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8.75" f/13 doublet, I kid you not

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#51 jragsdale

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Posted 20 December 2021 - 11:49 AM

Could I do DPAC with this thing? Just need to get ahold of a 10" flat, put this thing on some sawhorses and I'd be all set.

You can do an oil flat as well with a pan full of oil and the lens pointing straight down onto it. 


 

#52 Augustus

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Posted 20 December 2021 - 11:49 AM

You can do an oil flat as well with a pan full of oil and the lens pointing straight down onto it. 

I'd need a 10 foot ladder to look down into it!


 

#53 William Whitaker

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Posted 20 December 2021 - 12:43 PM

Could I do DPAC with this thing? Just need to get ahold of a 10" flat, put this thing on some sawhorses and I'd be all set.

Would somebody please enlighten me about what DPAC is? Else direct me to a resource to help explain it and to not distract from this thread.

Inquiring minds and all that...

TIA~


 

#54 Garyth64

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Posted 20 December 2021 - 02:30 PM

I just googled it, lots of information out there.


 

#55 ccwemyss

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Posted 20 December 2021 - 02:32 PM

It's amazing how quickly recent topics roll way down the list here and seemingly disappear.

 

https://www.cloudyni...c#entry11568821

 

Chip W. 


 

#56 Darren Drake

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Posted 20 December 2021 - 02:44 PM

If you don't have a flat you can do what I do and test using a HQ scope and artificial star to produce a star at infinity.  Here's an article I wrote a long time ago about this.  Today I have several focuser insertable artificial stars I use to routinely test scopes indoors...

https://www.cloudyni...null-test-r1586


 

#57 Darren Drake

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Posted 20 December 2021 - 02:47 PM

Here are 2 recently taken pics that show us testing 2 newly acquired scopes for the first time before they even saw starlight. 

Attached Thumbnails

  • IMG_20211220_56559.jpg
  • IMG_20211220_56708.jpg

 

#58 DAVIDG

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Posted 20 December 2021 - 03:00 PM

Could I do DPAC with this thing? Just need to get ahold of a 10" flat, put this thing on some sawhorses and I'd be all set.

 Yes I highly recommend double pass autocollimatiion and test in the green.  Just because it is an Unitron doesn't guarantee that the lens is well figured. There are a number of Unitron  lenses that have tested excellent and some that have not been so good. 

 You don't want to spend time and money to only be disappointed in the image quality. If the lens tests well and the image quality is not what you expect then you know to look elsewhere.

 

                          Happy Holidays,

                              - Dave 


 

#59 Augustus

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Posted 20 December 2021 - 03:09 PM

If you don't have a flat you can do what I do and test using a HQ scope and artificial star to produce a star at infinity.  Here's an article I wrote a long time ago about this.  Today I have several focuser insertable artificial stars I use to routinely test scopes indoors...

https://www.cloudyni...null-test-r1586

 Yes I highly recommend double pass autocollimatiion and test in the green.  Just because it is an Unitron doesn't guarantee that the lens is well figured. There are a number of Unitron  lenses that have tested excellent and some that have not been so good. 

 You don't want to spend time and money to only be disappointed in the image quality. If the lens tests well and the image quality is not what you expect then you know to look elsewhere.

 

                          Happy Holidays,

                              - Dave 

I just realized that my next Dob projects are using 5.5" secondary mirrors which would be a hair under 8" on the major axis. Would using one of these work for a DPAC flat?


 

#60 William Whitaker

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Posted 20 December 2021 - 03:11 PM

 Yes I highly recommend double pass autocollimatiion and test in the green.  Just because it is an Unitron doesn't guarantee that the lens is well figured. There are a number of Unitron  lenses that have tested excellent and some that have not been so good. 

 You don't want to spend time and money to only be disappointed in the image quality. If the lens tests well and the image quality is not what you expect then you know to look elsewhere.

 

                          Happy Holidays,

                              - Dave 

Oh, OK. Double-Pass Autocollimation.

That's the answer I was looking for. Thank you.


Edited by William Whitaker, 20 December 2021 - 03:16 PM.

 

#61 DAVIDG

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Posted 20 December 2021 - 03:41 PM

I just realized that my next Dob projects are using 5.5" secondary mirrors which would be a hair under 8" on the major axis. Would using one of these work for a DPAC flat?

 Yes that would work to give a  you at a look at a good  part of the surface. A majority of optical errors are figures of revolution so if you test across a diameter you would see those errors. I would just be sure to test the area were the chip is to see if there is any problems in that area. 

 

                     Happy Holidays !

                       - Dave  


 

#62 norvegicus

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Posted 22 December 2021 - 06:26 PM

So cool.  Congratulations.  I hope you're still looking through it when you're old and gray.


