William Optics GT102 sanity check

codemonkey (Lee) · #1 20-06-2015, 05:35 PM

Hey guys,

Looks like I've got the wife on my side and I'm considering the acquisition of a William Optics GT102.

The scope will be used for DSO photography with a small chip, mono CCD (Sony ICX674). I'm mostly interested in galaxies and I already have a WO GT-81 for wider field work (2"/px). Aiming for something to get me between 1.3"/px and 1.5"/px -- the GT102 will put me at 1.3" and that's about as deep as I'm comfortable going.

I'm only interested in refractors as there's always a plethora of things going wrong so I don't want to have to worry about collimation, I just want something that's plug and play.

So... any thoughts? Am I about to make a mistake that will eventually result in me selling yet another scope? Anything else that's better for my purposes, for the same money? Based on my calculations I think it'll cost me just shy of $2300 to land in Aus buying direct from WO.

Cheers,
Lee

LewisM · #2 20-06-2015, 05:39 PM

Go for it Lee. These are decent scopes.

codemonkey (Lee) · #3 20-06-2015, 06:00 PM

Thanks Lewis!

I forgot to mention... if purchased, this will replace my Saxon ED100 F/9 doublet. The assumption behind the purchase being that a well-corrected triplet at the right focal length is going to be better than a doublet with a focal reducer.

Shiraz (Ray) · #4 20-06-2015, 06:26 PM

Hi Lee. Sorry to be a wet blanket, but suggest that you get the opinions of Peter (PRejto) and Greg Bradley before you buy. Peter had problems with halos around stars that could have been due to chromatic aberration in his good quality refactor. It seems that refractors generally have deep blue/UV point spread that is not visible by eye (the eye just does not see it very well), but that stands out in an image from a Sony camera because of the very high blue/UV response of those chips.

I don't know about the WO, but it seems that an AP or TOA may be the best refractors for minimising this problem - many other high end designs are great for visual use, but may have problems when used for broadband imaging.

If you have that sort of money, maybe you could give the SW Mak Newt a look - it isn't a refractor, but should (just) work OK on your EQ6, be pretty much plug and play (fully corrected out of the box) and with no CA issues on your chip.

regards Ray

codemonkey (Lee) · #5 20-06-2015, 10:45 PM

Thanks Ray, appreciate that.

I've had a look around for images taken with the scope and so far I haven't seen anything bad, but I also haven't seen any Sony CCD + GT102 images either. Hopefully this is one that minimises the effect you speak of.

Sadly, true premium refractors are out of the budget (unless I'd like a new wife to go along with it) and this is about all I can afford.

The SW Mak Newt is definitely out due to the 1.4m focal length. I had actually considered the ES 700mm MN but after reading a bit I got the impression this design is sensitive to collimation issues and it sounded like just another thing to go wrong.

gregbradley · #6 20-06-2015, 10:58 PM

It was Peter that was having blue halos around bright stars. He tried various filters to minimise that effect. That was with a TEC140.

Yuri was told about this problem and he has tweaked the colour correction of the latest run of TEC140s and they are now a lot better corrected in blue than before.

The icx694 is about twice as sensitive as most Kodak sensors in the blue region. This is great for O111 imaging as they are the most sensitive sensors in that band.

I wouldn't fret over it. Its an FPL53 triplet, sounds good. F6.9 is kind of fast and faster scopes are more likely to have trouble than slower ones. Worst case is you change to a KAF8300 camera if its a big issue.

Greg.

codemonkey (Lee) · #7 21-06-2015, 10:22 AM

Ok cool, thanks Greg. Looks like I'll be locking it in shortly.

Camelopardalis (Dunk) · #8 21-06-2015, 02:36 PM

I've been thinking about one of these scopes myself but decided I wanted something a little faster.

If it shows up the blue halos you could always try a fringe killer or something with a more aggressive blue/uv cut off. So long as you're getting maximum transmission by about 475nm I'd expect you'd be OK, but I've yet to try this myself.

Btw, just IMO but I'd be surprised if you would be affected by this with the 102 if the 81 is fine with the same sensor.

codemonkey (Lee) · #9 21-06-2015, 03:21 PM

Cheers Dunk.

I've not actually had the GT-81 that long and due to the targets I've been imaging and the short time that I've had it, it's actually been relegated to being an over-priced guidescope... I don't actually know how it performs yet for imaging. One day when I get a few clear nights in a row, or the moon's full, I'll have a crack and see how it goes.

PRejto (Peter) · #10 21-06-2015, 03:39 PM

Hi Lee,

Sorry to be a bit late in seeing your post. Yes, I do have some blue fringing issues with my TEC140/Trius combination. However, on many targets it isn't a big deal. On bright blue stars it can be but there are ways to process this out. I've tried the fringing type filters: Baader Fringe Killer, Baader Semi-Apo, Schott GG420, GG435, and Hutech IDAS. The Hutech helped some and I still had good colour balance with my Baader filters. The others caused quite a bit of loss in blue and they tended towards green and were difficult to process. The Baaders transmit further into blue than I believe Astronimik filters so you might want to look at the blue transmit freq. before you decide which to buy. On the other hand with my KAF8300 CCD I didn't have any blue issue...and it's a larger, but noisier, CCD to boot. Perhaps it would be a better match, though a little lower resolution.

Nobody else has questioned your desire to photograph galaxies with this scope. I'd seriously question if ca 700mm is a long enough fl to make you happy. There are only a few "big" galaxies; a lot are pretty small! Before you plunge down the money you might want to reconsider if a fast newt with larger apeture (like what Ray is doing!) might better suit. Just my 2 cents.

Peter

codemonkey (Lee) · #11 21-06-2015, 05:11 PM

Hi Peter,

Thanks for posting! Wow, sounds like you had quite the time trying to resolve the issues causing the halos.

