155 replies to this topic

[barbie]

My 6" F8 newt(synta) is definitely a keeper as the optics are outstanding as are the mirror cell, secondary support and focuser.  I've also seen albedo features on Ganymede as well as plenty of detail in Jupiter's cloud bands and even Encke's minima in Saturn's outer ring this past summer under excellent skies.  Cassini's division is always jet black in appearance and nicely rendered for observation.

[bratislav]

Hi Bratislav! 

 

Thanks for the good data! If you're so inclined, it might be interesting how the star tests generally correlated with the bath results. I still don't think I've seen a perfect star test but been close a few times.  I've looked through some very good Chinese reflectors, also. When I was making a lot more mirrors, it was nice to make a batch as it seemed once things were dialed in with the machine and laps, I could make substantial progress. One off usually seemed to take a lot more work to get results that were "time profitable". Bob Goff told me once to, "make 'em in batches - you'll be happy!" 

 

Best,

Mike Spooner

Hi Mike,

unfortunately not all mirrors were available for me to star test afterwards. But few that did, easily corresponded to what Bath was showing (no surprises there). What interferometer doesn't pick so well is smoothness (it probably would but environment for testing is far from ideal with lots of air current that doesn't really average out) but general faults are straightforward to pick up in star test. Zambuto for example is very smooth but slightly undercorrected (deliberately?) and smoothness actually helps seeing that in star test. Which again shows how sensitive star test is as discrepancy we are talking about in case of Zambuto is tiny (about 1/35 wave rms). 

Talking about smoothness, smoothest mirror that I've seen/tested belongs to an old Takahashi MT160 ( 6.3" f/6.1). Again tad undercorrected but unbelievably smooth.

 

And no, you are not alone in not seeing a perfect star test. Once you start looking at extrafocals you will find faults with every telescope. I call it a curse !

 

Bratislav

Edited by bratislav, 08 December 2018 - 01:46 AM.

[azure1961p]

Brat,

 

It's a sobering thing my C6 has a better star test than my 8" PARKS mirror, though both terrific.  The 8 is very very very smooth.  Roughest optics but a darling little scope is my C90 mak.  I agree star tests are ruthless and after a certain point you either make piece c with the fact that th e OTA is working f or you effectively or not.  I think an unspoken trend among a tract of observers is the uncompromising pursuit of the most perfect optic with a lesser degree of just how little these extremely c ostly incremented improvements can make and the rare nights t h e can be glimpsed at all.  I'm not sniffing at the top opticans and I never would ever.  Still, methinks some put the test before all else .  That said, oh he ll yeah I'd love to own a Zambuto (!)  but there comes a time when testing is done, peace is made either way and real astronomy begins.  Othe r I think s uffer from some kind of telescopic-dysmorphia syndrome where the compulsions to improve outstrip real world results.

 

Pete

Edited by azure1961p, 08 December 2018 - 09:17 AM.

[25585]

My rule of wallet is, that if an option for better wavelength mirror costs no more than a decent eyepiece, I will get the better mirror. But if a larger aperture also costs the same, I would choose that instead.

[GWING]

Brat,
 
It's a sobering thing my C6 has a better star test than my 8" PARKS mirror, though both terrific.  The 8 is very very very smooth.  Roughest optics but a darling little scope is my C90 mak.  I agree star tests are ruthless and after a certain point you either make piece c with the fact that th e OTA is working f or you effectively or not.  I think an unspoken trend among a tract of observers is the uncompromising pursuit of the most perfect optic with a lesser degree of just how little these extremely c ostly incremented improvements can make and the rare nights t h e can be glimpsed at all.  I'm not sniffing at the top opticans and I never would ever.  Still, methinks some put the test before all else .  That said, oh he ll yeah I'd love to own a Zambuto (!)  but there comes a time when testing is done, peace is made either way and real astronomy begins.  Othe r I think s uffer from some kind of telescopic-dysmorphia syndrome where the compulsions to improve outstrip real world results.

Well said. -Greg W
 
Pete

[martinw89]

telescopic-dysmorphia syndrome

 

This perfectly articulates how I feel very often. I think one major contributing factor is that we rarely get good nights for observing, and the rest of our time is spent thinking about how that precious little time could be improved.