imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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It's been almost a year since the idea to build this telescope first came about. Ideally this thread would have appeared several months ago when I first began working on the project. Technically it started back in October, but the concept and determination to build this instrument originated circa February of 2008. I spent the summer doing my homework, researching materials, construction methods, optical suppliers and various options for newtonian components. Then the project finally started moving.
In September I settled on a finalized set of dimensions using NEWT.
NOTE: This drawing contains errors and is now obsolete.
I've done my best to juggle the various needs/wants on my list while respecting the limitations of my mount and budget. I wanted a native (unbarlowed, sorry for the made-up terminology) focal length of at least 800mm but probably no more than 1250mm. I chose an 8" aperture at f/5 and worked with NEWT to find an arrangement that can fully illuminate the CCD chip in my QHY8.
Edited by imhotep (03/09/09 01:58 PM)
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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Later in October I pulled the trigger and ordered an 8" f/5 Royce conical primary, which is scheduled for delivery around March 18th. At the time I ordered my mirror (and to my knowledge this is still the case) there was only one commercially manufactured mirror cell that featured a center hole for mounting a Royce conical primary - http://astronomy-mall.com/regular/products/cpt/13inchshortclips.jpg
I think it's an impressive piece of engineering, but I wanted something simpler and if possible, homegrown. I started working on designs for a custom mirror cell with the intention to have the plates precision-cut on a Flow-Jet machine. The assembly would then be doable at home.
CAD drawings of the mirror cell:
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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Both halves of the mirror cell were cut from a single 12"x12" plate of 6061 aluminum.
I took the material and my CAD drawings to a local metal fabricator. The DXF files were used to drive a water-jet machine. The owner permitted me to snap some photos of the machine while it cut the aluminum. I was also able to chat with the technician about the whole process. The mahcine pictured below uses a 60,000 PSI jet of water measuring 0.030 inches in diameter.
I was very pleased with the result:
The hardware for my mirror cell all came from McMaster-Carr. Pictured below from left to right, top row first:
- 1/4-28 stainless knurled thumbscrews, 1.75" shank (collimation bolts)
- Stainless compression springs, 1.5" long, 0.375" OD, 17.68 lb/inch
- Stainless compression springs, 1.25" long, 0.6" OD, 37.49 lb/inch
- 1/4-20 stainless knurled thumbscrews, 1.5" shank, (locking bolts)
- 8-32 stainless socket-head machine screws, 0.75" shank, (mounting bolts)
For lack of experience building mirror cells I ordered two different sets of compression springs to feel them out. One set is substantially stiffer than the other, but both are fully capable of lifting the combined weight of the upper plate plus mirror with as little as 0.25" of deflection.
I threaded varous holes myself and assembled the mirror cell with each set of springs. The following photos show the 0.6" OD springs, but eventually I decided to use the 0.375" OD springs. The both seem fully adequate to lift a 3.5 lb mirror. I chose the narrower springs because they are a better fit to the collimation bolts.
Oblique view:
Bottom view:
Top view:
The project basically halted after finishing the mirror cell. I was waiting on two very important packages to arrive. Just recently one of them did, so the past ten days have seen lots of progress.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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Initially I wantd this instrument to have a CF tube. After researching the process of rolling your own CF I felt like it would be taking on too much at once. I've decided to make the CF tube a long-term goal and use a Hastings tube for the near future. Several folks in this community have done some amazingly good jobs on their own CF upgrades and I look forward to following in their footsteps later this year.
A fellow ATM/astorphotographer gave me the idea to build a dry-run OTA using a cheaper tube material in order to confirm that my camera reaches focus using all of the spacings and dimensions shown in NEWT. The purpose of this exercise is to ensure that I only drill one set of holes in the Hastings tube. A 10" sonotube will be used to build the dry-run OTA. All components will be installed followed by a rough collimation before having first light with a QHY8 CCD camera.
I knocked together this cradle which bolts to my drill press. The cradle provides support and aids in carefully positioning the tube under the bit. The tube can also be clamped or strapped to the cradle.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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One of my goals for the astrograph was to fill a wishlist of bells of whistles that my previous imaigng scope lacked. I chose Moonlight's motorized CR newtonian focuser because it will add several additional technologies to my imaging workflow. Motorized focusing will allow me to stay at the laptop and adjust focus remotely rather than having to lean way over and shake up the mount by manually adjusting focus with my hand. I can also employ various automated focusing utilities, create focusing profiles for multiple cameras and filters, and lastly have the benefit of automated temperature compensation.
