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

C8 why a 2" dielectric diagonal?

  • Please log in to reply
26 replies to this topic

#1 gspeed

gspeed

    Vostok 1

  • -----
  • topic starter
  • Posts: 101
  • Joined: 22 Apr 2020
  • Loc: Around Alps

Posted 01 April 2021 - 11:34 AM

I'm wondering what are the good reasons to invest in a 2" dielectric diagonal for this scope - and perhaps bigger brothers as well.

 

Most folks seems to go that way, while with  buying the 6.3 focal reducer one can use his 32mm plossl (everyone has a 32mm plossl) or a 24mm 68° (I have the ES one as well) to get the max tFoV this scope can deliver.

 

A 1.25" diagonal/eyepiece is easily a 10 pound lighter approach and for sure it is cheaper - what are the cons? I can think only about the additional piece of glass, but is it really a concern?

 

Thanks!


Edited by gspeed, 01 April 2021 - 11:35 AM.


#2 wrnchhead

wrnchhead

    Mercury-Atlas

  • *****
  • Posts: 2,878
  • Joined: 28 Aug 2017
  • Loc: NE Kansas

Posted 01 April 2021 - 11:37 AM

My 30mm-ish 2' EPs won't reach focus with a mirror 2" diagonal (and/or with the 6.3 reducer, it's been so long now I don't remember the specifics) on my C8. And with the mirror cranked that far forward, probably destroys the edges anyway. I'm wholly content with a 1.25 prism. 


Edited by wrnchhead, 01 April 2021 - 11:38 AM.

  • ShaulaB, Rustler46 and Old Speckled Hen like this

#3 Supernova74

Supernova74

    Vanguard

  • -----
  • Posts: 2,491
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 01 April 2021 - 11:47 AM

Well it’s relatively easy really to understand why folks go for 2” diagonals is because most longer focal length eyepieces are 2” thay  generally offer you the lowest power wide feild views possible however tho you are restricted to a certain extent in a sct cassagrain due to the optical design and long focal length of the telescope,however on the plus side most diagonals come with a 1.25” adapter also so you get the best of both worlds.



#4 Nippon

Nippon

    Skylab

  • *****
  • Posts: 4,369
  • Joined: 22 Oct 2009
  • Loc: Central Florida

Posted 01 April 2021 - 11:56 AM

In my opinion no. The baffle tube is too small to take real advantage of 2" eyepieces. A less costly and more effective way to coax a wider field out of a C8 is the f/6.3 reducer/corrector.


  • ShaulaB, Rustler46 and gspeed like this

#5 cst4

cst4

    Viking 1

  • *****
  • Posts: 755
  • Joined: 06 Jun 2018

Posted 01 April 2021 - 12:14 PM

I use a 2" diagonal and eyepieces in my 6" SCT to gain wider views.  I also have the reducer.  I mainly use the 2" accessories because I like my 2" eyepieces which I use in all my scopes.  I've never noticed any vignetting if that is happening at all.  If I only had 1.25" format eyepieces then I would just be content with the reducer.  Those 6.3 reducers flatten the field as well which is nice.  Using the reducer or long focal length 2" eyepieces both expand your true FOV by near the same amount.. just a matter of which way you want to do it.


  • wrvond and kksmith like this

#6 Supernova74

Supernova74

    Vanguard

  • -----
  • Posts: 2,491
  • Joined: 25 May 2020
  • Loc: Epsom surrey near (London)

Posted 01 April 2021 - 12:17 PM

In my opinion no. The baffle tube is too small to take real advantage of 2" eyepieces. A less costly and more effective way to coax a wider field out of a C8 is the f/6.3 reducer/corrector.

Well I think many will disagree on that one,as thay cause unwanted vignetting and only really should be used for AP,imaging.the less glass as possible in the optical train the better the image and unwanted distortion.


Edited by Supernova74, 01 April 2021 - 12:19 PM.

  • lee14 likes this

#7 photomagica

photomagica

    Viking 1

  • *****
  • Posts: 556
  • Joined: 23 Jun 2013
  • Loc: Calgary and Tucson

Posted 01 April 2021 - 12:53 PM

I use 2" eyepieces on my C8 with no problems. The widest I have is the 23mm 82 degree Luminos. It performs very well on the C8 and there is no noticeable vignetting due to the size of the baffle tube. In my view, it is worth having the 2" diagonal for the delightfully comfortable and appealing views that an 82 degree or wider eyepiece yields in the 20mm to 30mm focal length range. The 23 mm eyepiece yields 88x on the C8 and a 0.93 degree field of view. The ability to have a near one degree field at a power that is beginning to take advantage of the resolving power of the telescope is an appealing combination.