 

#63 Kiwi Paul

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Posted 03 January 2022 - 02:09 AM

Hi Augustus,
This has been an interesting read. Up until recently I had an 8” f/12 D&G optical tube assembly which was mounted on a GEM built by a fellow by the name of Rick Stager. I saw it advertised in a Sky & Telescope mag back in the early 90’s. It was quite a job to set it up on the GEM even though the mount on a tall 4 inch steel tube pier was was mounted on wheels. Initially to get a look through it I built an AltAz mount which now that I think of it was like a Fobsonian. I built cage that enclosed the tube with about an 18mm plywood. It used hinges so that the telescope could be taken out of the cradle. On each side of the cradle I used a couple of large 4 inch heavy plastic(?) pipe fittings (don’t know what to call them) where they screwed to the wooden cradle and cylindrical part protruded about 3 inches. Then I built a base that sat level on top of the pier with a large circular surface of melamine or such which rode on pieces of Teflon. Off that base I mounted a couple of pieces of ply wound with cutout lined with Teflon into which the cradle around the telescope fitted for the Alt axis. This is essentially a Dobsonian mount I just (I could post some pics of you like). Anyway, I found I could get the scope up there and was able to look through it. The images were fantastic. The definition was sub peen and to the eye the achromat didn’t really produce too much false colour. I hope yours will be ok too. Anyway, later on I did get it mounted on its gem. To cut a long story short, I found that the long tube and associated heavy counterweights lead to a fair bit of flexure or ease of movement of the fov if one touched the telescope. It was a good thing both axes were motorised. I will follow this with a photo of the scope pointed at Jupiter last year.
Cheers Paul
 

#64 Kiwi Paul

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Posted 03 January 2022 - 02:21 AM

I tried posting an image of the telescope but don’t know where it went! It’s not easy on a cell phone!
Paul
 

#65 dave brock

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Posted 03 January 2022 - 05:44 PM

  Just because it is an Unitron doesn't guarantee that the lens is well figured. There are a number of Unitron  lenses that have tested excellent and some that have not been so good. 

 

                            

I'm late to this thread.

Have there been some deleted posts or part thereof? I see no statement that this is a Unitron lens, just a picture with a

Unitron focuser. 


 

#66 Kokatha man

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Posted 03 January 2022 - 07:01 PM

...there were posts deleted as well as part of the thread's title Dave - mods felt that as the provenance wasn't authenticated it wasn't proper to retain that name in the title, nor in some of our replies like my own.....but for my money I'm a believer! (I am an optimist generally - although the admonition as to whether it is a good'un optically  like the one that fella in the Netherlands has - albeit with none of that ones bells & whistles that has - is still valid commentary!


 

#67 dave brock

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Posted 03 January 2022 - 10:28 PM

Thanks Darryl.
It's certainly an impressive beast in post #6. Kind of looks Unitronish. Pity the mount is no longer.
 

#68 Leia

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Posted 13 January 2022 - 05:32 PM

I've only read page 1, but this is one insane big fract


 

#69 deSitter

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Posted 13 January 2022 - 09:14 PM

If you don't have a flat you can do what I do and test using a HQ scope and artificial star to produce a star at infinity.  Here's an article I wrote a long time ago about this.  Today I have several focuser insertable artificial stars I use to routinely test scopes indoors...

https://www.cloudyni...null-test-r1586

Yes this might be a good role for a Questar or ETX! :) I don't have a Questar, but... I could never understand what that N-3C gunsight collimator was in Sam Brown - then one day, oh yeah! That works! :)

 

-drl


 

#70 Augustus

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Posted 14 January 2022 - 11:22 AM

I am about to order a 6" minor axis secondary for my 22" project which I will use to DPAC the lens. While I certainly could use a good scope to produce an artificial star I'd rather just do this with a flat, since I'm getting one anyway.


 

#71 norvegicus

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Posted 14 January 2022 - 11:43 AM

You'll need a ladder. Take pictures!


 

#72 Bowlerhat

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Posted 17 February 2022 - 04:56 AM

https://astromart.co...or-objectiveota

 

Is it for sale?


Edited by Bowlerhat, 17 February 2022 - 04:59 AM.

 

#73 Kasmos

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Posted 17 February 2022 - 06:00 AM

Yes, it was mentioned in the ads thread.


 

#74 jragsdale

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Posted 17 February 2022 - 03:49 PM

Now it's gone, says the account has been suspended. confused1.gif


 

#75 GoodAsh

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Posted 17 February 2022 - 03:56 PM

Now it's gone, says the account has been suspended. confused1.gif


There was a discussion about it on the Vintage Telescope FB site that has since been removed. The guy seemed very upset.
 


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