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WO ZS 73 vs WO GT 71

d0tpt
03 Mar, 2020 07:04
Hello there,

 I'm just about to buy APO for my portable deep sky astrophotography set-up, but I can't decide between William Optics Zenithstar 73 II APO and Gran Turismo 71 II, maybe you will help or have some advice.

My current set-up:
Tripod: Horizon 8115-2
Tracker: iOptron SkyGuider pro
DSLR: Nikon D5600, not modified yet
Guide scope: QHYCCD miniGuideScope (FL: 130mm, D: 30mm, W: 111g)
Guide camera: QHYCCD QHY5P-II-C
Dew Heater: 5V linear dew heater strip x 2
Mission Control: macbook pro, for now, will be looking into Raspberry Pi alternatives/control via phone or something alike, more portable

Maybe I'm missing something in my set-up? Or something is not right here? please comment on that as well.

Thanks for the advice in advance smile
Edited 03 Mar, 2020 07:04
Starstarter86
03 Mar, 2020 15:39
Hi there,

Not sure about these exact scopes, but I am using an TS-Optics Photoline F/6 which bears remarkable optical similarity to the zenithstar 73 II, albeit it's listed as a 72/432 rather than a 73/430…. might still have rolled off a very simillar assembly line. Im am very content wit this scope, but being a triplet the GT71 should be a tiny bit better optically. It's always good to compare images taken with either of these scopes here on astrobin, if you search by imaging telescope you will find images like this (WO GT 71)
or this (zenithstar73)
For my mind, the stars look a bit better on the GT71, but its hard to tell and also depends on the processing. Both look like good scopes to me. Bear in mind you will also need the fitting field flattener for either scope.

Also speaking as someone who has gone down the Star Tracker/small APO route: On my Star Adventurer, 3,2kg was the most I've ever put on the mount without loosing some precision. These WO scopes are already pretty close to this, as they've got some weighty accesoires like the sturdy tube rings+handle. You might be thinking about a sturdier mount which is still transportable like a Skywatcher EQM35 or an iOptron CEM25P. I have just upgraded to the latter one and after some testing I am quite content, but I haven't had the weather for any serious Imaging yet. And you could consider a mono guiding cam with bigger pixels for improved sensitivity, like the ZWO ASI 120mm mini.

Word to the wise: modify your camera. I started with a unmodified D5300 which I also use for daylight, and have now bought and modified an D3200 (all with the same sensor as your D5600) and the difference is huge! Not only Ha, overall sensitivity is increased as well, a 3min sub on M42 contains more signal than a 6min sub on the unmodified camera.

Hope that helps and clear skies,

Marc
d0tpt
03 Mar, 2020 16:08
Marc, thanks for detailed response! I think I will stick to GT 71, its bit heavier, but I like the quality difference.

Yeah I know I'll be pushing skyguider pro to the weight limits and might exceed them at some point, will be thinking about new mount in the comming months, and your suggestions fit well for my needs!

About the camera modification, can you tell me more about it?
You just removed the existing filter and left the sensor without any or you did put some other glass/filter there?
Where I live, people does not know what astromodification is and its hard to find one who will know what he's actually doing smile
So I might do it myself but I need more information.

Clear skies!
Starstarter86
03 Mar, 2020 22:03
Hi Lukas, the GT 71looks very crisp, and for a triplet the price is quite good!

Lukas
Yeah I know I'll be pushing skyguider pro to the weight limits
There's actually a nifty little trick here: Since the guiding on a tracker like the star adventurer or the skyguider pro is RA only, you can mount the guidescope on the counterweight side of the mount, as long as its pointing roughly the same way as your scope. This way you can keep the overall load down, as your guidescope acts as a counterweight. You will have to find a custom way to attach it though, maybe something 3D-printed.

Lukas
You just removed the existing filter and left the sensor without any or you did put some other glass/filter there?
I have just removed the filter without replacement, becasue I wanted to keep it as cheap and simple as possible. This brings the disadvantage that some Lenses won't reach infinite focus anymore… especially wide angle lenses. And the distance from the bayonet will be off by rougly 1/6 of a mm. On a scope this is not an issue, as you have tons of focuser travel to play with and the possibility to insert shimrings.

Keep in mind that modifying your camera will render it practically useless for Daylight Photography and the autofocus will need recalibration as well if you want to use it. For that reason I rather modified a cheap D3200 I got off ebay with a broken viewfinder/diopter (Non-Issue in AP, where you only use the screen or your computer) rather than my good Nikon D5300.
I used instructions from Lifepixel
I haven't found the instructions for the D5600 there though, it's too new probably.

Clear Skies, Marc
LMVA
25 Mar, 2020 00:17
I have had both and I would keep the 71 that I currently have, smile
Carl_Weber
25 Mar, 2020 02:21
Lukas,
I've been a happy user of a GT71 for the past couple of years.  For imaging, I use a ZWO ASI1600mm Pro.  My mount is a Paramount MX+.

