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

GSO 8-inch TRUE CASSEGRAIN

  • Please log in to reply
38 replies to this topic

#1 Larry Carlino

Larry Carlino

    Sputnik

  • *****
  • topic starter
  • Posts: 41
  • Joined: 05 Jun 2006

Posted 13 December 2019 - 02:44 PM

GSO Cass can be regarded as somewhat of a specialist instrument. Its excellent overall optical quality, fine lunar and planetary performance, reasonable size and weight, and bargain price make it a fine choice where sterling deep-sky and rich-field capability are not a priority. It is a good alternative to much pricier 5 to 6-inch apochromatic refractors for both visual and ccd work. In the price- to- performance ratio, I don't see anything in its price range that comes close (except, perhaps for a high-quality long-focus Newtonian [but try to find one!]) as a dedicated lunar and planetary instrument.

Click here to view the article
  • zjc26138, retroman2, NZRoger and 1 other like this

#2 nirvanix

nirvanix

    Vanguard

  • -----
  • Posts: 2,130
  • Joined: 07 Jun 2007
  • Loc: Medicine Hat, AB

Posted 13 December 2019 - 04:41 PM

Nice review. I think this scope fills a niche of color-free lunar/planetary/doubles work without having to take out a second mortgage.


  • Venetia2004 and EverlastingSky like this

#3 RichA

RichA

    Vanguard

  • *****
  • Posts: 2,323
  • Joined: 03 Jun 2010
  • Loc: Toronto, Canada

Posted 14 December 2019 - 08:07 PM

Good review, thank you.  However, the claimed effective reduction in aperture would not account for noticeable differences between it and the C8, especially since the C8 has  two more optical surfaces.  It's likely the mirror reflectivity is less than claimed, or less than implied at least.  It does sound like a good scope though.


  • Live_Steam_Mad, jgroub and mikeDnight like this

#4 highfnum

highfnum

    Fly Me to the Moon

  • *****
  • Posts: 5,016
  • Joined: 06 Sep 2006
  • Loc: NE USA

Posted 16 December 2019 - 08:31 AM

got one

good scope real snap focus

but very sensitive to any alignment errors 

imho more so than sct's


  • nirvanix likes this

#5 csean97

csean97

    Lift Off

  • *****
  • Posts: 5
  • Joined: 20 Jun 2009
  • Loc: Ohio

Posted 16 December 2019 - 04:11 PM

Having read the Dec 19 S&T test report of the 10 inch cass, I'm wondering if it has the same reduction in aperture issue as the 8 inch.  The review does mention a "small annulus of light around the secondary mirror."  So is this because the secondary is too small?



#6 project nightflight

project nightflight

    Vostok 1

  • *****
  • Posts: 111
  • Joined: 14 Sep 2014
  • Loc: Vienna, Austria

Posted 16 December 2019 - 05:31 PM

Thank you for your review! I have been waiting for some time for a decent review of the GSO 8" Cass. Your comparison of the OTA with other instruments is especially appreciated.

 

BTW, there is a report on CN that the focuser of these GSO scopes comes with a tilted tube: https://www.cloudyni...egrain-focuser/

Can you confirm the focuser problem or was this merely a single flawed unit?

 

Again, thank you, and all the best.


  • BinoGuy likes this

#7 james7ca

james7ca

    Cosmos

  • *****
  • Posts: 8,871
  • Joined: 21 May 2011
  • Loc: San Diego, CA

Posted 16 December 2019 - 11:33 PM

One thing that never seems to get any notice is that these scopes apparently come with quartz mirrors (both primary and secondary). That's often a very expensive option and it should aid with cool down and help to prevent focus/figure change with temperature.

 

I've not found a single mention of this on CN nor in any online reviews. However, both Orion (who second sources this same scope) and a few other retailers mention this in the specs (including Astronomics, for their rebranded Astro-Tech CC).

 

Oddly, however, this scope seems to be missing from the GSO website, so there is no way to confirm this from the actual manufacturer.

 

Could the quartz mirrors just be a false report that has been repeated by a few of the retailers?



#8 nateman_doo

nateman_doo

    Viking 1

  • *****
  • Posts: 918
  • Joined: 17 Sep 2017
  • Loc: Dirty Jersey

Posted 27 December 2019 - 05:22 PM

So can I ask a simple question.... why do you call it a true cassegrain? Its an R/C scope.  



#9 Larry Carlino

Larry Carlino

    Sputnik

  • *****
  • topic starter
  • Posts: 41
  • Joined: 05 Jun 2006

Posted 27 December 2019 - 09:09 PM

An R/C design uses hyperbolic primary AND secondary mirrors.  The true Cass has a parabolic primary and hyperbolic secondary.

LC


  • Venetia2004 and Live_Steam_Mad like this

#10 careysub

careysub

    Mercury-Atlas

  • *****
  • Posts: 2,626
  • Joined: 18 Feb 2011
  • Loc: Rancho Cucamonga, CA

Posted 28 December 2019 - 04:33 PM

The solution: Use a smaller secondary [still a roughly 33% obstruction] and reduce the effective aperture of the scope to 7.34 inches(186.5 mm).