 

I have not used a focal reducer so I can't comment on going that route. I've avoided that because I'd have to remove it when I want to go to higher powers or put up with the extra glass in the light path.

 

BTW - I decloaked and internally flocked my 23mm Luminos to take care of Edge of Field Brightening (EOFB). If EOFB doesn't bother you too much the Luminos represents very good value and performs well on the C8. I bought mine used on CN. In fact almost all my eyepieces have been bought used and this enables me to have higher quality eyepieces than I'd otherwise e able to afford.

Bill


Edited by photomagica, 01 April 2021 - 09:50 PM.

  • jimandlaura26, eros312 and lee14 like this

#8 TOMDEY

TOMDEY

    Cosmos

  • *****
  • Posts: 9,982
  • Joined: 10 Feb 2014
  • Loc: Springwater, NY

Posted 01 April 2021 - 12:57 PM

All dielectric tend to be high throughput and typically better ~select~ quality. The only downside is they have polarization effects. So, if you happen to be using your scope for such things as polarimetry, that would screw up your data. Otherwise, they're good!    Tom


  • wrnchhead likes this

#9 wrvond

wrvond

    Gemini

  • *****
  • Posts: 3,162
  • Joined: 25 Sep 2014
  • Loc: West Virginia

Posted 01 April 2021 - 01:07 PM

Because I can. waytogo.gif

 

It's not just 2" eyepieces - most of my filters are 2" as well. 

Additionally, I really like the Baader Click Lock system and, as far as I've been able to determine, it doesn't exist in 1.25" (except as a 2" to 1.25" adapter).

 

back001

Edited by wrvond, 01 April 2021 - 03:13 PM.

  • jimandlaura26 likes this

#10 brentknight

brentknight

    Gemini

  • *****
  • Posts: 3,152
  • Joined: 29 Dec 2014
  • Loc: Foley, Alabama

Posted 01 April 2021 - 01:10 PM

If you have a significant investment already in 2" barrels, then it's definitely in your favor to go the 2" diagonal route on the C8.  My 31T5 and Ethos all seem to work really well, and my 1.25" pieces are mounted in 2" to 1.25" adapters - so no swapping adapters.

 

I have heard that there can be major balance and stress on some C8 mounts with the added weight of a 2" light path and heavy eyepieces.


  • gspeed likes this

#11 jallbery

jallbery

    Mercury-Atlas

  • *****
  • Posts: 2,649
  • Joined: 01 Nov 2015
  • Loc: Southeast Michigan

Posted 01 April 2021 - 01:43 PM

In my opinion no. The baffle tube is too small to take real advantage of 2" eyepieces. A less costly and more effective way to coax a wider field out of a C8 is the f/6.3 reducer/corrector.

If the baffle tube is too small to take advantage of 2" eyepieces, it is also too small to take advantage real advantage of an F/6.3 reducer/corrector.   In either case, the primary limiting factor is the diameter of the rear baffle tube.   The R/C turns an eyepiece with a 27mm diameter fieldstop (i.e., the widest TFoV 1.25" eye[oeces_  into the the virtual equivalent of  an eyepiece with a 43mm fieldstop.   Once you consider the degree to which the typical SCT 2" diagonal extends the focal length of the SCT, it makes it rather close to the equivalent of a 45-46mm fieldstop (approaching the widest 2" eyepieces).  

 

 

Well I think many will disagree on that one,as thay cause unwanted vignetting and only really should be used for AP,imaging.the less glass as possible in the optical train the better the image and unwanted distortion.

The Celestron F/6.3 R/C has a clear aperture of 41mm-- 10% larger than the rear baffle of of the C8.  With a C8 or smaller it does not add vignetting.  Furthermore, it provides some mild field flattening (albeit at the cost of a slight reduction in transmission and an increase in scatter).  It was designed to be used both visually and photographically.  Quite frankly, I consider more useful for visual use than photography (there are better, albeit more expensive, photographic options). Visually, I consider the flattening vs scatter to be about a wash.   YMMV.

