90mm APO for astrophotography advise

[drjolo]

I am currently looking for upgrade of my travel astroimaging setup, and searching for a 90-100mm triplet that will fit my 0.8x FF/FR https://www.teleskop-express.de/shop/product_info.php/info/p12209_TS-Optics-REFRACTOR-0-8x-corrector-for-refractors-up-to-102-mm-aperture---ADJUSTABLE.html and will cover APS-C format camera (QHY247C, but mono IMX571 in the future). 

My attention is on two triplets:
Sharpstar 94EDPH f/5.5 like this https://www.teleskop-express.de/shop/product_info.php/info/p12607_TS-Optics-94-mm-f-5-5-Triplet-Apo-Refractor-Telescope.html
Tecnosky 90/560 Owl like this https://www.teleskop-express.de/shop/product_info.php/info/p13566_Tecnosky-FPL55-90-560-mm-Apochromat--OWL-Series.html

 

First one contains two ED elements, but is a bit larger and a bit faster, and with reducer it will give f/4.4 - I am afraid that may be a little bit more demanding with regards to collimation, and also more susceptible for tilt.

So I am now a bit towards the Owl refractor - "regular" f/6 FPL-55 based triplet, that is also supplied with Strehl protocol. 

Do you know maybe any pros or cons for these two models? Or maybe there is somewhere a better alternative around 1500EUR price?

Many thanks in advance!

Edited January 29, 2022 by drjolo

[Louis D]

The Technosky is Long Perng made, I believe, and many reports are claiming better quality control, consistency, and spherical aberration correction than the Sharpstar.  Long Perng is Taiwanese while Sharpstar is Chinese if that matters to you.  Even then, it's likely Long Perng sources some parts from China.

I have the older TS-Optics Photoline 90mm APO FPL-53 triplet for visual use, and the rear end of it looks almost identical to the 94EDPH.  They use really beefy thumbscrews that seem to be solid steel or brass instead of pot metal.  The machining and fit of everything is first rate as well.  I really like the camera angle adjuster for aiming my diagonal off to the side without loosening the retention screws or rotating the focuser.  I haven't tried removing the rear tube section to use binoviewers natively yet, though I'm sure it works fine.

Now my peeves with it:

It annoys me a bit that it shows spherical aberration on one side of focus.  I get a nice Airy pattern on only one side of focus.  The other is just an even blur.  I guess we'll leave it to AP, Tak, TMB, TEC, and LZOS to nail that detail.

The focuser is a bit mushy under heavy loads as well.  I have to overshoot best focus a bit near zenith with a 3+ pound load in the focuser because the fine focus knob tends to counter rotate a fraction of a turn once let go of.  This might not be an astrophotography issue with a motorized focuser, though.  I don't know if it would be up to the task of demanding AP.

It also shows pinched optics while acclimating (spiky stars).  This can take 30 minutes or more to settle down.  Again, I find this annoying in a smallish refractor.

False color seems well controlled in focus even on bright objects.  However, it shows green tinge on one side of focus and red tinge on the other, so it's no reflector.

Otherwise, it's been a decent enough visual-only scope, but it's far from what I would consider perfect for visual use.  Would it be good enough for AP?  I can't say.  For what I paid used for it, I can't complain too much.  It was way cheaper than an AP Stowaway.