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

1.25 vs 2"

  • Please log in to reply
10 replies to this topic

#1 Don-richardo

Don-richardo

    Lift Off

  • -----
  • topic starter
  • Posts: 23
  • Joined: 12 Jun 2018
  • Loc: tupelo ms.

Posted 09 November 2019 - 09:36 PM

hello folk's. I have an older C8 "79 model" and was thinking of going to a 2" visual back and EP's is there a really advantage to switching to 2"? 

thanks! 



#2 noisejammer

noisejammer

    Fish Slapper

  • *****
  • Posts: 3608
  • Joined: 16 Sep 2007
  • Loc: The Uncanny Valley

Posted 09 November 2019 - 10:04 PM

Don, the short answer is "Yes, but..."

 

The long answer is the C8 is internally constricted to about 38mm. This means that any eyepiece with an entrance pupil larger than 38mm will be a bit vignetted. On the other hand, a 38mm entrance pupil would allow you to have a max true field of about 1.1 degrees while the 1.25" format limits you to about 0.8 degrees. You could achieve pretty much the same if you used a 0.63x reducer.

 

The second reason to consider the 2" format is that you can use eyepieces with 2" barrels or robustly install binoviewers. Be aware that this can make a large dent in your savings.


  • Jon Isaacs, bbqediguana and Ohmless like this

#3 Don-richardo

Don-richardo

    Lift Off

  • -----
  • topic starter
  • Posts: 23
  • Joined: 12 Jun 2018
  • Loc: tupelo ms.

Posted 09 November 2019 - 11:07 PM

Lol. I thought as much. Thanks

#4 SeattleScott

SeattleScott

    Fly Me to the Moon

  • *****
  • Posts: 6096
  • Joined: 14 Oct 2011

Posted 09 November 2019 - 11:29 PM

Max FOV of C8 is 1.22 degrees. You can get there with 2” diagonal or focal reducer. The advantage of focal reducer is it is cheaper, and cleans up a little field curvature at the edges. The disadvantage is you lose a little contrast for high power lunar/planetary observing, so many will remove the reducer for planetary. Just adds a little bit of hassle taking it on and off.

Scott
  • Viktorious likes this

#5 Sky Muse

Sky Muse

    Fly Me to the Moon

  • *****
  • Posts: 7130
  • Joined: 26 Oct 2012
  • Loc: Mid-South, U.S.

Posted 10 November 2019 - 12:35 AM

I got a 127mm(5") f/15 Maksutov back in May... 

 

Maksutov5b.jpg

 

It has a focal-length of 1900mm(!), and quite close to that of a C8 Schmidt at 2032mm.  I got the Maksutov knowing full well and accepting of its limitations as an all-rounder; long focal-length = rather narrow views at medium-to-high powers.  It is what it is.

 

Back in 2012, I got this 6" f/5 Newtonian...

 

6 f5z2.jpg

 

Now, that one is an all-rounder, and with a 750mm focal-length.  Powers range from a low and wide 23x, up to 200x and beyond with the aid of 2x and 3x barlows, or very short focal-length eyepieces.  But mine is very basic, Synta's most basic, and with a plastic 1.25" focusser.

 

This one, however, has a 2", metal focusser, and configured to make the best use of 2" eyepieces...

 

https://www.highpoin...tonian-ota-6f5n


  • Ohmless likes this

#6 faackanders2

faackanders2

    Fly Me to the Moon

  • *****
  • Posts: 6022
  • Joined: 28 Mar 2011

Posted 10 November 2019 - 12:48 AM

hello folk's. I have an older C8 "79 model" and was thinking of going to a 2" visual back and EP's is there a really advantage to switching to 2"? 

thanks! 

Yes for wider TFOV eyepieces than 24mm Panoptic 68 AFOV, up to 40mm 70 AFOV.

 

P.s. Answer may not be appropriate to a SCT that I do not have.


Edited by faackanders2, 10 November 2019 - 12:50 AM.