 

 

With that out of the way...

 

2" diagonal advantages with an 8" SCT:

  • Convenience-- don't have to put on and take of the R/C
  • Stronger/Sturdier, which can be very important if you are using physically large eyepieces (like UWAs and wider). Even longer 1.25" focal-length ultra/mega wide eyepieces are big and heavy enough that a 2" diagonal may be preferable.
  • Potentially better contrast due to elimination of R/C.
  • Big eye lenses with many 2" eyepieces make for an impressive presentation.
  • If you already have 2" eyepieces (e.g., because they are needed to take full advantage of that telescope's capabilities), you can use them with the SCT.  

2" diagonal disadvantages:

  • Cost:  good 2" eyepieces are expensive
  • Weight: almost all 2" eyepieces are very heavy, and can create balance issues, particularly when switching to short focal length eyepieces.
  • Most 2" diagonal setups more back the focal plane beyond the designed optimal backfocus.  This increases focal length and causes an over-correction of spherical aberration (very slightly degrading optical performance). In extreme cases it can cause loss of effective aperture.
  • At F/10, a prism diagonal will outperform a mirror diagonal of comparable optical quality (prisms have CA, mirrors have scatter-- at higher focal ratios the scatter outweighs the the CA).

 

1.25" + R/C advantages:

  • Significant cost savings:  The R/C costs about the same as a decent 2"  SCT diagonal.  If you already have a 32mm plossl and/or a 24mm Pan/ES68/SWA, you don't need to buy additional eyepieces to max out your field of view.   It's like getting a 55mm-ish plossl and a 40mm-ish Pan/ES68/SWA for free!  You get double duty out of your medium to to long FL 1.25" eyepieces.
  • The R/C+1.25" diagonal and eyepiece generally weighs substantially than 2" equivalent: it's easier on your mount AND your wallet, and you have fewer scope balancing issues.
  • SCTs have significant field curvature, and the R/C provides some modest field flattening
  • The C8 was designed around the use of a 1.25" prism diagonal, and you get optimal performance with the setup.

1.25"  + R/C disadvantages:

  • Some people find putting on and taking off the R/C to be hassle.
  • The R/C adds some scatter

 

My primary C8 setup uses a SCT 2" diagonal, but I could be quite happy with 1.25" eyepieces and the R/C, if my only telescope were an 8" or smaller SCT.  If I'm trying to squeeze out every last bit of planetary performance, I'll  go back to a good 1.25" prism, though.   For my C6 and C5, I stick to 1.25" and use the R/C when I want wider fields.  Quite frankly, I'm surprised how well it works on these smaller scopes with their even more constrictive rear baffles.

 

One more word on the C8's 37-38mm rear baffle tube...  the baffle is far enough away from the focal plane that it does not cause any abrupt vignetting-- just gradual falloff.  People sense brightness logarithmically, and most people are pretty good at ignoring this sort of falloff.   And it's not like there isn't already measurable falloff before you get to the 37mm image circle (or even the 27mm circle) point.  This whole idea that it's a waste of time to use an eyepiece with a fieldstop larger than 37mm with a C8 is nonsense as far as I'm concerned: I'd much rather have a flawed 1.2-degree-ish field than NOT have the option.   And FWIW, it's not like that field would not be flawed if we eliminated the constraint of the rear baffle:  it would still have falloff (just not as much) and still have just as much field curvature and coma.  The latter two are much more noticeable to me than the former, anyway.


Edited by jallbery, 01 April 2021 - 07:32 PM.

  • jimandlaura26, eros312, deepwoods1 and 3 others like this

#12 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,730
  • Joined: 14 Oct 2011

Posted 01 April 2021 - 03:05 PM

In my experience the reducer can cause reflections that can be problematic for lunar/planetary viewing, and it is a hassle to take on and off.

But the #1 reason to me is that I already have 2” eyepieces for my Newts and refractors that can’t use a SCT reducer. Might as well use them on the SCT too! It might be different if my only scope was a SCT. Many people who own a SCT also own a refractor for wide field views. So if you also have a 4” Apo with a 2” diagonal and a 35 Panoptic, why wouldn’t you also use it with the SCT rather than messing with a reducer?