 

Seems like that makes it an F/13.0, since the literature gives an explicit focal length of 2436 mm.


  • jonbosley and BinoGuy like this

#11 GeneT

GeneT

    Ely Kid

  • *****
  • Posts: 15,812
  • Joined: 07 Nov 2008
  • Loc: South Texas

Posted 29 December 2019 - 06:14 PM

Great review!


  • Venetia2004 likes this

#12 Astrojedi

Astrojedi

    Aurora

  • *****
  • Posts: 4,822
  • Joined: 27 May 2015
  • Loc: SoCal

Posted 29 December 2019 - 08:38 PM

Thanks for posting. Excellent, well balanced and objective review.



#13 EverlastingSky

EverlastingSky

    Apollo

  • *****
  • Posts: 1,030
  • Joined: 12 Sep 2006
  • Loc: Vancouver Canada

Posted 01 January 2020 - 02:31 PM

Good review of this interesting instrument waytogo.gif 

 

Just my observation: It appears the GSO 8" Cassegrain has the venerable C8 as its main rival to "overcome" so to speak. And the thing that is off-putting more than reduced brightness is the weight. I sure wish GSO could have made the weight of their 8" Cassegrain more competitive with the C8 which can be as little as 11 pounds bare OTA. I imagine that maybe all those baffles and huge focuser are adding many pounds to the weight?



#14 Live_Steam_Mad

Live_Steam_Mad

    Viking 1

  • -----
  • Posts: 946
  • Joined: 24 Jul 2007

Posted 02 January 2020 - 08:27 PM

I see that the Teleskop Service rebranded version of this scope has 99 per cent reflective Dielectric mirror coatings instead of 96 per cent Enhanced Aluminium. I'd like to see a test of the TS 8" CC versus the Celestron C8 XLT or Meade 8" ACF UHTC to see how "dim" or not the 8" CC is of TS. Also I am not keen on the non-moving primary mirror focus in this CC that won't allow large changes in back focus to accomodate a Binoviewer or and Atmospheric Dispersion Corrector (ADC) for planets viewing. The weight of this CC isn't a concern to me until the aperture gets to 10" and over, then the Celestron SCT seems to be much lighter.


  • eq3d likes this

#15 Charles Kirk

Charles Kirk

    Lift Off

  • -----
  • Posts: 20
  • Joined: 23 Dec 2019
  • Loc: London, UK

Posted 08 January 2020 - 02:26 PM

Having read the Dec 19 S&T test report of the 10 inch cass, I'm wondering if it has the same reduction in aperture issue as the 8 inch.  The review does mention a "small annulus of light around the secondary mirror."  So is this because the secondary is too small?

I don’t think so, it looks like they kept the secondary baffle size quite small to reduce the secondary obstruction to improve contrast on fine lunar and planetary detail. If the specification is for 100 percent illumination over a 15mm circle that implies no aperture reduction other than that implied by having a secondary mirror and baffle in the light path.


  • eq3d likes this

#16 David Knisely

David Knisely

    Hubble

  • *****
  • Posts: 17,022
  • Joined: 19 Apr 2004
  • Loc: southeastern Nebraska

Posted 16 January 2020 - 03:53 AM

I think the Cave Cassegrains were actually Dall-Kirkhams rather than the pure Classical Cassegrain design.



#17 Wildetelescope

Wildetelescope

    Vanguard

  • -----
  • Posts: 2,116
  • Joined: 12 Feb 2015
  • Loc: Maryland

Posted 16 January 2020 - 10:02 AM

Thank you for the interesting comparison.  What is the advantage to the classical cassegrain over the Mak and the SCT, if the central obstruction is of a similar size and the Focal ratios are more or less comparable (f10-F15)?   It has always been my understanding that what made the CC of interest was that you could get Focal ratios of 15-20 in a relatively short package, enabling the use of a smaller central mirror.  Is there something inherent in the design of the cassegrain optics themselves that you believe is responsible for what you saw at the EP? Or is it just superior execution of the design relative to the Synta  Mak, which SHOULD have the longest Focal ratio at f15, an smallest CO of the compound scopes you tested.   Just curious!  Really appreciate the review.

 

JMD


  • Venetia2004 likes this

#18 Larry Carlino

Larry Carlino

    Sputnik

  • *****
  • topic starter
  • Posts: 41
  • Joined: 05 Jun 2006

Posted 16 January 2020 - 10:46 AM

According to my 1974 Cave Optical catalog, the 8-inch Cass was a true parabolic/hyperbolic design. I owned one of these many years ago. The 10, 12, and 16-inch scopes were of the DK configuration with true Cass optics being an extra-cost option.

Thanks one and all for your kind comments on my GSO Cass review.

Larry C.