After getting the bugs worked out,  I am totally sold on the GT71.  I did find that it was necessary to use a field flattener.  I use WO's Flat 6a flattener.  It was absolutely critical to get the backspacing correct.  A millimeter or two wrong in either direction made a huge difference in the image.  For my setup,  I had a Precise Parts adapter made (28.5mm).  The ZWO EFW is 20mm thick and the camera needs 6.5mm.   All that totals up to the very critical 55mm of backspace needed for the Flat 6a.

With the backspacing correct,  I had pinpoint stars across the entire field.  The pesky corners of the full frame are great and there is no vignetting.

I only used the ZWO camera and have no experience with a DSLR.  Look at my Astrobin page for examples as well as William Optics Facebook page.  They posted all of the images I've sent them.

Good luck,
Carl
kajouman
25 Mar, 2020 05:56
Marc Agostini
Hi there,Not sure about these exact scopes, but I am using an TS-Optics Photoline F/6 which bears remarkable optical similarity to the zenithstar 73 II, albeit it's listed as a 72/432 rather than a 73/430…. might still have rolled off a very simillar assembly line. Im am very content wit this scope, but being a triplet the GT71 should be a tiny bit better optically. It's always good to compare images taken with either of these scopes here on astrobin, if you search by imaging telescope you will find images like this (WO GT 71)
or this (zenithstar73)
For my mind, the stars look a bit better on the GT71, but its hard to tell and also depends on the processing. Both look like good scopes to me. Bear in mind you will also need the fitting field flattener for either scope.

Also speaking as someone who has gone down the Star Tracker/small APO route: On my Star Adventurer, 3,2kg was the most I've ever put on the mount without loosing some precision. These WO scopes are already pretty close to this, as they've got some weighty accesoires like the sturdy tube rings+handle. You might be thinking about a sturdier mount which is still transportable like a Skywatcher EQM35 or an iOptron CEM25P. I have just upgraded to the latter one and after some testing I am quite content, but I haven't had the weather for any serious Imaging yet. And you could consider a mono guiding cam with bigger pixels for improved sensitivity, like the ZWO ASI 120mm mini.

Word to the wise: modify your camera. I started with a unmodified D5300 which I also use for daylight, and have now bought and modified an D3200 (all with the same sensor as your D5600) and the difference is huge! Not only Ha, overall sensitivity is increased as well, a 3min sub on M42 contains more signal than a 6min sub on the unmodified camera.

Hope that helps and clear skies,

Marc

hey marc,what flatner do you use ? i have a short scope too,but i cant get it flat enough,even  when i use  the good distnce  from sensor to flatner.
d0tpt
25 Mar, 2020 06:54
Carl Weber
Lukas,I've been a happy user of a GT71 for the past couple of years.  For imaging, I use a ZWO ASI1600mm Pro.  My mount is a Paramount MX+.

After getting the bugs worked out,  I am totally sold on the GT71.  I did find that it was necessary to use a field flattener.  I use WO's Flat 6a flattener.  It was absolutely critical to get the backspacing correct.  A millimeter or two wrong in either direction made a huge difference in the image.  For my setup,  I had a Precise Parts adapter made (28.5mm).  The ZWO EFW is 20mm thick and the camera needs 6.5mm.   All that totals up to the very critical 55mm of backspace needed for the Flat 6a.

With the backspacing correct,  I had pinpoint stars across the entire field.  The pesky corners of the full frame are great and there is no vignetting.

I only used the ZWO camera and have no experience with a DSLR.  Look at my Astrobin page for examples as well as William Optics Facebook page.  They posted all of the images I've sent them.

Good luck,
Carl

I went with GT71 II, here are some example photos I managed to take recently (more will come soon), polar alignment was very bad.
https://www.astrobin.com/w9djcg/B/
https://www.astrobin.com/a9ppuw/B/

Btw, Is there any way to check if the flattener spacing is correct 100% for my DSLR? (I can get focus with bathinov mask on, but I'm not sure if the spacing I made is correct and a ways to check it.)
matthew.maclean
25 Mar, 2020 11:06
Hi,
good choice. I have a GT81 and it’s a terrific telescope.  It looks like you are using their Flat6Aiii also. I’m not aware of an exact way to check the spacing is right, but if you have a chance, try taking a similar image with and without the flattener. You should notice a huge difference. I did this when I bought my setup last year and, without the flattener, all the stars look like “shooting stars” in the corners (very obvious).

The WO documentation is a bit vague, but it looks like your GT71 wants 64.1mm total back focus. I use a Canon, but it looks like Nikon also adhere’s to the standard 55mm from sensor to front of T-ring, so if your flattener is set to just past the 9mm mark (assuming no extra filters in line), you should be fine.
 
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