Scott

Edited by SeattleScott, 01 April 2021 - 03:19 PM.

  • RAKing, ewave, jjack's and 1 other like this

#13 jallbery

jallbery

    Mercury-Atlas

  • *****
  • Posts: 2,649
  • Joined: 01 Nov 2015
  • Loc: Southeast Michigan

Posted 01 April 2021 - 03:18 PM

In my experience the reducer can cause reflections that can be problematic for lunar/planetary viewing, and it is a hassle to take on and off.
 

Never noticed that with mine.  Now, I'd never use it for planetary, but I used it plenty with with a 35mm Ultima on the full lunar disk (to give it some breathing room).  I have the original made in Japan one.

 

Totally agree, though, that if you have 2" eyepieces already,  by all means, get a diagonal that lets you use them on the SCT.



#14 gspeed

gspeed

    Vostok 1

  • -----
  • topic starter
  • Posts: 101
  • Joined: 22 Apr 2020
  • Loc: Around Alps

Posted 01 April 2021 - 03:20 PM

Many thanks for all your valuable feedbacks - and especially to @jallbery for the most "comprehensive" one.

 

Surely I don't want to start a flame about what is the larger tFoV with the C8, as explained above it is subjective since vignetting fades slowly - the generally accepted range of 1.1°-1.2° is achievabe both with R/C&1.25"  or with a 2" setup.

 

I'm pending towards keeping the 1.25" setup and getting the R/C  - surely I'll not put it on for moon, planetary, globular or other hi-res, but just for giving a better context to winter open clusters or other wide objects.   I don't have any 2" eyepiece now, I don't think it worth investing on that for my current scope.


Edited by gspeed, 01 April 2021 - 03:38 PM.

  • brentknight likes this

#15 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,730
  • Joined: 14 Oct 2011

Posted 01 April 2021 - 03:24 PM

Never noticed that with mine. Now, I'd never use it for planetary, but I used it plenty with with a 35mm Ultima on the full lunar disk (to give it some breathing room). I have the original made in Japan one.

Totally agree, though, that if you have 2" eyepieces already, by all means, get a diagonal that lets you use them on the SCT.

It seemed like it was sensitive to the angle. Like it could be an issue when putting the edge of the Moon in the center, zoomed in at high power. Straight on it seemed to be ok. Now maybe it is just an issue with my particular scope and reducer combination? It’s possible. But certainly I have heard reports that people feel like planetary contrast is a little less using the reducer.

#16 jallbery

jallbery

    Mercury-Atlas

  • *****
  • Posts: 2,649
  • Joined: 01 Nov 2015
  • Loc: Southeast Michigan

Posted 02 April 2021 - 09:47 AM

It seemed like it was sensitive to the angle. Like it could be an issue when putting the edge of the Moon in the center, zoomed in at high power. Straight on it seemed to be ok. Now maybe it is just an issue with my particular scope and reducer combination? It’s possible. But certainly I have heard reports that people feel like planetary contrast is a little less using the reducer.

Just curious... was this with an SCT or your Mak-Cass?    

 

There have been various production changes on the Celestron R/C over the years.   Some people swear the original ones from Japan are the best.  I don't know if that is true, but that's the kind I have.    The new ones probably have better coatings, but it would not surprise me if the old Japan ones have better polishing and stray light control.

 

I would expect the some loss of planetary contrast-- the R/C itself will cause some,  the necessary focus change required to use the R/C will add negative spherical aberration, and reduction in focal ratio will cause a prism diagonal to produce more chromatic aberration. 

 

I would not use the R/C with eyepieces shorter than 15mm.  If I want that much magnification, it is only going to be a detriment

 

--Jim



#17 jimandlaura26

jimandlaura26

    Viking 1

  • -----
  • Posts: 551
  • Joined: 19 Nov 2003
  • Loc: San Diego, CA

Posted 05 April 2021 - 01:08 AM

If the baffle tube is too small to take advantage of 2" eyepieces, it is also too small to take advantage real advantage of an F/6.3 reducer/corrector.   In either case, the primary limiting factor is the diameter of the rear baffle tube.   The R/C turns an eyepiece with a 27mm diameter fieldstop (i.e., the widest TFoV 1.25" eye[oeces_  into the the virtual equivalent of  an eyepiece with a 43mm fieldstop.   Once you consider the degree to which the typical SCT 2" diagonal extends the focal length of the SCT, it makes it rather close to the equivalent of a 45-46mm fieldstop (approaching the widest 2" eyepieces).  

