Which is better of the two?
I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
🤗🤔

On Saturday, September 9, 2023 at 4:45:15 PM UTC-7, StarDust wrote:
> Which is better of the two?
> I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> 🤗🤔

I already, had an older 11/4" prism diagonal, Celestron, Made in Japan.
Just bought a mirror, dialectic diagonal coated, of the same size.
I've been testing them in my 80 mm f/6.25 APO refractor daytime, lookin it trees, high power, low power, but can't see any difference?
The prism type add focus distance, I have to to crank it out more for things to come into focused than the mirror diagonal.
Any input, which one should I keep?

On Saturday, September 9, 2023 at 7:57:28 PM UTC-4, StarDust wrote:
> On Saturday, September 9, 2023 at 4:45:15 PM UTC-7, StarDust wrote:
> > Which is better of the two?
> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> > 🤗🤔
> I already, had an older 11/4" prism diagonal, Celestron, Made in Japan.
> Just bought a mirror, dialectic diagonal coated, of the same size.
> I've been testing them in my 80 mm f/6.25 APO refractor daytime, lookin it trees, high power, low power, but can't see any difference?
> The prism type add focus distance, I have to to crank it out more for things to come into focused than the mirror diagonal.
> Any input, which one should I keep?

Compare them on stars and planets. Daytime views have turbulence and low contrast that will hide defects.

On Tuesday, September 12, 2023 at 12:33:57 AM UTC-7, RichA wrote:
> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust wrote:
> > Which is better of the two?
> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> > 🤗🤔
> I've never used a prism that was as good as a good mirror and I owned Baader's best prism diagonal.

Prism makes the light path longer, so if focusing travel distance is a problem, some thing to think about?

On Tue, 12 Sep 2023 04:19:46 -0700 (PDT), StarDust <csoka01@gmail.com>
wrote:

>On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA wrote:
>> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust wrote:
>> > Which is better of the two?
>> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
>> > ??
>> I've never used a prism that was as good as a good mirror and I owned Baader's best prism diagonal.
>
>Prism makes the light path longer, so if focusing travel distance is a problem, some thing to think about?

Prism diagonals do not make the light path longer. In fact, prism
diagonals usually have a somewhat shorter light path.

Prism diagonals will be slightly brighter which is a plus. They might
introduce a small amount of chromatic aberration with short focal
length telescopes, which is a minus.

In reality, in actual use, it is unlikely that you would notice any
difference between the two, assuming they were quality products.

On Tuesday, September 12, 2023 at 6:40:36 AM UTC-7, Chris L Peterson wrote:
> On Tue, 12 Sep 2023 04:19:46 -0700 (PDT),
> wrote:
> >On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA wrote:
> >> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust wrote:
> >> > Which is better of the two?
> >> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> >> > ??
> >> I've never used a prism that was as good as a good mirror and I owned Baader's best prism diagonal.
> >
> >Prism makes the light path longer, so if focusing travel distance is a problem, some thing to think about?
> Prism diagonals do not make the light path longer. In fact, prism
> diagonals usually have a somewhat shorter light path.
>
> Prism diagonals will be slightly brighter which is a plus. They might
> introduce a small amount of chromatic aberration with short focal
> length telescopes, which is a minus.
>
> In reality, in actual use, it is unlikely that you would notice any
> difference between the two, assuming they were quality products.

In my experience comparing the two, using the prism diagonal, I have to crank the focuser out more to come into focus, using the same eyepiece, same OTA!
I assume, my prism diagonal. Celestron, marked Made in Japan, should be a fairly good quality?

On Tuesday, September 12, 2023 at 5:49:33 PM UTC-4, StarDust wrote:
> On Tuesday, September 12, 2023 at 6:40:36 AM UTC-7, Chris L Peterson wrote:
> > On Tue, 12 Sep 2023 04:19:46 -0700 (PDT),
> > wrote:
> > >On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA wrote:
> > >> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust wrote:
> > >> > Which is better of the two?
> > >> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> > >> > ??
> > >> I've never used a prism that was as good as a good mirror and I owned Baader's best prism diagonal.
> > >
> > >Prism makes the light path longer, so if focusing travel distance is a problem, some thing to think about?
> > Prism diagonals do not make the light path longer. In fact, prism
> > diagonals usually have a somewhat shorter light path.
> >
> > Prism diagonals will be slightly brighter which is a plus. They might
> > introduce a small amount of chromatic aberration with short focal
> > length telescopes, which is a minus.
> >
> > In reality, in actual use, it is unlikely that you would notice any
> > difference between the two, assuming they were quality products.
> In my experience comparing the two, using the prism diagonal, I have to crank the focuser out more to come into focus, using the same eyepiece, same OTA!
> I assume, my prism diagonal. Celestron, marked Made in Japan, should be a fairly good quality?

The tube where the eyepiece fits could perhaps be a bit shorter on the prism diagonal, requiring that the assembly be moved outward to get to achieve focus. The prism could also be smaller than the mirror version.

If the diagonal says "Japan" on it, I would keep it.