Several controller options are available with Moonlight's motorized focusers. I chose the Cercis controller because it can control two focuser simultaneously while running separate temperature profiles for each one. In fact, I believe the Cercis controller is the only choice for those who want temperature compensation. Two temperature probes can be connected as well.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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The focuser hole was cut using a 2-3/8" hole saw attached to a corded hand-drill. I wasn't able to use the drill press because the hole saw is too tall.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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My plan for mounting the mirror cell has evolved over time. Originally I was going to drill and tap threaded holes into the edges of the lower plate. The problem was that it required me to know the ID of the Hastings tube within a very small margin of error. After talking to Andy over at Hastings Pipe CO I realized this was asking too much of irrigation pipe. I opted to leave somewhere between 0.10" and 0.25" of space between between the arms of the mirror cell's base plate and the inner walls of the tube. I had to make this decision back when the mirror cell was being designed in CAD. The revised plan for mounting the mirror cell is as follows.
Mounting brackets for the mirror cell were fabricated using three 1" sections of aluminum L-channel. I bought these pieces at the local metals supplier for less than a dollar each. One side is longer than the other, measuring 1.5" and 0.75" respectively. The brackets are fastened to both the tube wall and the mirror cell using 10-32 screws. I wanted a small amount of adjustablility with respect to the tube's inner diameter, so I routed slots in the face that abuts the mirror cell (rather than single holes).
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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The spider I ordered from Protostar features a couple of bells and whistles. It comes with an integrated dew heater that runs off a 12V power source. Two of the vanes actually act as positive and negative conductors. Wires can be attached on the outside of the tube and then run to the power source. In my case I'll be using a standard RCA plug so I can connect the integrated secondary dew heater right alongside my other dew heater strips (for guidescope, telrad, etc).
The second enhancement is built-in offset. I simply took the offset amount provided by NEWT and requested that Protostar build it into the spider. This is a standard option they offer and well worth it IMHO.
I haven't glued the secondary to the holder yet. This will probably get done either tonight or tomorrow.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Curt,
For a fully illuminated field diameter of 1.313", your diagonal should be offset 0.106" away from the focuser and toward the primary. That centers your FOV on the diagonal aperture (see attached diagram).
Your tube wall thickness looks to be a lot more than the 0.064" value you used in NEWT. But as long as the distance from the primary vertex to the focuser hole plane is 32.45" as you used in NEWT, the diagonal offset of 0.106" is right.
S'gonna be a nice instrument!
Mike
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56 mirrors, lenses, 16" f/6 Newt, 6" f/10 refractor, TOA-130S, Tinsley 5" f/15 Mak, 6" f/4 RFT, Coronado PST. Still to build: 24" f/10 Modified Dall-Kirkham, 10" f/26 Mak, 8" f/12 apo, spectrohelioscope, Herrig, Schupmann, and others.
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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The sonotube dry-run has already paid off. Yesterday when I drilled holes for the spider I forgot to account for the built-in offset. The holes I drilled in the sonotube are spaced 90º apart, but the spacings will be a little off from 90º for this custom spider. The two vanes on either side of the focuser are slightly longer than the other two which accounts for the offset. I'm not sure exactly how far off from 90º the other spacings will need to be, but I can already see my current holes aren't correct. The vanes are flexed slightly to reach them.
Here's the dry-run OTA sitting next to the Hastings tube.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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With all of that said, there's not a whole lot left to do between now and March 18th. I'll have the secondary installed and aligned to the focuser by then. First light will probably be somewhat anticlimactic. I'll confirm that the QHY8 reaches focus, then start transferring the hole pattern to the Hastings tube.
Knock on pyrex for a timely delivery of the primary 
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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imhotep
Pooh-Bah
Reged: 02/14/07
Posts: 1429
Loc: Tampa, FL
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Mike, thanks for posting the illustration. I'm glad you did because it made me realize I posted the wrong NEWT drawing! I've edited the original post to show the correct one. That drawing went through several revisions and I got mixed up when I posted it.
Using the correct drawing, I'm curious if you come up with the same offset of 0.13" that I did.
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Curt
8" F5 Newtonian Astrograph
Orion 80ED CUSTOM
Edmund Optics 4.25" 'Palomar' Newtonian, 1968 vintage
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charles genovese
super member
Reged: 02/04/06
Posts: 132
Loc: Madisonville Louisiana
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To adjust the amount of back focus you simply drill more than one set of holes for the primary mirror - can slide it back and forth to adjust parameters. I would suggest setting the focal plane .5" above the minimum focuser height and have a smaller secondary for planets/visual - a 1.6 or 1.8" secondary (all you need is about .5" 100% illumination). For the astrograph you will need a secondary 2.1 to 2.6 inches , and I would suggest being able to get 3.5 " of back focus (ie the second set of primary mirror mount holes will be 3" farther foreward) to allow for a coma corrector and possibly an off axis guider. Would also suggest the tube extend at least 5-6" of tube beyond the focuser so the imager can't "see" any light from outside the tube. A (Also helps keep the secondary from dewing). Flock the tube- baffles are unnecessary.
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Don't forget to take the real, fabricated mirror focal length into your design. Royce should be able to tell you the final focal length to 1mm accuracy or better.