#19 Charles Kirk

Charles Kirk

    Lift Off

  • -----
  • Posts: 20
  • Joined: 23 Dec 2019
  • Loc: London, UK

Posted 16 January 2020 - 04:08 PM

What is the advantage to the classical cassegrain over the Mak and the SCT, if the central obstruction is of a similar size and the Focal ratios are more or less comparable (f10-F15)?

I think that there are two main advantages, the first is there is no corrector plate to dew up.

The second is that there is very little coma compared to the contentional SCT.

At these f ratios the Classical Cassegrain performs as well as or better than an RC http://www.dreamscop...04/ccvrc-04.htm.

Maks in the Gregorian configuration are limited to a maximum aperture of around 7 inch / 180mm.

Of course there is one disadvantage which is the presence of diffraction spikes from the secondary support.


Edited by Charles Kirk, 16 January 2020 - 04:10 PM.


#20 David Knisely

David Knisely

    Hubble

  • *****
  • Posts: 17,022
  • Joined: 19 Apr 2004
  • Loc: southeastern Nebraska

Posted 22 January 2020 - 12:13 AM

According to my 1974 Cave Optical catalog, the 8-inch Cass was a true parabolic/hyperbolic design. I owned one of these many years ago. The 10, 12, and 16-inch scopes were of the DK configuration with true Cass optics being an extra-cost option.

Thanks one and all for your kind comments on my GSO Cass review.

Larry C.

Actually, some of their optics were made, not by Cave, but by 3B Optics, and those may have been the Dall-Kirkhams that were put out.  We had a fairly nice 1960's vintage 8 inch f/15 Cave Cassegrain at Hyde Observatory on long-term loan from Nebraska Wesleyan University in Lincoln, NE, and it was the first scope I ever saw the companion to Sirius in (the 2nd was our C14).  We eventually replaced it with an 11 inch Celestron NexStar SCT, which outperformed it notably.   Clear skies to you.


  • Jon Isaacs likes this

#21 David57

David57

    Lift Off

  • *****
  • Posts: 23
  • Joined: 19 Sep 2015
  • Loc: Eastern North Carolina

Posted 24 January 2020 - 10:31 PM

Great review! Thanks for posting. 



#22 Upstate New Yorker

Upstate New Yorker

    Vostok 1

  • -----
  • Posts: 123
  • Joined: 27 Nov 2018

Posted 16 February 2020 - 10:00 AM

This is a clear, authoritative review that explains the technical specs of the revived classical Cassegrain telescopes, as well as their strong and weak suits.  The discussion of the resistance to dew by these scopes was an added bonus, one that was especially helpful for those of us living east of the Mississippi.  Thanks.  



#23 CGAstronomer

CGAstronomer

    Lift Off

  • *****
  • Posts: 7
  • Joined: 12 Mar 2007
  • Loc: Peoria, IL

Posted 05 March 2020 - 08:26 PM

I reviewed this same telescope in the December 2019 issue of Astronomy Technology Today (Vol 13, Issue 5).  I did not see the lack of image brightness with the scope I purchased that Lawrence described in his review.  The secondary mirror obstruction in the GSO is 33%, approximately the same as in an 8-inch SCT.  Any reflector is going to have an effective light gather diameter smaller than the primary mirror due to the secondary obstruction.  I found the view in the GSO crisper and better contrast than an 8-inch SCT.


  • edl, Live_Steam_Mad, eros312 and 1 other like this

#24 Live_Steam_Mad

Live_Steam_Mad

    Viking 1

  • -----
  • Posts: 946
  • Joined: 24 Jul 2007

Posted 06 March 2020 - 06:26 AM

I reviewed this same telescope in the December 2019 issue of Astronomy Technology Today (Vol 13, Issue 5).  I did not see the lack of image brightness with the scope I purchased that Lawrence described in his review.  The secondary mirror obstruction in the GSO is 33%, approximately the same as in an 8-inch SCT.  Any reflector is going to have an effective light gather diameter smaller than the primary mirror due to the secondary obstruction.  I found the view in the GSO crisper and better contrast than an 8-inch SCT.

Did you try the GSO brand one specifically and not a rebranded one?

 

Regards

AG



#25 NZRoger

NZRoger

    Lift Off

  • -----
  • Posts: 21
  • Joined: 02 Aug 2019
  • Loc: Whangaparaoa New Zealand

Posted 08 March 2020 - 10:07 PM

That review was good reading thank you.

I have recently bought one of these from AstroNZ here in North Auckland, New Zealand. Only managed to use it once so far but am very happy. The only problem I have at the moment is that I haven't been able to put my 2" optics on it from my Dob. Have been using my 1.25" diagonal and eyepieces. Had a talk to AstroNZ and they say I will be able to use the 2" stuff once I buy a 2" diagonal. I have put the larger supplied spacer in before the focuser to achieve focus for my general use, but couldn't get near achieving focus with all the supplied spacer rings when trying to use my 2" eyepieces when I plugged them straight into the back with no diagonal.
Regards Roger

Edited by NZRoger, 08 March 2020 - 10:09 PM.



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