On Wednesday, September 13, 2023 at 7:49:13 AM UTC-7, W wrote:
> On Tuesday, September 12, 2023 at 5:49:33 PM UTC-4, StarDust wrote:
> > On Tuesday, September 12, 2023 at 6:40:36 AM UTC-7, Chris L Peterson wrote:
> > > On Tue, 12 Sep 2023 04:19:46 -0700 (PDT),
> > > wrote:
> > > >On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA wrote:
> > > >> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust wrote:
> > > >> > Which is better of the two?
> > > >> > I read some where, mirror is better for short f ratio telescopes, f/6 and under, prism works better for longer f/ratios?
> > > >> > ??
> > > >> I've never used a prism that was as good as a good mirror and I owned Baader's best prism diagonal.
> > > >
> > > >Prism makes the light path longer, so if focusing travel distance is a problem, some thing to think about?
> > > Prism diagonals do not make the light path longer. In fact, prism
> > > diagonals usually have a somewhat shorter light path.
> > >
> > > Prism diagonals will be slightly brighter which is a plus. They might
> > > introduce a small amount of chromatic aberration with short focal
> > > length telescopes, which is a minus.
> > >
> > > In reality, in actual use, it is unlikely that you would notice any
> > > difference between the two, assuming they were quality products.
> > In my experience comparing the two, using the prism diagonal, I have to crank the focuser out more to come into focus, using the same eyepiece, same OTA!
> > I assume, my prism diagonal. Celestron, marked Made in Japan, should be a fairly good quality?
> The tube where the eyepiece fits could perhaps be a bit shorter on the prism diagonal, requiring that the assembly be moved outward to get to achieve focus. The prism could also be smaller than the mirror version.
>
> If the diagonal says "Japan" on it, I would keep it.

Thx!
Think, I keep both.
I also have an older 2" dielectric diagonal (William optics?), it's just too have on my 80 mm OTA to use 2" stuff.
At F/6 it's all ready wide angle.

On 12/09/2023 22:49, StarDust wrote:
> On Tuesday, September 12, 2023 at 6:40:36 AM UTC-7, Chris L Peterson
> wrote:
>> On Tue, 12 Sep 2023 04:19:46 -0700 (PDT), wrote:
>>> On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA
>>> wrote:
>>>> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust
>>>> wrote:
>>>>> Which is better of the two? I read some where, mirror is
>>>>> better for short f ratio telescopes, f/6 and under, prism
>>>>> works better for longer f/ratios? ??
>>>> I've never used a prism that was as good as a good mirror and I
>>>> owned Baader's best prism diagonal.
>>>
>>> Prism makes the light path longer, so if focusing travel distance
>>> is a problem, some thing to think about?

>> Prism diagonals do not make the light path longer. In fact, prism
>> diagonals usually have a somewhat shorter light path.

+1
>>
>> Prism diagonals will be slightly brighter which is a plus. They
>> might introduce a small amount of chromatic aberration with short
>> focal length telescopes, which is a minus.
>>
>> In reality, in actual use, it is unlikely that you would notice
>> any difference between the two, assuming they were quality
>> products.
>
> In my experience comparing the two, using the prism diagonal, I have
> to crank the focuser out more to come into focus, using the same
> eyepiece, same OTA! I assume, my prism diagonal. Celestron, marked
> Made in Japan, should be a fairly good quality?

That isn't too surprising AOBE. What matters is the location of the
optical surface where the reflection actually takes place.

On a prism that is by total internal reflection off the glass surface
right at the very back of the right angle prism assembly with perhaps a
1mm air gap behind.

For a front silvered optical flat mirror you have the physical thickness
t of the mirror x1.4 extra length to the whole assembly when the
reflecting surface is in the right position behind the scope.

--
Martin Brown

On Thursday, September 14, 2023 at 3:08:20 AM UTC-4, Martin Brown wrote:
> On 12/09/2023 22:49, StarDust wrote:
> > On Tuesday, September 12, 2023 at 6:40:36 AM UTC-7, Chris L Peterson
> > wrote:
> >> On Tue, 12 Sep 2023 04:19:46 -0700 (PDT), wrote:
> >>> On Tuesday, September 12, 2023 at 12:33:57?AM UTC-7, RichA
> >>> wrote:
> >>>> On Saturday, 9 September 2023 at 19:45:15 UTC-4, StarDust
> >>>> wrote:
> >>>>> Which is better of the two? I read some where, mirror is
> >>>>> better for short f ratio telescopes, f/6 and under, prism
> >>>>> works better for longer f/ratios? ??
> >>>> I've never used a prism that was as good as a good mirror and I
> >>>> owned Baader's best prism diagonal.
> >>>
> >>> Prism makes the light path longer, so if focusing travel distance
> >>> is a problem, some thing to think about?
>
> >> Prism diagonals do not make the light path longer. In fact, prism
> >> diagonals usually have a somewhat shorter light path.
> +1
> >>
> >> Prism diagonals will be slightly brighter which is a plus. They
> >> might introduce a small amount of chromatic aberration with short
> >> focal length telescopes, which is a minus.
> >>
> >> In reality, in actual use, it is unlikely that you would notice
> >> any difference between the two, assuming they were quality
> >> products.
> >
> > In my experience comparing the two, using the prism diagonal, I have
> > to crank the focuser out more to come into focus, using the same
> > eyepiece, same OTA! I assume, my prism diagonal. Celestron, marked
> > Made in Japan, should be a fairly good quality?
> That isn't too surprising AOBE. What matters is the location of the
> optical surface where the reflection actually takes place.
>
> On a prism that is by total internal reflection off the glass surface
> right at the very back of the right angle prism assembly with perhaps a
> 1mm air gap behind.
>
> For a front silvered optical flat mirror you have the physical thickness
> t of the mirror x1.4 extra length to the whole assembly when the
> reflecting surface is in the right position behind the scope.

None of that matters since the reflection occurs at a surface with an infinitesimally small thickness.

It's the housing of the optic that determines how much back focus the device consumes.
The OP is comparing just TWO such devices and can draw no general conclusions from that small sample.