View Full Version here: : Diagonal upgrade - prism or mirror
17-02-2012, 09:32 AM
I am using a Celestron 102SLT (achro refractor), and upon advice from this community have now purchased a zoom lens that I am extremely happy with. Thanks.
I am now considering upgrade of the provided Celestron 1.25inch star diagonal (prism), to a 2inch variety, and need advice as to whether or not an improvement will be evident with a quartz mirror diagonal.
The reason, I seek your advice is that I have read articles that state:
"Assuming the same quality of manufacture, prisms give less light scatter, a good thing. And unlike a mirror, they are also self collimating. But they also create false color in scopes with fast focal ratios"
But have also read atricles to the contrary indicating that the prism is used to add correction to the false colour. Interestingll, it appears that were of use in ED refractors...........hmmmm........t hought that they prodced no false colour in the first place...........oils aint oils and obviously EDs aren't EDs.
Therefore in my F6.5 scope (fast?) was the prism diagonal (as supplied) beneficial to the scope's overall performance, or would that be otherwise enhanced in the adoption of a 2inch quartz diagonal?
Thanks in advance for any advice offered.
17-02-2012, 09:37 AM
If the prism was supplied, than the whole optical system was designed to use the prism.
Yes, unlike flat mirrors, prisms are optical elements - very thick piece of glass - that do affect the light path and contribute to the formation of the image in the focal plane.
17-02-2012, 10:14 AM
If you're doing low to medium power viewing you could consider a 2" correct image diagonal which is nice if you're panning across the milky way:
Otherwise a standard Bintel/GSO dielectric diagonal should be fine in that scope too.
17-02-2012, 01:24 PM
There is actually a very good article on cloudy nights which explains in a very easy to follow manner the relative merits and recommendations of a prisim v mirror diagonal. Naturally, I can't seem to find it at the moment - but I am sure if you hunt around you will locate it.
As a very simple rule of thumb (and there are exceptions/caveats to this point) mirror diagonals are generally better for short focal ratio scopes (f/9 and less) and prisims are generally better for long focal ratios.
17-02-2012, 01:53 PM
It was that artilcle (refer link) by Gary Hand located in Cloudy Nights as attached ithat has me thinking of a change to a mirror diagonal
But the thread below adds some confusion;
and I therefore had concern, that removal of the prism from the system would introduce more false colour.
17-02-2012, 05:00 PM
In adding to the previous post, provided is the following extract to explain my dilemma.
"A well-made conventional 90-degree prism star diagonal can transmit as much or more light as a mirror, and do so with higher image contrast since there is no possibility of light scattering from a reflective metallic surface as in a mirror diagonal. Also a prism will never degrade over time as a mirror will since there is no reflective metal coating to degrade from oxidation. However prism diagonals may introduce chromatic aberration when used with short focal-length scopes although this isn't a problem with the popular Schmidt-Cassegrain and Maksutov Cassegrain telescopes, which have long focal lengths. On longer focal ratio telescopes a well-made 90-degree prism diagonal is the optimum choice to deliver the highest image contrast short of using the telescope without a diagonal entirely. However prisms seem to be falling out of favour probably due to marketing forces which have been favouring short focal length instruments which tend to function better with a mirror diagonal. In some special cases however, the colour dispersion effects of a prism diagonal can be used to advantage to improve the performance of undercorrected refractor objectives (regardless of focal length) by shifting the spherical and colour correction of the objective closer to the design optimum. The natural colour dispersion properties (overcorrection) of the prism works to lessen or nullify the undercorrection of the objective lens."
Of course, if money were no issue, would just buy a mirror diagonal, try it out, and either continue to use it or then buy a decent prism.
17-02-2012, 07:23 PM
There should be plenty of people in Adelaide with a 2" mirror diagonal that you could try out.
18-02-2012, 11:45 AM
Ok you are talking about 2 diff systems, a diagonal mirror & a diagonal prism.
The standard mirror diagonal is basically a 45 deg mirror to npbounce the light to the ep, it has just 1 surface & generally transmits light well
A prism diagonal is used to make a image upright & correctly oriented , it bounces the light over several surfaces & so light loss can result...this generally isn't a problem during the day cos here is so much light
For astro purposes the best is the baader 2" amici prism (amici is the best prism at transmitting light), however above about 50x you get a small diff spike on bright objects a bit like a newt mirror spider, a have afraid in Europe who uses one for low mag milky way sweeps but for high mag he always swaps it out for a standard mirror
22-02-2012, 10:30 AM
Thanks for all the comments.
I got brave last night and dismantled the diagonal...........to find.............a mirror!......and not a prism as stated in a specification that I found on the net.
Looks like as most have stated a mirror diaginal is more suited to a short focal length refractor.
So my upgrade will be to a Quartz di-electric, probably by GSO due to its price, and unfortunatley not the vernonscope (1/20 lamba) were money no object.
Therefore my kit will comprise 102SLT (refractor), 2inch mirror diagonal, apo 2.4x barlow and zoom lens. Interestingly, because the zoom is not parfocal, I can adjust it to then minimise the chromatic abberation bu choosing that sweet spot between the green and red halos.
Thanks to all for your time to reply,
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