AL_Woods 7"

Like I needed another telescope

    For the last year or so, I've been looking to remount my 8" F5 reflector in a Dobsonian mount for portability. At the same time, I was thinking about getting another cheaper one for use at public outreach events where things get knocked around pretty good. I was looking to my friend Jerry Loethen to assist in the building of the Dobsonian when he put up an advertisement for an 8" F5 Dobsonian for $100.00. Sometime in 2004/2005 he had acquired a primary mirror and telescope tube from the estate sale of the late, great Alfred (AL) Woods.

    Jerry assembled a bunch of spare parts (base, cradle, spider, diagonal mirror and focuser) and installed the whole contraption on one of his old Dobsonian mounts and put it up for sale. This was just the starting point for what I was looking for so I bought it. It does require a lot of work but hey, I've got time between looking for a job and waiting for the RETI project to start. So this is an online log of a telescope rebuild project. It gives you an idea of some things that need to be done to rebuild an old, used telescope.  

Telescope as received on 18 May 2010

    Note the strapped on Telrad, and the fixed tube. Positioning the focuser for better angle is not possible as is. I do like the slots to enable balancing. The mirror is in pretty bad condition (see images at end of this page). Lots of dirt, haze and pinholes in the coating. Only the Tube and the Mirror are original to Al. 

1 June 2010

    The mirror cleaned up really well with John Ducheks optics cleaning kit (). While there are tons of very small pinholes in the coating, this scope out performs my 8" F5 reflector from Orion.

(Click on images for larger versions)

New cradle

     First step is to get the thing more user friendly. This needs a new cradle. I decided to use a clamp style. This allows the tube to be shifted fore and aft for balancing as well as rotation for a comfortable viewing angle on either alt/az or equatorial mountings. As initially constructed, I used one of the original cradle sides to support the front and rear of the tube.

    As installed on the base. I've temporarily mounted the Telrad and it's position makes it a "right handed" scope.

Cradle V1.5

    The cradle worked great but looked odd. I replaced the original cradle support with a new one that looks a bit more organic with provisions for an equatorial mount adapter. It's now a "Left handed scope" with the Telrad repositioned as seen next.

    The Dobsonian mount is great for low power views but this mirror works really great to well over 300x. At this magnification, Saturn is tack sharp but drift requires constant movement of the tube. The EQ mount adapter allows this assembly to be used on a driven equatorial mount. Tube rotation is needed when various parts of the sky are viewed. so the cradle design really makes this easy to do

    With a "Left Handed" mounting, the viewers back is now faced to all the stray light sources down the street. The bushes in front block lights on the other side.

Spider and diagonal

    The original spider had cap screws One of which was bound up. There are two things I don't like about this type of spider:

    1] Allen wrenches are required to adjust the screws. The wrench can be dropped into the tube during adjustment.

    2] When one of the screws is loosened, the whole assembly can flop out of alignment.

    Rebuilt spider. The clearance hole for the bound screw was opened up and it now adjusts smoothly. An acorn nut was installed internally to act as a bearing and a spring was added for positive pressure during adjustments.

    The diagonal mirror was originally 2.6 inches and this was ok but it blocked a bit more of the light path at low powers (< 80x), its' shadow could be seen and was annoying. It was replaced with a 2.14 inch diagonal. The drawback is the 100% field diameter. Right now its about .8 degrees which is just fine for planetary work. The light drop off outside this region is not too bad all things considered - I'm still able to see mag 7 objects in the light polluted city skies.

Focuser upgrades

    I liked how this helical focuser worked. The problem is that with this short tube, the eyepiece needs to be way out (like five to six inches) and since the focuser is 1 1/4 inch, severe vignetteing of the optical path was encountered. Ok for high magnifications but horrible for wide field views.

    The focuser was replaced by a 2" Crayford on a special built pedestal. The focuser is from JMI and its' curved base was removed and a new mounting plate machined up.

