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New Dob Project - Home Grown 8" f/10!

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#1 jgibson1@emich

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Posted 05 July 2012 - 10:08 AM

So here it goes………………

I have finally pulled the trigger on a pet project I have wanted to build for quite some time. An 8” f/10 dob will be mine soon. :grin: I have a Primary being ground and will be in my hands shortly. I am looking at using sono-tube or BlackLite tubes from Protostar. I may use Sonotube initially to get the design (ie-dimensions) nailed down and then transfer this to BlackLite tubing. I have an inexpensive 2” rack & pinion focuser to use initially for testing. I have been looking at 0.75 – 1.00 inch secondary’s from Protostar. A friend is offering to let me use a couple of his spare secondary mirrors for testing. Unfortunately he does not have a 0.75” mirror.

This all came about after having the pleasure of observing the moon, Jupiter, and Saturn with a friends homegrown 8” f/10 dob. The views were superb and the best I had ever seen in a Newtonian. I am essentially modeling my design after his. I know a 0.75” secondary is a little small if I wanted to observe wider field of views. However, my friend’s scope, with a 0.75” secondary, produced amazing images at “moderate to high” magnifications. I did not observe any fall off of image brightness.

The build will take some time as I need to collect the other components over the coming months. It should be complete by the end of the year at the latest. I will certainly add updates as I progress.


And finally here is my question / request from this forum…………

One thing I am looking for are any online tutorials or “real” books on telescope making. Any of y’all have suggestions? I am playing with Dale Keller’s Newt Software. The design I have in mind looks good but second sources of info and many second opinions are always welcome. I want to get the initial design and setup as close to ideal before building the scope. I do not want to get the darn thing together and out for first light just to learn that it cannot come to focus!!! :tonofbricks: That would make Jason not so happy!

Thank you in advance for any suggestions, references, opinions, or ramblings you may want to throw my way.

Clear Skies,
Jason

#2 starman345

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Posted 05 July 2012 - 10:14 AM

Hi Jason, sounds like a great project, I'll be watching. I can suggest having a look at the Kriege & Berry "The Dobsonian Telescope". There is a chapter in there on building an 8" dob plus lots of other invaluable information about telescope building.

#3 Pinbout

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Posted 05 July 2012 - 10:19 AM

Hey Jason,

I have questions... :question:

do you have any plans for the primary mirror cell? are you going to use clips or are you going to glue it?

do you know what your going to use for a secondary mirror spider/stalk? are you going to make it or buy it?

#4 Al8236

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Posted 05 July 2012 - 10:23 AM

Another would be Neale Howards "Standard Handbook For Telescope Making", Some what dated, but if you can find one a good source of information.

#5 jgibson1@emich

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Posted 05 July 2012 - 11:35 AM

Thank you for the book suggestions! I will look those up asap.

Pinbout,

As for the mirror cell I am not sold but I suspect I am going to go with a glued mirror. It works well with my 12" dob and it is simple. Would you agree? Why or why not?

I have looked at some commercial options. However, I do have access to a machine shop. A billet aluminum cell may be in the future. I need to check on the availability of the milling machines and operators. If not I will likely opt for a commercial option.

Another idea I had for a mirror cell was one made of wood. I do have access to an top-notch wood working shop too. One where I can use the equipment at my leisure. I have seen 6-10 inch scopes with cells made of marine grade Plywood. Any thoughts or opinions on the use of wood? The mirror will not weigh much. I know with a wood cell the pieces will be thicker and a bit more bulky. However this will likely save a bit on weight ultimately. Not to mention that this would keep the design simple too!!

I am basically sold on a Protostar curved spider!! I thought about making this but Protostar will custom make the thing for their tube. No trial and error. The lack of diffraction spikes is really attractive. I am trying to achieve the highest possible contrast with this scope / design. My ultimate goal is to have an ultimate planetary / lunar / solar scope.

Thanks again for the input guys!

