|
Post by Viktor Z. on May 19, 2014 16:24:11 GMT -5
Thinking about eyepieces, binoviewers and other components of a refractor, it is easy to forget about the importance of diagonals. I apologize posting here at eyepieces, but eyepieces are very close to diagonals;)
An excellent refractor should also have an excellent diagonal, otherwise the whole optical system is compromised no matter how great our eyepieces are. I could do streight-through viewing but it can result very uncomfortable body postures, and I am not so flexible, only so-so flexible; therefore I need to use a diagonal most of the time.
Now my dilemmas: - What wavelength error should a diagonal have for refractors: 1/4, 1/10, 1/20 lambdas? or there is another consideration that I should be aware of? - How to test a diagonal at home?
|
|
|
Post by Mike on May 19, 2014 21:07:42 GMT -5
I've seen diagonals from $129 up to $799. I just talked to a company at the Arizona show (ASAE) that sells 2" diagonals for $800 and include an interferometer report and some other certification. I own an Astro-Physics, Televue, Stellarvue and Istar. To save my life I cannot see the difference whether visual or imaging. You are not supposed to image through a diagonal. However, on a 14" SCT pointed to the zenith with a crayford focuser, filter wheel, 2" Powermate and a camera trying get the planet on a small chip you must get down on your knees and wrench your neck to see through a parfocal eyepiece. I started using the AP diagonal to make it easier. Then I went back to no diagonal because a friend saw me using one and cautioned me not to. I can discern no difference in the final planetary image other than it's mirrored in orientation. I know there is a difference because testing reveals one. Quality mirrors are flatter and have better thermal properties. The question is can you SEE the difference. Ales had our diagonals tested and he says they are every bit as good as a $399 diagonal yet they are priced at $129. We sell so many diagonals they are always out of stock. By the time I get them they are usually sold. I will have to look for it but Yuri Petrunin posted a simple yet reliable way to test diagonals. I will see if I can find the link.
|
|
|
Post by istarmullet on May 20, 2014 13:25:00 GMT -5
800 smackers for a diagonal is way to much; that sucker better open beers, rub my feet, and cook me dinner for that price. I use an ISTAR diagonal, Denk, OPT, and a couple others...all good and all get the job done near as I can tell.
|
|
|
Post by bn1777 on May 21, 2014 4:43:49 GMT -5
I have tested my $129 GSO Quartz dielectric 2 inch against diagonals costing 3 times as much and like Mike cant see the difference , this is in my Takahashi , a 127mm TV and my 127mm Istar . Food for thought for sure Viktor .
Brian.
|
|
rgm
Full Member
Posts: 65
|
Post by rgm on May 21, 2014 7:15:32 GMT -5
I agree with everyone else. It is very difficult to see differences in mirror diagonals. I can though, see small differences with prism diagonals. I find slightly less scatter with prisms in my refractors. A prism also allows me use binoviewers without an OCS due to the shorter light path.
|
|
|
Post by borispider on May 21, 2014 12:25:18 GMT -5
I have a WO diag. + the one that came with my C4-R. The only thing ppl would mention to me in getting 1 was is it collimatable. I never check mine since the images it threw looked good to me.
|
|
|
Post by Mike on May 21, 2014 13:46:14 GMT -5
Oh come on! What happened to "Only the best will do."
$800 Diagonal
|
|
|
Post by Viktor Z. on May 21, 2014 17:47:17 GMT -5
Well, if I chose, I would definitely save for food rather than spending $800 for a diagonal;) Vernonscope's silver diagonal must be awesome, but $800 would surely buy me quite a few burgers AND an Istar diagonal. However, if prisms REALLY have smaller light scatter, than a prism diagonal should be considered too...
I would like to ask you Mike or Ales if Istar Optical has any plans to produce prism diagonals, as well? A 2" or even larger prism at affordable price would be a killer and for a good use for large refractors IF prisms really better than mirrors in means of light scatter...
|
|
|
Post by Mike on May 21, 2014 23:23:59 GMT -5
We never say never but it's unlikely that we'll get into diagonals anymore than we already are. Maybe in the future but right now our concentration is on telescopes. We offered quartz mirrors at one time and there was no interest. Prism diagonals better? Depends on the application. Reflectivity, flatness of the reflecting surface, material i.e. quartz, Zerodur, Pyrex and how the housing was made. All these things need to be considered. Prism diagonals are good for binos. Since passing light through a material slows it, prism diagonals always have a much shorter optical path than mirrors allowing binos to come to focus without shortening the tube. The thinking used to be use mirror diagonals on short focal length refractors and prism diagonals on long focal length refractors. Prisms supposedly cause false color in short focal length scopes. A lot of theory but how much practice?
|
|
|
Post by Viktor Z. on May 22, 2014 12:15:34 GMT -5
Thank you, Mike for your reply, it is very interesting new knowledge for me about prisms in relation to refractor focal lengths and binoviewing. I used WO and Orion 2" dielectric mirror diagonals and I never had any issues with them. I also noticed difference in the light path even without measuring between the Tak 1.25 and televue 1.25 diagonals: the tak requires much shorter light path and I had a telescope that could not come to focus with the televue only the tak 1.25 diagonal.
|
|
|
Post by mrkellytaylor on Aug 1, 2014 3:40:09 GMT -5
I can highly recommend the Istar's diagonals. see my review on Istar website. when seeing allows it will show you what your optics will pass. it will be one less thing to worry about. get one and have fun. I bought 5 five of them so far. everyone that tried them won't give them back. Great product!!
Mr. Kelly Taylor
|
|
|
Post by Mike on Aug 1, 2014 14:39:38 GMT -5
Thanks for the "high five" Kelly! I still say Kevin should spring for the $800 diagonal then do a side by side comparison and let us know if it was worth it. Anybody else think this should happen?
|
|
|
Post by skybender on Aug 1, 2014 18:59:27 GMT -5
You can make your own. Go on surplusshed.com and pick up one of the 1/20th wave mirror flats. That is the best you are going to get for next to nothing. For the housing just buy a cheapo on ebay and take out the mirror and replace with the new one.
Telescope manufacturing is simple. Find a supplier. Done.
|
|
|
Post by skybender on Aug 1, 2014 19:02:28 GMT -5
Quartz overcoat is SiO2. Fused silica = quartz.
|
|
|
Post by skybender on Aug 1, 2014 19:09:33 GMT -5
I just checked surplus shed, looks like they sold out the 1/20th wave aluminized + sio2 overcoated mirrors.
Sorry about that.
|
|