Your transverse offset is 0.106", but with your vanes rotated 45 degrees to the focuser, the spider vane mounting hole offsets are (0.106)sin(45)=.075" away from the focuser, not much but enough to keep from bending the spider vanes.
REALLY good idea to prototype-drill a disposable Sonotube before final drill on the Hastings tube. I need to go tell Frank in the Yahoo Astrograph group about your idea. He's putting together a 10" f/3.8 astrograph that uses an MPCC to correct the coma, and neither of us are 100% sure how much the MPCC shifts the DSLR focus position. Sonotubing it together first will answer that question with confidence.
Mike
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Chris Curran
scholastic sledgehammer
Reged: 06/14/05
Posts: 827
Loc: Brandon, FL
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0.064 is correct for the final Hastings OTA. The Sonotube of course is not 0.064....
Quote:
Your tube wall thickness looks to be a lot more than the 0.064" value you used in NEWT.
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cheers & beers,
Chris
Homepage | Over 40?
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Chris Curran
scholastic sledgehammer
Reged: 06/14/05
Posts: 827
Loc: Brandon, FL
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Nice work Curt.
BTW, you can use a hand drill and hole saw for the Hastings tube. It'll actually cut easier than the cardboard sonotube...
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cheers & beers,
Chris
Homepage | Over 40?
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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I changed the ZEMAX model to the 31.74" mirror spacing and reduced the FOV diameter to 1.194", as you show in the new diagram. I'm getting the transverse offsets to be 0.112" to the nearest thousandth. The elliptical aperture offset in the plane of the diagonal face is 0.158", and 0.158cos(45)=0.112". Did NEWT do the calculation for the offset? It may be using an approximate formula for decentration rather than using exact raytracing. At lower focal ratios, like Frank's 10" f/3.8 astrograph over in Yahoo, the extra precision of raytracing really matters.
Here's a ZEMAX shaded layout of your astrograph using the values from NEWT above. I only drew half the tube for clarity.
I am getting disturbing results that don't jive with NEWT, though. Using all the values in your NEWT graphic, I'm only getting a focal plane clearance of 0.618 inches above the 2.0125" focuser at its minimum height. Let's go through the math:
Focal length = 1000mm = 39.37 inches
Spacing to diagonal face = 31.74 inches
Diagonal face to focus = 7.63 inches
Tube ID = 9.872, so ID radius = 4.936 inches
Wall thickness = 0.064 inches, so
Tube OD radius = 5.000 inches
So the image plane is 7.63-5.00=2.63 inches above OD
Focuser minimum height = 2.0125 inches,
So image plane clearance above the focuser is 2.63-2.0125=0.6175 inches, which agrees with the ZEMAX model exactly.
Is this enough clearance for you? Or, are there any errors remaining in the NEWT graphic?
Jus' checkin', seems kind of low.
Mike
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Strgazr27
Vendor - Deep Space Mods and Composites
Reged: 10/04/04
Posts: 6984
Loc: StonyHill Observatory
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Curt,
I'm working on a similiar design only F/4 so this is perfect timing. Looking at your NEWT drawing I see a 100% FOV of 1.19 inches which is roughly 30mm. This is still smaller than the diagonal distance of the QHy's chip. I know it's only about 2mm shy but will this make a difference to you? I will be using an MPCC with mine and was curious if my 100% FOV needs to be bigger than your 1.19. Maybe Mike could answer this.
That hastings tube is nice! When your ready for a CF tube for it let me know.
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Bobby
StonyHill Observatory
Skywatcher EQ6 Pro (On it's way)
AT 130 F/6 Prototype
Royce 10" f/4 Astrograph (Under Construction)
Self Modded 40D
70-200 F2.8 L IS
SSAG
YAHOO TMB 130SS Group
Astronomy Technologies Yahoo Group
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Mike I. Jones
Carpal Tunnel
Reged: 07/02/06
Posts: 1572
Loc: Fort Worth TX
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Found some information on the MPCC at http://www.baader-planetarium.com/pdf/mpcc_e.pdf. It says the MPCC doesn't change the primary EFL, but didn't indicate if the position of the primary focal point is shifted or by how much.
Losing a little illumination out in the corners isn't a big deal as it can be compensated for by flat fielding, up to around 1 to 1 1/2 stops max.
If you have any detailed information on the MPCC that would be most helpful here.
Mike
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Strgazr27
Vendor - Deep Space Mods and Composites
Reged: 10/04/04
Posts: 6984
Loc: StonyHill Observatory
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Mike,
Baader says it adds about 10mm to the backfocus but doesn't change the in focus requirement. That and it supposedly will produce 12 micron stars at the edge of a 35mm chip is about all I could dig up.
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Bobby
StonyHill Observatory
Skywatcher EQ6 Pro (On it's way)
AT 130 F/6 Prototype
Royce 10" f/4 Astrograph (Under Construction)
Self Modded 40D
70-200 F2.8 L IS
SSAG
YAHOO TMB 130SS Group
Astronomy Technologies Yahoo Group
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