I already have Astronomik filters so it sounds like I'm in luck there.

As for the imaging scale I'd achieve with the 700mm, maybe you're right. That would put me at ~1.3"/px I'm actually a bit confused at the moment with what I'm capturing. I'm using a focal reducer and not sure of the exact, effective focal length given my recent images... PixInsight tells me they're 1.4"/px and Astrobin tells me over 1.5" so I'm not entirely sure.

What I might do tonight is try imaging without my focal reducer and see how I go. My current "big scope" will give me 1.04"/px without reduction... I had massive problems trying to image at 0.73"/px recently and basically gave up so I'm trying to keep within my capabilities. A small, sharp galaxy is better than a large, soupy one.

Cheers,
Lee

LewisM · #12 21-06-2015, 05:44 PM

Peter is right - I tried imaging GX's with an f/5.6 scope, and it was pretty disappointing. You could see them, but the detail was very sorely lacking. I also tried f/6.4 scope with not much improvement. an f/8 scope was starting to do it...

And now I go and get an f/5 refractor astrograph

GX imager never to be

I could TRY the extender I guess...

Alchemy (Clive) · #13 21-06-2015, 05:50 PM

The advertising blurb says and I quote

The stunning new GT 102 APO is the new jewel of the apochromatic refractor world. From the pristine white finish on the CNC to the elegant gold trim, this beauty has more than just good looks. The new GT 102 features a 3-element objective using high-quality FPL-53 glass and combined with other extra-low dispersion glass that reduces any secondary spectrum to an unbelievable degree.

*
Optically unbeatable

Three-element air-spaced primary objective (FPL-53) in adjustable CNC machined ultra high precision lens cell.
Extremely sharp with excellent color correction.
Each lens is Fully Multi-Coated with a special super-high transmission coating (STM Coating) on all surfaces.
This f/6.9 is great for astrophotography.

Enough of the promo stuff .....

I have a William FLT 132 pretty much same optics... Triplet APO , and I have zero problems with halos from the scope, that being the blue halos from your typical acro scopes, any further residual halos around only the brightest stars are from the filters in train at the camera end, an old astronomik Ha and a cheap ir/UV which have now both been replaced.
It's been a good scope

PRejto (Peter) · #14 22-06-2015, 09:07 AM

Quote:

Originally Posted by Alchemy

T
I have a William FLT 132 pretty much same optics... Triplet APO , and I have zero problems with halos from the scope, that being the blue halos from your typical acro scopes, any further residual halos around only the brightest stars are from the filters in train at the camera end, an old astronomik Ha and a cheap ir/UV which have now both been replaced.
It's been a good scope

Clive,

You seem to be speaking both about visual and photographic. The issue I see is not visual at all as our eyes don't perceive blue below a certain freq. (ca 435??). But a sensitive CCD like the Sony in particular will see way down (380 and below). So, you don't say which camera you use on your FLT 132 and that will make a huge difference. Whilst the KAF8300 might show very good blue filter images the Sony 694/674 may very well not. One would really need to see the corrected specs of the scope to know for sure.

Peter

gregbradley · #15 22-06-2015, 11:33 AM

That German guy Mr Rohr, he does analysis of scopes. I wonder if he had done this one as its new.

But FPL53 triplet and air spaced - wow, that's high spec.

If F6.9 is not fast enough you can get a reducer. Most are .75X so that would make it F5.175 that's almost the same as an FSQ106 F5.

I can't see you having a problem. ED80's have shown many sharp halo free images and that's only an FPL53 doublet.

As I say worst case is switching to a camera using the KAF8300 sensor.
I doubt it will be needed. The TEC140 just got caught out by advancing cameras. Yuri does not do imaging and optimises his scopes for visual, that's why. AstroPhysics and Tak have always considered the imager first.

Greg.

codemonkey (Lee) · #16 22-06-2015, 06:40 PM

Thanks for the advice everyone. I think the consensus here is spot on and 1.3"/px is probably not going to cut it. Having said that, I don't know how low I'll be able to go here before I become seeing-limited.

Last night I took a couple of captures of M83 again, this time at 1.04"/px. When I consider that most galaxies I see in Stellarium are around 2' and M83 is ~11', even 1.04"/px might not be enough.

For some reason I had terrible guiding last night... usually I'm getting around 0.65 rms lately, but it was up as high as 1.16 last night, with DEC corrections seeming to have no effect at all.. and then I noticed the objectives were dewing up and then I packed up. If I can get guiding back to normal I think I might be able to handle the 900mm and so I might just keep that for now.

Tip for anyone in the market though... WO apparently have a GT102 they've been using as a demo that should be available for a good price... they advertised one a month ago and that's sold, but this current one hasn't been advertised so get in quick if you're keen...

LewisM · #17 22-06-2015, 06:59 PM

I'll swap you my ST-8300C for the SX

codemonkey (Lee) · #18 23-06-2015, 01:54 PM

Quote:

Originally Posted by LewisM

I'll swap you my ST-8300C for the SX

lol I'm good, but thanks for the offer ;-)

RobC (Rob) · #19 23-06-2015, 03:22 PM

Have a look at these. They are the same price here in AUD and have got exceptional reviews overseas.

http://www.skywatcherusa.com/ed-apo-...efractors.html

Rob

LewisM · #20 23-06-2015, 04:46 PM

Indeed they are excellent Rob, but if I am reading it correctly, Lee already has his W.O and using it.

The Esprits are a mixed bag - either superb or poor. Most of the isues stem from the rubber O-ring they use in the objective which can interfere in the light bath giving diffraction spikes. Also, collimation seems very sensitive with them too. There is a dealer in Europe who completely rebuilds the Esprit objective cell compnonents to eliminate these issues.