Jason

#6 Pinbout

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Posted 05 July 2012 - 11:46 AM

here's a thought, if you want to travel with the scope that's pretty long, 80" tube.

what about a split truss dob? it would pack up very nicely in a small car, better than most scopes.

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#7 Wes James

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Posted 05 July 2012 - 12:07 PM

An 8" f/10 scope will certainly be an interesting project, though f/7-8 would probably be a bit more reasonable when it comes to transporting/reaching the eyepiece.
Wood mirror cells work fine at this size- cabinet grade plywood and adequate venting will work nicely. Easy to build.
Re. the truss configuration- there are those who would say that a truss type configuration- while desireable for transportability- is less than desireable due to the issue of tube currents. There has been many discussions discussions of the "ultimate" planetary reflector, and most agree that a solid tube is better at controlling tube currents. (Try searching "ultimate" in the "Reflectors" telescope section here on C/N)
One way of ensuring that you get the mirror placement correct (if you're going to start with a cardboard tube- to "get it right" first- a good idea) is to start building at the front of the scope.... place your secondary and focuser at the front of the tube- then leave yourself some extra at the rear of the tube, and make yourself several sets of primary mount screwholes, spaced perhaps 1" apart- or 1.5" apart... then you can have different placement options for your primary mirror, and ensure you get the correct distances set up so the scope will come to focus where you want it to- with the eyepieces you want to use.
Another classic telescope building book is Sam Brown's "All about Telescopes"- from Edmund Scientific... published back in the '50's- and still a great deckplate-level book. Good luck!!
(Better get you a taaaaal ladder!! ;))

#8 Mirzam

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Posted 05 July 2012 - 12:13 PM

Here is a focuser idea for you--Use a KineOptics HC-1 mounted on a metal plate inside the tube. This will allow placement of the focal plane 1" or less above the outer tube surface. With a secondary to focal plane distance of 6" you can use a 0.75" diagonal!

Here's a shot of an HC-1 on a machined plate. The hole placement for the HC-1 is a bit critical, as is the diameter of the hole for the focuser tube.

JimC

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#9 Mirzam

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Posted 05 July 2012 - 12:14 PM

Here is a shot of the plate inside an old 12" tube.

JimC

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#10 Mirzam

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Posted 05 July 2012 - 12:15 PM

Here it is from the outside. This is just an old beat up tube that I had laying around for this illustration. I do plan to try this approach with my recent 10" Delmarva mirror.

JimC

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#11 Ksearles62

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Posted 05 July 2012 - 12:17 PM

JGipson..You can check out Richard Berry's book on telescope making. It has a lot of good information. Also check out the Stellafane website. It has plans for a Birch primary cell holder. My first build was a Z8 Truss conversion. Be forewarned...Once you build your first it is hard to stop..Good Luck and clear skies

#12 dan_h

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Posted 05 July 2012 - 01:10 PM

So here it goes………………
I have an inexpensive 2” rack & pinion focuser to use initially for testing. I have been looking at 0.75 – 1.00 inch secondary’s from Protostar. A friend is offering to let me use a couple of his spare secondary mirrors for testing.


Unless this is a very unusual focuser, the focal plane will probably be about 2.5"-3.0" above the tube. With a 10" OD tube, that will put the focal plane about 8" above the primary axis.

I know a 0.75” secondary is a little small if I wanted to observe wider field of views. However, my friend’s scope, with a 0.75” secondary, produced amazing images at “moderate to high” magnifications. I did not observe any fall off of image brightness.


Given the dimensions I stated above, a 0.75" diagonal is not going to provide full illumination of any point in the image. A 0.8" is the absolute minimum size you can use and still utilize the full 8" mirror. Any smaller diagonal will vignette. You won't notice any falloff in the light but you will not be using the full 8" of the primary.

Of course, if you can lower the focal plane by going to a very low profile focuser, you can get by with a slightly smaller diagonal. But you do not want the focuser barrel so low it enters into the light path.