#7 stevew

stevew

    Now I've done it

  • *****
  • Posts: 6135
  • Joined: 03 Mar 2006
  • Loc: British Columbia Canada

Posted 10 November 2019 - 01:56 AM

I have an F-6.3 reducer for my C8, but in all honesty I prefer a good 2 inch 40mm eyepiece over the reducer for low power views.

2 inch eyepieces typically have a larger field of view than 1.25 eyepieces.


  • Ernest_SPB likes this

#8 Don-richardo

Don-richardo

    Lift Off

  • -----
  • topic starter
  • Posts: 23
  • Joined: 12 Jun 2018
  • Loc: tupelo ms.

Posted 10 November 2019 - 06:18 AM

Thank you folks. I may have to ask santa for one lol.

#9 Bataleon

Bataleon

    Vostok 1

  • -----
  • Posts: 162
  • Joined: 29 Aug 2019

Posted 10 November 2019 - 06:56 AM

If you're strictly doing "tight work" like planetary, lunar, globs and doubles, you probably won't get much out of going 2", but if you like nebula hopping like me, it does open up a wider field of view. 2" eyepieces won't do much for you for higher magnification of the aforementioned objects, mainly because large, wide angle eyepieces distort the image around the edges unless you're talking about very expensive high end units and also, if you're doing planets, the moon, stars and such, you're not looking for wide angle anyway. Magnification and contrast trumps FOV in this case. However, for those lower power, wide field targets like nebulae and open clusters, this is where 2" equipment really shines. I have a newer C8 and while I do have a 2" diagonal in it, my only 2" EPs are a TV 55mm plossl and a Meade 5000 UWA 20mm and that's primarily what I use them for; glassing nebulae and open clusters.

Edited by Bataleon, 10 November 2019 - 07:02 AM.


#10 Ronofthedead07

Ronofthedead07

    Vostok 1

  • *****
  • Posts: 152
  • Joined: 07 Oct 2018
  • Loc: New Hampshire

Posted 11 November 2019 - 10:17 AM

I have an F-6.3 reducer for my C8, but in all honesty I prefer a good 2 inch 40mm eyepiece over the reducer for low power views.

2 inch eyepieces typically have a larger field of view than 1.25 eyepieces.

When I had a C8 I put a 2" SCT diagonal on it and plopped in a 35mm Panoptic. It was easily my most used EP with that scope, followed by the 22mm Nagler (another 2" only eyepiece). Granted, I do more DSO observing than anything else.

 

I've heard that using the f/6.3 reducers with longer focal length eyepieces (like a 24mm Panoptic) can result in severe vignetting, although I've never tried myself.

The 35 Pan has a field stop diameter of 38.7 mm which seems nearly tailor made for the rear baffle of a C8...but several have used wider 40+mm eyepieces without an issue.

 

Any 2" eyepiece with a field stop diameter larger than 27mm or so will have a larger field of view than a 1.25" barrel can accommodate. This is important if you want to view fainter objects requiring a larger exit pupil without constricting yourself to a narrow apparent field. With a 2" setup you can use 50 degree eyepieces up to 56mm focal length, 68-70 degree eyepieces up to about 40mm focal length and 80-82 degree eyepieces up to about 30mm focal length. In 1.25" you are restricted to about 50 degrees at any focal length between 25mm and 32mm, and about 40-45 degrees at focal lengths longer than 32mm.


  • stevew likes this

#11 noisejammer

noisejammer

    Fish Slapper

  • *****
  • Posts: 3608
  • Joined: 16 Sep 2007
  • Loc: The Uncanny Valley

Posted 11 November 2019 - 11:14 AM

I've heard that using the f/6.3 reducers with longer focal length eyepieces (like a 24mm Panoptic) can result in severe vignetting, although I've never tried myself.

There is slight vignetting. To see this, consider the 38mm entrance pupil of the reducer & multipy by .63 .. this gives 24mm.

The vignetting can be minimised by reducing the distance between reducer and eyepiece - at the cost of some field and some field flatness.

 

I do concur though that the 35Pan is very well suited to the C8. I've never used the 22T4 but my 22Pan was another excellent match for the instrument (even though it can in theory be used in a 1.25" diagonal.)




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