 

 

The Celestron F/6.3 R/C has a clear aperture of 41mm-- 10% larger than the rear baffle of of the C8.  With a C8 or smaller it does not add vignetting.  Furthermore, it provides some mild field flattening (albeit at the cost of a slight reduction in transmission and an increase in scatter).  It was designed to be used both visually and photographically.  Quite frankly, I consider more useful for visual use than photography (there are better, albeit more expensive, photographic options). Visually, I consider the flattening vs scatter to be about a wash.   YMMV.

 

 

With that out of the way...

 

2" diagonal advantages with an 8" SCT:

  • Convenience-- don't have to put on and take of the R/C
  • Stronger/Sturdier, which can be very important if you are using physically large eyepieces (like UWAs and wider). Even longer 1.25" focal-length ultra/mega wide eyepieces are big and heavy enough that a 2" diagonal may be preferable.
  • Potentially better contrast due to elimination of R/C.
  • Big eye lenses with many 2" eyepieces make for an impressive presentation.
  • If you already have 2" eyepieces (e.g., because they are needed to take full advantage of that telescope's capabilities), you can use them with the SCT.  

2" diagonal disadvantages:

  • Cost:  good 2" eyepieces are expensive
  • Weight: almost all 2" eyepieces are very heavy, and can create balance issues, particularly when switching to short focal length eyepieces.
  • Most 2" diagonal setups more back the focal plane beyond the designed optimal backfocus.  This increases focal length and causes an over-correction of spherical aberration (very slightly degrading optical performance). In extreme cases it can cause loss of effective aperture.
  • At F/10, a prism diagonal will outperform a mirror diagonal of comparable optical quality (prisms have CA, mirrors have scatter-- at higher focal ratios the scatter outweighs the the CA).

 

1.25" + R/C advantages:

  • Significant cost savings:  The R/C costs about the same as a decent 2"  SCT diagonal.  If you already have a 32mm plossl and/or a 24mm Pan/ES68/SWA, you don't need to buy additional eyepieces to max out your field of view.   It's like getting a 55mm-ish plossl and a 40mm-ish Pan/ES68/SWA for free!  You get double duty out of your medium to to long FL 1.25" eyepieces.
  • The R/C+1.25" diagonal and eyepiece generally weighs substantially than 2" equivalent: it's easier on your mount AND your wallet, and you have fewer scope balancing issues.
  • SCTs have significant field curvature, and the R/C provides some modest field flattening
  • The C8 was designed around the use of a 1.25" prism diagonal, and you get optimal performance with the setup.

1.25"  + R/C disadvantages:

  • Some people find putting on and taking off the R/C to be hassle.
  • The R/C adds some scatter

 

My primary C8 setup uses a SCT 2" diagonal, but I could be quite happy with 1.25" eyepieces and the R/C, if my only telescope were an 8" or smaller SCT.  If I'm trying to squeeze out every last bit of planetary performance, I'll  go back to a good 1.25" prism, though.   For my C6 and C5, I stick to 1.25" and use the R/C when I want wider fields.  Quite frankly, I'm surprised how well it works on these smaller scopes with their even more constrictive rear baffles.

 

One more word on the C8's 37-38mm rear baffle tube...  the baffle is far enough away from the focal plane that it does not cause any abrupt vignetting-- just gradual falloff.  People sense brightness logarithmically, and most people are pretty good at ignoring this sort of falloff.   And it's not like there isn't already measurable falloff before you get to the 37mm image circle (or even the 27mm circle) point.  This whole idea that it's a waste of time to use an eyepiece with a fieldstop larger than 37mm with a C8 is nonsense as far as I'm concerned: I'd much rather have a flawed 1.2-degree-ish field than NOT have the option.   And FWIW, it's not like that field would not be flawed if we eliminated the constraint of the rear baffle:  it would still have falloff (just not as much) and still have just as much field curvature and coma.  The latter two are much more noticeable to me than the former, anyway.