If you use a tube less than 10.0" OD you can squeeze a little more off the diagonal. However, you want room in the tube for air currents to fall outside the optical path so a 10.0" OD is not unreasonable.

IMHO, 0.75" diagonal doesn't cut it; it is too small to field the full primary. A 1.0" is far more reasonable and is only 12.5% obstruction. You probably won't discern any degradation of the images by increasing this to 15% or 1.2" and this still only provides a fully illuminated field about 0.25" diameter.

See Mike Lockwoods table of secondary sizing here:

http://www.loptics.c.../diagonals.html

dan

#13 Sean Cunneen

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Posted 05 July 2012 - 04:55 PM

Here is a link to an old thread in the Archives which will tell you everything you want to know. It's long, but a great read!

Reflector vs Refractor?

#14 dan_h

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Posted 05 July 2012 - 08:03 PM

Here is a link to an old thread in the Archives which will tell you everything you want to know. It's long, but a great read!

Reflector vs Refractor?


Somewhere within that thread is buried a link to this.

http://legault.perso...bstruction.html

<<For an amateur who builds his reflector, it may even be dangerous to minimize at all costs the size of the secondary mirror. If this mirror is undersized, the periphery of the light beam is lost, leading to a light decrease and an increased effective obstruction (the opposite of the goal !). Above all, many (industrial or home-made) mirrors suffers from defects at their periphery (turned edge). In this case, using the very edge of them can cause a deterioration of the performances far ahead what would have caused a small 'safety' supplement of obstruction. >>

I still vote for a secondary larger than 0.75"

dan

#15 Pinbout

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Posted 05 July 2012 - 08:08 PM

Here is a focuser idea for you--Use a KineOptics HC-1 mounted on a metal plate inside the tube. This will allow placement of the focal plane 1" or less above the outer tube surface. With a secondary to focal plane distance of 6" you can use a 0.75" diagonal!



that's a great idea! :)

#16 Jeff Morgan

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Posted 05 July 2012 - 08:53 PM

What a fun project, I envy you. The Kriege book is good for insight, but more of a guide to tail-heavy short focus truss scopes. At f/9, you have a very different dynamic. Here is a much better link IMHO.

A double truss would be workable, but you would need high-quality connectors, and twice as many. That all add$ up fast. A solid tube would still be manageable at that length. Consider adding a sleeve in the center to increase rigidity.

Transportability might not be as bad you think. For my 10" f/9 I used a Yakima kayak roof rack. The curved hull fittings offered decent support for the tube. The mirror cell was on a bayonet mount (easy removal for travel) and I had a cover for the front of the tube. The mount rocker arms detached from the base and both would fit in the trunk of a Honda Accord. Attached is a picture of that old scope. Still the only mirror that showed me spokes in Saturn's rings, but I have high hopes for my new Zambuto.

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#17 Tom and Beth

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Posted 06 July 2012 - 01:31 AM

Hi Jason,

I hand rolled a 7 inch F10. You just breathed on the sphere to parabolize it. As you already know the views in these long scopes are KILLER.

Add me to the list of those who would suggest you use a slightly larger diagonal. And unless you are going to use a wire spider, I doubt you'll notice the difference at the EP as compared to using a 1 inch diagonal. FWIW I used Hastings Irragation 9 inch tubing.

If you mentioned it, I must have missed it. So assuming you are using a full thickness pyrex as the mirror substrate you could use a 3 point support using silicone, even on sealed plywood. I attached a pic of mine.

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#18 jgibson1@emich

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Posted 06 July 2012 - 06:16 AM

WOW!!! I cannot write much now as I need to get my butt to work this morning to justify the salary that my employer seems to think that I am worth..... ;) But seriously some great information here guys. Thanks a ton!

I am glad to have the insight on the secondary size. I am really debating now. I should be able to position a focuser such that the .75" secondary would work but will require special needs! This certainly makes a 1" more attractive.

