my astromaster diagonal ?

[rory]

got me thinking while reading another thread here.

my diagonal is a correct image diagonal. so it has a prism in it.

although its handy for me as a relative novice(up is up, down is down ect)

would i possibly get a better image by having a mirror diagonal instead ?

thanks..

[Capricorn]

I think the answer is probably yes, but you could also need (again I think) an extender to compensate for the removal of the prism. The light will be folded inside the prism and also the refractive index of the glass used adds to the path length.

In effect do not expect a straight replacement.

[rory]

mmm, good point capricorn. shall leave alone then. ive managed okay with it so far.

[CGolder]

in fairness i would try another diagonal, the one you get with the astromaster is poor and doesnt allow a full 1.25" of light to pass though it, a very worthwhile upgrade to a stock skywatcher or celestron.

[rory]

in fairness i would try another diagonal, the one you get with the astromaster is poor and doesnt allow a full 1.25" of light to pass though it, a very worthwhile upgrade to a stock skywatcher or celestron.

i would like to get one so i can compare views, but not if i need to start adding extenders and the like. if i see one going cheap ,i'll give it a go i think.

thanks.

[Capricorn]

I would agree that if you get the chance to try one then do so and see if it works.

If someone has done their job right then the prism diagonal you have should be shorter to allow for the extra path created by the glass and the folding. Simply reducing the the tube into which the eyepiece drops should the job. So easy.

Someone somewhere may have accounted for it, and so a straight swap would be possible. Having the prism in the path will I believe be upsetting the overall view. Not only is the prism giving extra path, that can be accounted for, but the light inside the prism will be further refracted (split) and this I doubt very much is accounted for. So you will experience a little additional CA that otherwise would be absent.

[rory]

thanks for the info capricorn. makes sense yes.

[Mark Scott]

I have a 90 mm Astromaster for my son and have swapped the prism for an Antares 1.25" diagonal (one I don't mind him dropping [£30]) with no problems bringing a 32,24,17,15,10 and 8mm EP'S into focus.

[rory]

I have a 90 mm Astromaster for my son and have swapped the prism for an Antares 1.25" diagonal (one I don't mind him dropping [£30]) with no problems bringing a 32,24,17,15,10 and 8mm EP'S into focus.

thanks for that info mark, was there any improvement in the brightness /viewing ? thanks.

[John]

A mirror diagonal still shows the image the right way up. Left and right are reversed though.

[Mark Scott]

thanks for that info mark, was there any improvement in the brightness /viewing ? thanks.

Hi Rory,

I found the prism was throwing reflections from light getting in through the eyepiece. The light came from a streetlight overhanging my garden rendering my prism totally useless so I could not do a comparison. Now with the 1.25" diagonal we can at least use the scope. Next job will be to add a propper finder.

Here's my write up.

Celestron Astromaster 90mm review - Review

[rory]

thanks for that mate. yeah , i whacked a telrad on mine, and never looked back.

i read your review, i agree the focuser is smooth,the mount is adequate for the job. i only used one weight too, i think the other is reserved for when/if theres more equipment to be added.

all in all for a budget scope the main two things that matterd were the mount and the actual glass. and they work fine.

i hardly ever use the fine movement controllers ,so that dont bother me. ive been happy with mine on the whole.

clear skies.....