    Note that the pedestal allowed mounting of the Telrad in a more accessible and comfortable location. It works well at this position but would work better when it is mounted higher

V 1.10 - 4 Aug 2010 Laser finder

I added a green laser pointer from Z-Bolt. The mount is actually from Lumicon but Z-bolt had this "Astronomy package". It consists of a 4.5 - 5 mw green laser, remote switch and mount. This photo shows a standard laser mounted up. Note the arrow. I had to modify the mount by adding a screw so that it would actuate the momentary on switch of the laser. Putting the scope on the field of the target object is just too easy! Now I can get rid of that overweight Telrad. See also the note down in the "To Do" list regarding adding an integral finder to the pedestal - that'll be really sweet.

That's a full moon in the upper left with Orion in the center. No more contortions and stiff neck from craning to use the Telrad. It was removed and sold off. All my scopes have now been retrofitted with laser pointer holders (I now have a number of standard 30mm and 50mm finders with mounts gathering dust). For more information on laser alignment and usage, go to my AstroScan page.

Simulated view of laser in eyepiece FOV

V 1.20 - 6 Sept 2010 integral Finder

    Finished the initial construction of the integral finder - and how sweet it is! The final TFOV is 7 degree 45 minutes. Not as much as expected but far more than the five degrees of a standard 8x50 finder. First light under severely light polluted skies revealed mag 7 stars. More than sufficient for a finder of this type.

    Since no provisions were made for collimation as well as adjustments for alignment with the eyepiece (perhaps V1.30), the field shows asymmetric coma and the eyepiece FOV is offset by a degree or so from the center of the finder FOV. The Laser mitigates the alignment error in that it is aligned to the eyepiece view and it shows up in the FOV. Simply setting the end of the laser beam on the target in the finder is all that is needed to place the object in the FOV of the eyepiece. No need for aligning the finder to the eyepiece or adding crosshairs.

  A $20.00 precision first surface mirror is used as a diagonal and is mounted in a drawer. Slide the drawer in for wide angle finder and pull it out for normal scope use.

Completed Focuser assembly:

Results:

     The aperture is stopped down to 41.25mm to increase the sharpness of the resulting image. So, with a focal length of 190mm and the Astro-Tech 25mm paradigm (60degree FOV) eyepiece, we get the following performance: 

Focal ratio        = objective fl / aperture                         = 190mm / 41.25mm = f 4.6

Magnification      = objective focal length / eyepiece fl            = 190mm / 25mm    = 7.6x

True field of view = Eyepiece apparent field of view / magnification = 60degrees / 7.6 = 7.894 degrees (7 degrees, 53.75 minutes)

Limiting magnitude =                 (pretty involved calculations)               = 8.3 under mag3 skies

Exit Pupil         = aperture / magnification                        = 41.25 / 7.6     = 5.42mm

    The result closely approximates an 8x50 finder with one exception: a gain of almost three degrees over the standard 5 degree FOV of stock 8x50 finders.

    During testing, the resulting field of view, using a 25mm Astro Tech Paradigm eyepiece, completely covered the constellation Lyra (careful checking against stars yielded a useable FOV of 7.75 degrees - very close to the calculated 7.89 degrees). This is a simulation of that view (including the magnitude 7 limit). For comparison purposes, the simulated view includes the FOV circles of a Telrad finder and the 25mm Paradigm in normal use in the scope.

FOV circles superimposed on a target rich area of Sagittarius...

     It should be noted that the 32mm Sirius Plossel from Orion does give a true FOV of 8 degrees 6 minutes which is about 15 minutes larger than the Paradigm but the quality of the view is not nearly as good. Serious distortion in the field occurs at about 70% from the center whereas the Paradigm begins to drop off at 80%. The Paradigm offers a completely useable field over the Plossels' 90%.