Now I cannot wait to get out of work so I can read through everything here.

Clear Skies,
Jason

#19 Jon Isaacs

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Posted 06 July 2012 - 06:55 AM

Transportability might not be as bad you think. For my 10" f/9 I used a Yakima kayak roof rack



With a 90 inch focal length, it must have required a substantial stool or probably a ladder. How did you carry that?

Jon

#20 Jeff Morgan

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Posted 06 July 2012 - 10:00 AM

Transportability might not be as bad you think. For my 10" f/9 I used a Yakima kayak roof rack



With a 90 inch focal length, it must have required a substantial stool or probably a ladder. How did you carry that?

Jon



The tube was fairly light with the mirror cell removed, it was just a Hastings aluminum irrigation tube. My car was an Accord, so the roof rack was at a manageable height. I used to have a photo of the car with the scope atop, it looked like a large JATO rocket. At that time I lived in downtown Chicago, so it was a little awkward to swing the tube around corners (i.e., going up the stairway to my condo). You learn not to be in a hurry when carrying it. The mirror cell had it's own transport box which went on the backseat floor.

For high elevation viewing I used a short wood step ladder. It could also fit in the trunk. (At the zenith I only needed about 20-24" of lift.) In hindsight it was a very workable arrangement, I even drove it down to the Winter Star Party. Selling that scope was the single largest mistake I have made in this hobby. But I guess most of us have one of those sellers remorse moments, don't we?

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#21 Mark Harry

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Posted 06 July 2012 - 10:05 AM

Yeah, once or twice a year!
M.

#22 Mr Magoo

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Posted 06 July 2012 - 12:14 PM

Nice Jeff! I like the finder placement.

#23 Pinbout

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Posted 06 July 2012 - 09:37 PM

here's another take on a split tube.

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#24 rgm40

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Posted 08 July 2012 - 04:11 PM

Lots of good info here: http://stellafane.or.../dob/index.html

#25 jgibson1@emich

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Posted 11 July 2012 - 12:01 PM

So, after a fair bit of research and scratching my head I have 2 designs roughly sketched out on paper. Both designs are similar and differ in the secondary size. One option, my initial idea, incorporates a 0.75” secondary. The other design utilizes a 1.0” secondary. In both cases the views should be great. Yes, the 0.75” setup would probably not give as nice a view at low magnification due to light fall-off (& I have another question about this later!) but this scope’s intended use is for high power planetary, lunar, solar, and binary observing. As such this shouldn’t play a vital role in image quality.

The 1” secondary option is pretty straight forward and will be easy to build in a conventional manor (ie- focuser attached to the outside of the OTA). Now the 0.75” secondary setup will be a bit different story. The focuser will be offset into the OTA as suggested by Jim C. I am looking at utilizing the Kine Optics HC-2 focuser. I have drawn up three variations on this design. The three variations differ only by the amount of offset into the OTA. I am looking at 0.25, 0.50, & 0.75 inch offsets. I like the 0.5” offset as this provides nearly a half inch between the base of the mounting plate for the focuser and the edge of the primary.

@Jim C – Which focuser tube do you have in your HC-1? How far into the OTA / mounting plate does the focuser protrude when racked completely in?

I apologize if I am missing something that was detailed in all the information provided. I cannot find anything on how a 100% and 75% illuminated field is calculated. I really would like to understand this and if I missed it please call me a moron and point me to the location!!! No offense would be taken. ;) I can draw and calculate right triangles all day using the appropriate dimensions of my designs. They all show that the secondary will “see” the entire primary for all possible configurations I am looking at (ie- the light cone from the primary will fit on the entire secondary with as much as 0.117” to spare around the light cone). I just want to understand how this concept of light-fall off and illumination works. I can handle math up to mild calculus………

Thank you everyone for the wonderful input and encouragement!

Clear Skies,
Jason






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