Good thoughts here.  To add on a bit and confirm the advantages of a 2" eyepiece configuration... I have been using a Televue 22mm Type 4 2" Nagler mounted in a Denkmeier S-2 Power Switch diagonal that gives three magnifications for a given eyepiece (0.6X, 1.0X & 2x) using sliding levers; for many years. No discernible visual vignetting with an 82 degree AFOV eyepiece possessing a field stop of 31.1mm in my C-8 EdgeHD. A great immersive experience even at only a nominal 0.9 degree TFOV; and delivering three powers and TFOVs with a single premium eyepiece mounted in the Power Switch.


Edited by jimandlaura26, 05 April 2021 - 01:16 AM.


#18 PJBilotta

PJBilotta

    Mariner 2

  • *****
  • Posts: 220
  • Joined: 02 Sep 2017
  • Loc: Portland, Oregon

Posted 06 April 2021 - 11:18 AM

One other benefit I find when using a R/C is that you can vary the reduction factor by shortening or lengthening the R/C to EP distance. By withdrawing the diagonal or EP slightly from their holders, one can increase the reduction to f/5.5 or so to "go all the way" for extended objects. In essence, it's the reverse of increasing barlow spacing, which boosts magnification further.

Sure, this is not perfect and introduces some edge aberrations, but this allows you to completely max out the FOV on an 8" SCT. Using this method, I am actually able to yield a larger TFOV in 1.25" format with a 24mm 68-degree than I can with a 2" 40mm 72-degree EP, with better edges - great for extended objects like M44 & 45, the Veil and other large nebulae, or just general wider star field sweeps.

Like I said, not perfect, but makes my C8 a triple threat - f/10, f/6.3, and an imperfect, but serviceable f/5.5 or so - all without having to leave 1.25" and without the focus issues 2" diagonals & EPs sometimes encounter.
  • gspeed likes this

#19 MrJones

MrJones

    Mercury-Atlas

  • *****
  • Posts: 2,740
  • Joined: 15 Sep 2010
  • Loc: Indiana

Posted 06 April 2021 - 12:33 PM

I got a C8 in 2019 and spent a year REALLY trying to stick with 1.25". I've had 4 of the f/6.3 reducers for my 3 previous SCTs (!) and rebought the Antares one. I even built my own 1.25" diagonals:

 

https://www.cloudyni...nals/?p=9736595

 

The good is you're keeping the system around f/10 (it'll go to f/11 with 2" and a little more SA) and the lighter weight is nice for many reasons. The biggest negative IMO is what SeattleScott said, the hassle of adding and removing the reducer depending on what you want to observe. I have a ES68 24mm and often wound up observing what I could with it without the reducer because I got tired of dealing with it.

 

Fast forward to 2021. I gave in and got a short 2" adapter (which I recommend it's been great):

 

https://www.bhphotov...CT_Adapter.html

 

I've really enjoyed the added flexibility of a 2" diagonal with 1.25" adapter and I'm mostly going to stick with it. I've been out with my ES68 34mm recently and observed the Leo Triplet and M81/82 for example, that both barely but nicely fit into the FOV. I will say there is definitely a little more noticeable field curvature vs. say the ES6824mm + f/6.3 reducer but it's not too bad.

 

Anyway IMO if you plan to get a C8 and use just 1.25", a f/6.3 reducer is a decent addition even just for visual, but expect to get tired of swapping it in and out.



#20 Mark9473

Mark9473

    Voyager 1

  • *****
  • Posts: 12,018
  • Joined: 21 Jul 2005
  • Loc: 51°N 4°E

Posted 07 April 2021 - 01:38 PM

I really like the Baader Click Lock system and, as far as I've been able to determine, it doesn't exist in 1.25" (except as a 2" to 1.25" adapter).

 

https://www.baader-p...2-part-08).html

I have both sizes, and actually like the 1.25" version better.



#21 Mark Lovik

Mark Lovik

    Explorer 1

  • -----
  • Posts: 91
  • Joined: 09 Nov 2020

Posted 07 April 2021 - 04:12 PM

I bought a used 8" SCT with a 2" diagonal and no visual back.  Most of the time the field stop of the scope is listed at 38mm.  I commonly use a 32mm 70 degree eyepiece with a lens field stop that is a bit larger ... 40mm.  This is my finder eyepiece.  It works fine for wider field view measured at around 1.1 degrees.  My back focus distance is not much changed from nominal 1.25 visual back and a diagonal.  So yes ... 2" diagonals are useful and are a visual alternative to the 6.3 reducer.