4 October update - A 14 degree finder!

    I've had this 38mm, 70deg AFOV Astro Tech Titan 2 inch eyepiece in the back of the box for a year now. I don't use it much because it's a bit large and heavy. It unbalances the tube in dob mode so I have to rebalance the tube between eyepiece changes. Earlier in the evening I was running some numbers and it occurred to me to try the Titan with the finder. Turns out the eyepiece yeilds a magnification of 5 and a True Field of View of 14 degrees. A quick check under very clear skies confirms that not only will it come to focus but the calculation results are accurate . The field is very sharp to the edge and mag 6 stars are easily seen even under very light polluted skies. I now have my very wide FOV finder!

1 March 2011: Star hopping is amazing. Under really light polluted skies, no object is more than three hops from a mag 2 star. Zeroing in on the field of the target object is simplified by successively higher magnifications. Generally, one hop with the 38mm and a field centering with an 18mm Paradigm is sufficient. Sliding the drawer out with the 18mm still in place almost always shows the object I'm looking for. If I can't see it, it is either to dim for the seeing conditions or it is mis-plotted.

14 degrees (yellow circle) is sufficient to cover he teapot asterism! The white circle is the original 8 deg fov with the 25mm paradigm

15 July 2010:

 More professional circles were provided by Larry Walton from ASEM. A very nice job Larry! Note the magnetic pointer. On deck is an led light pipe based version.

Azimuth circle

15 July 2010 

The updated Azimuth circle from Larry. Mighty nice and easy to use. The text is water based so we are testing three different spray on sealants for dew resistance. On the first outing, all worked well except the fact that the poster board backing warped. Didn't see that coming! The board will be remounted on more rigid 1/8" material.

March 15 2012 

The Azimuth setting circle was replaced and a new one remounted on 1/8" fiberboard. A 1/32" plastic sheet was mounted over the circle as a dew shield. The  setting circles have been a real joy to use on this scope.

To do list

1] Add tube baffles 

Primary mirror condition

Some would say the main mirror is in sad shape. Side by side comparisons with my 8" f5 scope from Orion says differentlly. Saturn is tack sharp in the 7" while it is soft in the 8". It's an "ain't broke" so I don't think I'm going to "fix it" kind of thing. Here's a shot from the front and with a backlight:

Left image is mirror with front lighting while the right image is with mirror backlighting

April 13 2012 

After seeing the performance of recoating the 8" F7 mirror from Jim Roe by Nova Optical (who has since closed doors to the general public), I immediately shipped this mirror off for recoating. Steve Dodds called 23 April and said that not only is the figure between 1/10 and 1/15 wave but that is across the full width of the mirror! There is apparently no turned edge on this thing. Waiting return of the mirror. 

2013 update: I was talking with Mark Jones who knew Al well. When I related this little story, Mark said Al may have wanted a higher f ratio so he got there by masking off the edge. Makes a lot of sense. With a focal length of 914mm, an 8" (203.2mm) diameter yields an f ratio of 4.49 while 7" (177.8mm) is f 5.2

2020 update: I was talking with Bill Neubert about this masking thing and he mentioned that "He might have wanted a higher f ratio so some of his older eyepieces would perform better." and with that, everything fell into place. On the side of the mirror, there are what appear to be 2 dates. 5-67 (scratched out) and 9-78. These dates make sense as Al was in his heyday at those points and way back then, fast ratios like 4.5 were not very kind on the eyepieces of the time. But why he wanted a focal length of 36 inches is a yet to be answered mystery. It just goes to show, you can't answer one question without raising another ;) 

April 30 2012

I Got an email from Steve at Nova. No estimate on mirror return but he did include the Ronchi-gram of this mirror (the wait continues...):

August 2013

Alas, this mirror is no longer mounted in the original tube assy. For various reasons, I remounted it into my older 8" Orion tube. The tube is mounted on the EQ6 GEM which is attached to the shortened Roe pier. It makes for a compact, short height, high magnification goto telescope without the need for constant manual movement (I mean really, when you can take this thing beyond 250x, who wants to sit there bumping a dob along all night).

I'm looking to get another 8" mirror with a 914mm focal length to install into this tube assy.