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23.5" f/3.6 Dob

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#1 Augustus

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Posted 05 June 2020 - 02:49 PM

So a while back some of you may have seen Steve Dodds's chipped 24" f/3.5 mirror. I've been interested in it from the get-go and was very happy to see this DPAC image today, upon which I immediately bought it:

 

dpaccropped.jpg

 

Apart from the miniscule 1.5" x 3" chip out of the mirror and some very slight TDE that needs to be masked, it's very good - certainly what I'm happy with, especially for what I paid. I plan to mask the outer 1/4" so the scope is basically going to be a 23.5" f/3.6. I'm ignoring the additional 3.75 in2 area loss from the chip because that's roughly equivalent to enlarging my secondary mirror from the planned 4.5" or 4.7" to 5".

 

Now, you're probably wondering: "Zane, don't you have a hatchback and not an SUV? Didn't you literally say you thought it was impossible to fit this scope in there?" Yes, indeed, for a while I thought this project was impossible (mostly for financial reasons, but truth be told I really didn't think I could fit it in my car), until I stumbled upon tommm's fantastic build thread, which I highly recommend checking out. His 22.5" f/3.6 has a complete Servocat system and fits in a Chevy Bolt, which has a trunk that's several inches shorter than mine (and again, he has a Servocat that adds to the height by a couple inches) so in some respects I will be very much emulating his design with a slight scale factor. However, my scope will use a drum shell for the UTA.

 

The mirror cell will be constructed and welded by me. Currently the plan is an 18-point cell made out of steel with 90-degree edge supports (which the Cruxis site says are more than sufficient). I know a couple folks locally who I am hoping can help with machining a spider and altitude bearings, and I plan to buy the New Moon CTS for my trusses. Total weight is something like 220 pounds if I recall correctly, but I'll be loading it into the car on ramps and moving it around on wheels.

 

This is easily the hardest and most complicated project I've ever done and the biggest telescope I will likely ever build. The idea is to use this for around 50% of my observing with the 14.7" for around 35-40% and all other scopes filling in the remainder. Will start building once I've got the mirror in hand (so probably a couple weeks).

 

 

 

 


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#2 Barlowbill

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Posted 05 June 2020 - 03:22 PM

I know it will be fantastic.  I enjoy your threads.


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#3 Tangerman

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Posted 05 June 2020 - 03:37 PM

Will you be updating this thread with your progress?


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#4 Augustus

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Posted 05 June 2020 - 03:39 PM

Will you be updating this thread with your progress?

Yep!


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#5 Pinbout

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Posted 05 June 2020 - 04:26 PM

 

Apart from the miniscule 1.5" x 3" chip out of the mirror and some very slight TDE that needs to be masked, it's very good -

dpac makes it look 2x's worse than it really is.


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#6 Diego

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Posted 05 June 2020 - 05:48 PM

Sounds like a cool project and specially the price for such a huge mirror (according to the ad few hundred dollars!)

You could even completely mask the outer 1.5" edge and still have an excellent 20.5" telescope.

Good luck on the new job!
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#7 N3p

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Posted 05 June 2020 - 06:09 PM

Very nice project Augustus.


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#8 starspangled

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Posted 05 June 2020 - 09:54 PM

Do you know the frequency of the grating used in that DPAC test ?


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#9 Steve Dodds

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Posted 06 June 2020 - 08:46 AM

100 LPI DPAC


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#10 tommm

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Posted 06 June 2020 - 11:32 AM

You are a busy guy Augustus!  Thanks for the kind words.  The Goto system is not a Servocat, it is diy using OnStep, and adds 5/8" height to the scope (due to the Az drive disk), not 2". Well, actually a bit less I guess because if the rocker sat on teflon pads on the ground board (push-to scope), those would be around 3/16" thick, so 3/16" spacing between rocker and grd bd.  The scope has 5/8" spacing. The mirror box passes within about 1" of the rocker box bottom when rotated.  The head of the 1/2" diam Az pivot bolt in the center sticks up about 1/4".

 

Keep in mind that you have to account for the back of the grd bd being lifted up as you roll the scope into the car when considering height, and that also lifts the front of the Alt bearings a bit since their front is typically behind the wheels.  You can of course set the UTA on the scope AFTER the scope is in the car so the combination doesn't have to fit through the back opening if it is limiting.  You have a young agile mind so you will figure all this stuff out.

 

I'm looking forward to seeing how it works out for you!


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#11 Augustus

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Posted 06 June 2020 - 11:41 AM

You are a busy guy Augustus!  Thanks for the kind words.  The Goto system is not a Servocat, it is diy using OnStep, and adds 5/8" height to the scope (due to the Az drive disk), not 2". Well, actually a bit less I guess because if the rocker sat on teflon pads on the ground board (push-to scope), those would be around 3/16" thick, so 3/16" spacing between rocker and grd bd.  The scope has 5/8" spacing. The mirror box passes within about 1" of the rocker box bottom when rotated.  The head of the 1/2" diam Az pivot bolt in the center sticks up about 1/4".

 

Keep in mind that you have to account for the back of the grd bd being lifted up as you roll the scope into the car when considering height, and that also lifts the front of the Alt bearings a bit since their front is typically behind the wheels.  You can of course set the UTA on the scope AFTER the scope is in the car so the combination doesn't have to fit through the back opening if it is limiting.  You have a young agile mind so you will figure all this stuff out.