 

Ergonomics are much better than using the reducer (which does not do anything to change the FOV available to the scope.  It just allows shorter focal length eyepieces to provide the wider fields).  I just add the 2" - 1.15 adapter and that's it.  If you pick the right system, the system can be nearly parfocal.  I like that much better than doing the visual back - reducer - diagonal dance, and playing with the focus. 

 

Weight - my diagonal and wide field eyepieces weigh <= 2 pounds.



#22 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,730
  • Joined: 14 Oct 2011

Posted 07 April 2021 - 05:28 PM

Oh yeah focus too, valid point.

So you remove eyepiece and 1.25” diagonal. Unscrew VB. Screw on FR, about ten turns or so, careful not to cross-thread. Screw VB onto reducer. Insert diagonal. Insert eyepiece. Turn focus knob five turns before you realize you are turning focus wrong direction. Turn focus knob back five turns, plus another ten turns to refocus with the FR. Now you are ready to observe with the FR.

Or just get a 2” diagonal and call it good.

Scott
  • ewave, deepwoods1, brentknight and 2 others like this

#23 AOHANA

AOHANA

    Lift Off

  • -----
  • Posts: 21
  • Joined: 16 Feb 2021

Posted 08 April 2021 - 11:14 AM

Anyway IMO if you plan to get a C8 and use just 1.25", a f/6.3 reducer is a decent addition even just for visual, but expect to get tired of swapping it in and out.

On SCT, why swapping it in and out if using 1.25" eyepieces from 5mm to 24 ?

Does it not only depend on what eyepieces you own, and what magnifications you need ?

I am near to buy a reducer. I think it will often be screwed on, using 5 to 24mm 1"25 eyepieces, except for planetary only observings.

 


Edited by AOHANA, 08 April 2021 - 11:42 AM.


#24 SeattleScott

SeattleScott

    Voyager 1

  • *****
  • Posts: 11,730
  • Joined: 14 Oct 2011

Posted 08 April 2021 - 11:56 AM

On SCT, why swapping it in and out if using 1.25" eyepieces from 5mm to 24 ?
Does it not only depend on what eyepieces you own, and what magnifications you need ?
I am near to buy a reducer. I think it will often be screwed on, using 5 to 24mm 1"25 eyepieces, except for planetary only observings.

Yes that is the typical way to do it, leave it on for DSO and remove it for planetary. However it is a fair effort to swap it out for planets, and typically there might only be one target. You basically have Jupiter, Saturn, Moon, and on rare occasions Mars. Maybe only one of those is up on a given night. Are you going to spend 1-2 minutes swapping equipment for a slightly better view of one target? Maybe two? So some just leave reducer on all the time even though they lose a little contrast. Others opt for 2” diagonal to avoid the hassle. But certainly your approach will work. It just isn’t as convenient.

Granted many people already have a 2” diagonal and eyepieces for other scopes, so then avoiding the hassle by using 2” format is kind of obvious.

If you are leaning towards adding a refractor in the future for grab and go and wide field views, it might make sense to invest in 2” format now if it will benefit you later, rather than buying a $150 reducer which might go by the wayside if you move to 2” format later.

Scott

Edited by SeattleScott, 08 April 2021 - 12:00 PM.


#25 AOHANA

AOHANA

    Lift Off

  • -----
  • Posts: 21
  • Joined: 16 Feb 2021

Posted 08 April 2021 - 12:35 PM

If you are leaning towards adding a refractor in the future for grab and go and wide field views, it might make sense to invest in 2” format now if it will benefit you later, rather than buying a $150 reducer which might go by the wayside if you move to 2” format later.

Scott

Interesting. I also own a 4" APO f/7 for wide fields, and have planed to buy a 2" diagonal for it.

Only one dilemma currently : go for a 400$ 30 or 40mm 2" long focal length EP (XW30 or 40), or deal with a cheapest 150$ FR, and 1.25" EPs on both scopes (though limited to 24mm). 2" filters are more pricey too.

Will i often use this 2" wide eyepiece on both scopes ?

Not certain (SCT are not wide field scopes, and the APO is F7).

The only limit is nearly 3.5 exit pupill on the APO....


Edited by AOHANA, 08 April 2021 - 01:18 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