temp backup

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 slapped together 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 (need pics). 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. Bad for high magnifications and 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 August 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.

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 metrics: 

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%.

Setting circles

These things make locating objects in mag 2 skies possible. One needs to only use Starry night Pro to display the altitude and azimuth of the target object and then set the circles accordingly.

These are temporary to test concept, installation and operation. Testing showed that all targeted objects fell within three degrees of the center of the field of view. The 25mm Astro Tech paradigm has a 1.6 degree True Field of view so the targets were easily found with a small sweep of the field.

The circles are being replaced with more accurate and durable versions. Note that the indicator will be rebuilt and installed on a magnet. Small metal plates will be mounted opposite the setting circles and the magnet will allow movement of the indicators. This is for alignment of the scope as follows:

1] Level the scope and set the scope and baseboard pointing roughly South (+/- 60 degrees).

2] Point the scope at a suitable star and find its altitude and azimuth. Move the magnetically mounted indicators to indicate the Alt Az for the object.

That's all there is to it. Rather than move the dials or mount, move the indicators.

Altitude Circle

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.

To do list

1] Repair, clean and coat setting circles as this past IDSSP trip really showed susceptibility to dew.

2] Add tube baffles

3] Rethinking this - Send main mirror in to be recoated (do the following while mirror is out of town):

    2a] Seal and finish all wood surfaces (Done 7 September 2010)

    2b] Clean tube exterior

    2c] Fill in all extraneous "Custom ventilation" holes

    2d] Get matching paint and touch up all dings and filled holes.

4] Add an integral finder to the pedestal (Done 7 September 2010) - This will be really sweet. I have a 54mm x 190mm achromat. By mounting it in the front of the pedestal and using a flip mirror and a 32mm 54 degree plossel, a nearly 10 degree wide true field of view finder can be had with the telescope focuser. Flipping the mirror in provides a 6x 54mm (F4) finder! The true field of view is a little over nine degrees. Flip the mirror out and use the primary mirror yields a 28x 1.89 degree TFOV. So now locating objects will be a breeze: Put the scope in the field with the laser, flip the mirror in with the 32mm eyepiece and center up in the field, flip the mirror out and center the object. Can't get much better than that! Oh wait, yes it can: It's a 2" F4 optic after all. The scope will work from less than 6 x to over 400x and has ultra wide field capability! Beat that Questar!  Here's a simulation of the field of view of the finder and the widest angle eyepiece I have (Click to enlarge):

Proposed 9degree FOV finder simulation

Here's a full field view - More than enough stars to find a pattern for object location (and just look at all those targets!):

All this and you don't have to take your eye off the eyepiece.