 

I'm looking forward to seeing how it works out for you!

Thanks!

 

About how much would you say it cost to add that system? If it's not too pricey I might consider it for this scope.



#12 jtsenghas

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Posted 06 June 2020 - 12:40 PM

How will you actually get that mirror from Utah to Connecticut.  I saw Steve's comments about not wanting to ship it again in his heartbreaking UPS story in his thread last week. 



#13 mconnelley

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Posted 06 June 2020 - 01:46 PM

Hello:
 

   I imagine that a 24" should fit fairly easily into a hatchback.  I have a 20" f/4 and a Honda Fit.  With the scope (and me) in the car, there is still room in the car for my wife, some chairs, warm clothes, snacks, etc.  I think I've even thrown in another small scope into the car with the 20".  A friend of mine is going to have a much more 'interesting' time trying to fit his 20" into his Tesla.  

Cheers

Mike



#14 Pinbout

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Posted 06 June 2020 - 01:49 PM

How will you actually get that mirror from Utah to Connecticut.  I saw Steve's comments about not wanting to ship it again in his heartbreaking UPS story in his thread last week. 

uber optics... ha ha ha



#15 Steve Dodds

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Posted 06 June 2020 - 01:58 PM

It will be shipped UPS or FedEx It's in a fairly bulletproof box., the only reason it got broke is UPS opened it and through away all the packing material. It will be fine unless they do it again.



#16 Pinbout

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Posted 06 June 2020 - 02:00 PM

 

UPS opened it and through away all the packing material.

who does that.... smh



#17 grzesznypl

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Posted 06 June 2020 - 02:35 PM

who does that.... smh

UPS, lol

I would not ship mirror via UPS, those people are unenlightened halfwits. I have limited experience shipping it via USPS, nothing ever got damage.



#18 Pinbout

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Posted 06 June 2020 - 03:06 PM

UPS, lol

I would not ship mirror via UPS, those people are unenlightened halfwits. I have limited experience shipping it via USPS, nothing ever got damage.

i still like uber optics.... 


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#19 Steve Dodds

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Posted 06 June 2020 - 04:45 PM

UPS, lol

I would not ship mirror via UPS, those people are unenlightened halfwits. I have limited experience shipping it via USPS, nothing ever got damage.

Can't ship it USPS it's 120 lbs.  Maybe FedEx.


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#20 tommm

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Posted 06 June 2020 - 05:07 PM

Thanks!

 

About how much would you say it cost to add that system? If it's not too pricey I might consider it for this scope.

I paid about $50.00 each for the Steppersonline "precision" planetary gear motors, with < 25 arcmin backlash. Alt is 30:1, Az is 50:1. Overall reduction on both axes is a bit over 900:1, which is not good enough for AP but fine for visual at about 0.45 arcsec/step. Timing pulleys, belts, shaft, bearings, etc., added close to $100.00 to that. 

 

The OnStep PC board and components I think were less than $150.00. I ordered the minimum 5 boards from the supplier. Every once in a while someone offers some extra PC boards for sale on the OnStep groups.io site, so you might get one cheaper. 

 

Howard supplies the Gerber files for the boards.  Howard also supplies the firmware for the board (Arduino sketch) and the App for interfacing to the scope for free.  So altogether, as best I recall it was around $350.00 - $400.00, less than a new P2.


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#21 Augustus

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Posted 06 June 2020 - 11:12 PM

I paid about $50.00 each for the Steppersonline "precision" planetary gear motors, with < 25 arcmin backlash. Alt is 30:1, Az is 50:1. Overall reduction on both axes is a bit over 900:1, which is not good enough for AP but fine for visual at about 0.45 arcsec/step. Timing pulleys, belts, shaft, bearings, etc., added close to $100.00 to that. 

 

The OnStep PC board and components I think were less than $150.00. I ordered the minimum 5 boards from the supplier. Every once in a while someone offers some extra PC boards for sale on the OnStep groups.io site, so you might get one cheaper. 

 

Howard supplies the Gerber files for the boards.  Howard also supplies the firmware for the board (Arduino sketch) and the App for interfacing to the scope for free.  So altogether, as best I recall it was around $350.00 - $400.00, less than a new P2.

Okay, that's pretty tempting.....



#22 grzesznypl

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Posted 07 June 2020 - 01:07 AM

Can't ship it USPS it's 120 lbs.  Maybe FedEx.

How thick is that mirror???



#23 a__l

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Posted 07 June 2020 - 05:53 AM

This chip will most likely develop with changes in temperature and will generate astigmatism. Increasing over time.
The best option is to make a wide bevel and perhaps(?) re-polish the mirror.

The first photo has a similar problem. 

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  • скол.jpg

Edited by a__l, 07 June 2020 - 06:04 AM.


#24 a__l

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Posted 07 June 2020 - 06:05 AM

In the second photo, the optician made a wide bevel. The surface of the finely ground mirror is protected by a film.

 

By TDE. It is better to make the frame a smaller diameter in order to cover it with aluminum in a vacuum chamber. The outside diameter of the mirror remains clean glass.

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Edited by a__l, 07 June 2020 - 06:36 AM.


#25 Augustus

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Posted 07 June 2020 - 09:57 AM

How thick is that mirror???


2” but I think the crate is about fifty pounds.
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