Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

Surplus Shed 27MM x 285mm objective

  • Please log in to reply
8 replies to this topic

#1 texcoco

texcoco

    Mariner 2

  • *****
  • topic starter
  • Posts: 200
  • Joined: 19 May 2018
  • Loc: Mountains of Mexico & visits to the US

Posted 23 February 2021 - 11:50 AM

Anyone used the "27MM DIA OBJECTIVE IN BRASS CELL 285MM FL" (Product Code: L1403) Surplus Shed is offering for use with STEM projects?

 

I'm specifically interested in the quality of the image it produces. I'm thinking of combining it with a 7-21mm zoom and at 285/7, or 40x mag, that might be a bit too much for the lens.

 

 



#2 Diego

Diego

    Viking 1

  • -----
  • Posts: 603
  • Joined: 29 Jul 2003
  • Loc: Cordoba, Argentina South America

Posted 28 February 2021 - 08:07 AM

Hi Texcoco, sounds like a good project.

I don't have access to SS (and that's a good thing otherwise I'd have tons of optical parts).

Actually I'm planning to make something similar for quick views at lowish powers, 30 to 60 x max.

From the focal length described, the objective seems to be around f10 which should have acceptable color correction, and at low powers any CA probably won't show.

In my case I am going to buy one of those cheap monocular zooms and see what I can get out of it after fixing all it flaws.

In theory I can't get only objectives as they seem to be prohibited for import in my country, but a finished product like a monocular or telescope is not an issue 🤷🏼‍♂️

Good luck!

#3 texcoco

texcoco

    Mariner 2

  • *****
  • topic starter
  • Posts: 200
  • Joined: 19 May 2018
  • Loc: Mountains of Mexico & visits to the US

Posted 28 February 2021 - 11:08 PM

We will find out shortly. I went ahead and ordered one. It arrives tomorrow.

 

Keep in mind that cheap monocular have really short focal lengths. The bodies are lengthened to accommodate erecting optics and zooms. I just tested one the other day and the objective tested out to be about f4.7 and the image matched that f ratio.



#4 Diego

Diego

    Viking 1

  • -----
  • Posts: 603
  • Joined: 29 Jul 2003
  • Loc: Cordoba, Argentina South America

Posted 01 March 2021 - 05:19 AM

We will find out shortly. I went ahead and ordered one. It arrives tomorrow.

Keep in mind that cheap monocular have really short focal lengths. The bodies are lengthened to accommodate erecting optics and zooms. I just tested one the other day and the objective tested out to be about f4.7 and the image matched that f ratio.

Yes I'm quite aware of the short FL for moniculars... I'm not expecting anything great, but it will be used for quick peeks at the moon and maybe the planets at low power. I want to leave it permanently set of on a photo tripod, so it has to be relatively compact. A longer OTA will get in the way of storage and portability.

Did the monocular you test have a horrible image? Can you maybe provide a link to the one you have?

I've also considered buying a cheap 40 mm x 400 fl or 30 mm x 300 mm used telescope. A lot are available but I suspect some may have plastic objectives. Unless I can find one locally and test it...I don't have many options.

Keep us posted with your project, I'm quite interested of the outcome! Good luck!

Edited by Diego, 01 March 2021 - 05:22 AM.


#5 luxo II

luxo II

    Gemini

  • -----
  • Posts: 3,163
  • Joined: 13 Jan 2017
  • Loc: Sydney, Australia

Posted 01 March 2021 - 07:26 PM

For a STEM project I wouldn't be fussy about image quality. Doesn't hurt to show kids what's poor, vs what's good (binoculars will do). The really clever kids will start to figure out why the image isn't so great, and what separates the crap from  really good optics. And they'll learn something about optics, in the process, which might captivate them.

 

The first scope I assembled as a kid started with a simple magnifying glass for the objective. You can guess how awful that was, but it did work, even at 5X. But the next attempt I mated a 135mm camera telephoto lens to a 20mm Erfle, and at the time that was something pretty nice.

 

Next came a 50mm finderscope someone gave me. 

 

Then came the scope that got me hooked - discovering the pieces of a beautiful 1880 Cooke 4.5" refractor scattered in the science labs at school, followed by a year refurbishing it and setting this up (with help from the school). The sight of Saturn in that scope was simply jaw-dropping.

 

This might be why I'm now really, really, picky about image quality...


Edited by luxo II, 01 March 2021 - 08:08 PM.

  • Adam Long likes this

#6 texcoco

texcoco

    Mariner 2

  • *****
  • topic starter
  • Posts: 200
  • Joined: 19 May 2018
  • Loc: Mountains of Mexico & visits to the US

Posted 01 March 2021 - 07:35 PM

For a STEM project, I would agree. In my case I'm prototyping a special-use scope for production and need a sharp image.
  • Diego likes this

#7 texcoco

texcoco

    Mariner 2

  • *****
  • topic starter
  • Posts: 200
  • Joined: 19 May 2018
  • Loc: Mountains of Mexico & visits to the US

Posted Yesterday, 04:05 PM

So, the report is in.

 

Long story short, the objective performs nicely. However, take note, it was pulled from a working environment and saw heavy use and abuse. The cell is soldered. The air-spaced achromat seems to be spaced by small metal discs. This particular objective was delivered with many scratches on the inside from being "cleaned" by what appears to be a metal rod. It also had significant debris along the outside edges of the objective.

 

It performed admirably under the circumstances.

 

d9da50fa-497b-49e2-ba6f-e3e05c92e1ea.jpg

c04be09f-db0f-4747-acac-a1fe041732b2.jpg

6c1b3246-fb8b-42ef-8f5a-ab0b32fa06ee.jpg

 

 

bea5340c-55f3-4755-9fd3-a18e74028798.jpg

c7a7915e-fe3c-4606-ba8e-8a81ec148aca.jpg

0d3cfe46-5d15-4cbd-8f7a-8f341c23445f.jpg

 

I compared to a Celestron 70x400mm stopped down to 42mm. Same zoom eye piece, higher quality diagonal.

d40d2aa1-7bac-4f87-a8c5-00f3a6bb7356.jpg

1f09e3e3-1681-4664-ba5f-875a892a57b2.jpg

a771e075-9d35-418b-85ce-6928289b05db.jpg


Edited by texcoco, Yesterday, 04:10 PM.


#8 Diego

Diego

    Viking 1

  • -----
  • Posts: 603
  • Joined: 29 Jul 2003
  • Loc: Cordoba, Argentina South America

Posted Today, 06:31 AM

Cool!! Love the quick set up for testing!

Image quality seem about the same as the Celestron, with less contrast but that is probably due to stray light in the open box configuration. Interested in seeing how it performs on the sky.

Edited by Diego, Today, 06:34 AM.


#9 texcoco

texcoco

    Mariner 2

  • *****
  • topic starter
  • Posts: 200
  • Joined: 19 May 2018
  • Loc: Mountains of Mexico & visits to the US

Posted Today, 11:01 AM

Absolutely...the stray light issue was more obvious at night with a street light shining into the open OTA. Just for fun I'll be keeping my eyes open for a closed tube to perform a star test with an astro camera.

Right now I'm looking for how to integrate a suitable focus mechanism. One of the nice things about the cell is that it's outside diameter is just shy of 1.25" so the whole thing fits into a visual back perfectly. You could easily use a helical focuser for the objective.
  • Diego likes this


CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics