives reading 
>nearly everything but the administrative posts.  I found Allan Killian's 
>"log" to be very inspiring, so I'm planning to do the same.
>
>HOWEVER, I realize that some will object on the grounds that "we've heard it 
>all before".  If you're one of those, then please e-mail me directly, and try 
>to dissuade me. Encouragment would, of course, also be welcomed. It is, 
>hwever, too late to stop the first installment, as you're already reading it 
>:-).  There are some embedded questions so please read this even if you don't 
>need the inspiration.
>                                                            
>I received the  abrasives kit yesterday from Willmann-Bell. I bought from 
>them based on positive remarks from this group, and got the same prompt, 
>accurate service others have reported.         
>
>I'm still trying to come up with a good grinding stand..Steel drums just 
>don't seem to be as common as when I was a kid.  Ideas for alternatives, or 
>suggestions as to where to find a drum, would be most welcome.
>
>I was quite anxious to see if my ceramic tool would work, so I put some wet 
>newspaper on top of my washing machine (ahhh...the joys of bachelorhood!) 
>Threw on some #60 (came with kit..books say start at 80??) and went at it.
>
>To my great relief, the tool seems to work great.  Books say to expect my 
>first wets to last "hardly a minute" with plate glass tool, but I got ~ 5-7 
>minutes out of each of the two wets I did, with a nice "crunchy" sound/feel 
>the whole time. I stopped there, because I can see that I am too lazy to turn 
>the tool often enough..barrel walking seems like a better approach.  
>
>The ceramic breaks down into clay, forming a runny paste with the water..this 
>seems to do a nice job of lifting the grit up out of the channels. 
>                                                           
>I have Sam Brown's book (Edmund) and have borrowed older editions of 
>Texereau, and Barry from public library.  Brown assumes I'm doing a 6".  I 
>presume I should pretty much double everything (pressure, time, amount of 
>grit, etc.) for my 8" due to ratio of surface area, and volume of glass to be 
>removed?? If this is wrong, somebody please clear this up for me. (I think 
>Tex or Barry may have had a table..I'll go to library and check them out 
>again tonight)
>
>f ratios???
>I was (niavly, I now think) first thinking of going for a f/4.5, but after 
>playing around with sketches, and doing a lot of reading, I have toned down 
>my aspirations to f/6.0, which may still be a bit ambitious for a first 
>mirror. Looking at the coma, and difficulty of figuring a fast mirror, it 
>hardly seems worth the FOV or slightly enhanced portability.  I was thinking 
>I wanted "rich-field" performance of the 4.5, but with the coma at field 
>edges that  this would have, the  extra FOV doesn't, now, seem to me to be 
>worth the bother.   I need to commit to a f/ratio before continuing with the 
>mirror, so here is my proposed design, for the group to throw rocks at:
>
>primary      8" , f/6
>diagonal:    1 7/8" minor axis (~5% obstruction)
>tube:        Plywood, octagonal, 11" minimum interior dimension
>             interior mounted focuser.
>
>other details "To Be Determined".
>                                                               
>I will glady pay 2 cents (less 1/2 buck shipping and handling!) for your 
>comments. But please don't delay, as I hope to finish rough grinding this 
>weekend.
>
>Enough for now...Clear skies,
>
>Kevin                                                              
>
>
Regards,
Mel Bartels
mailto:mbartels@efn.org
http://www.efn.org/~mbartels