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Zenithstar 61 Backspacing with Cooled Camera

William Optics ZenithStar 61 / ZS61 William Optics Flat61A ZWO ASI533MC CMOS Flattener
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Mark Germani avatar
Mark Germani avatar
Hi folks:

After receiving some excellent advice on this forum, I purchased the ASI533MC-Pro and have just received it in the mail. I might hook it up tonight and give it a go, but I'm very uncertain about the backspacing required between the camera and the Flat61A adjustable flattener.

First of all, my apologies for the rather rudimentary question, but after a fairly exhaustive search on various other forums, there doesn't seem to be a consensus on the exact settings to use here. Some forums say set the adjustable flattener to zero and use all the extenders included with the camera, some say 11.2mm and the extenders, one YouTube video says leave it where you had it for the DSLR (12.9mm) and use all the extenders. I believe the last suggestion is the correct one, but I'm wondering if anyone else out there might be able to confirm. I know there are folks on this website who use this OTA-Camera combo.

Thanks in advance! Looking forward to trying this thing out.

CS,
Mark
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Fenrisian avatar
Fenrisian avatar
I‘m assuming your are using the Flattener from WO..

Maybe this will help you:
Mark Germani avatar
Mark Germani avatar
I‘m assuming your are using the Flattener from WO..

Maybe this will help you:

Thanks Fenrisian - I appreciate it! Unfortunately this diagram appears to imply that I have my adjustable flattener set to 14.2mm plus the 55mm from the extenders to equal the 69.2mm asked for, and I can’t find anyone else online using this setting. Maybe I’m not reading the diagram correctly?

CS!
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Fenrisian avatar
Fenrisian avatar
As far as I remember the Image shows the Backfocus with the Flattener set to zero.

So your Target should be 67,9mm, including the XX mm you can get from adjusting your Flattener.

So you just add up the mm from the connection of your camera to the sensor + Spacers + Adjustment on the Flattener = 67,9

Cloudynights
Stargazers


ASI 533 
6,5mm+11mm
If you use the connection ring
Jens avatar
Jens avatar
Hey Mark
i just bought the asi2600 for the Z61 and the flat 61. I had the same question. It turns out that it's literally the same as noted in the camera manual. You can follow those steps. I think that's the 55mm everything should be included spacer wise to achieve that. If you need closer measurements, I can look what I have at home smile
Mark Germani avatar
Mark Germani avatar
Jens:
Hey Mark
i just bought the asi2600 for the Z61 and the flat 61. I had the same question. It turns out that it's literally the same as noted in the camera manual. You can follow those steps. I think that's the 55mm everything should be included spacer wise to achieve that. If you need closer measurements, I can look what I have at home

Hi Jens! Out of curiosity, what position is your flattener set to?

CS,
Mark
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Jens avatar
Jens avatar
From the top of my head it's 12.9mm. you can find that on the william optics website of the flat 61 to see how to set it up. There's two different positions, one for the Zenithstar 71 and one for the 61 since you can use the flat 61 for both.
Quinn Groessl avatar
Quinn Groessl avatar
I don't see the markings on mine (Z61 w/ Flat61A and a 1600mm pro), but measured with a calipers I have the adjustable bit so that there are 12.58mm worth of threads showing. Then I have an OAG (16.5mm) and a filter wheel (20mm) in front of the camera (17.5mm). There might be a touch of trailing, but I'm blaming it on tilt rather than backspacing since it's not even.

The really nice thing about the adjustable one is how quick and easy it is to change it. To get mine dialed in I just took 10 minute or so and took a picture, zoomed in. Adjusted. Repeat until I got something like below.

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Mark Germani avatar
Mark Germani avatar
Ok, sounds like I should keep my flattener at 12.9mm. Thanks everyone!
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Mark Germani avatar
Mark Germani avatar
Just finished the last run of RGB stars for my DSLR projects, nipped outside to swap out the camera and… clouds!

Well, at least I know how to set it up so I'm ready to go for the next clear night. Whenever that is smile

CS!
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Chris Ashford avatar
Chris Ashford avatar
Use a vernier caliper to measure all your components and simply add it all up and make it the right length per the spec.

When you have it ready to go, set up for imaging, point at something that has a lot of stars take an image and check the corners for eccentricity. Add or subtract small adjustments of the FLAT61A until eccentricity goes away completely and your stars are round.


example:
This set up has a M48 to M42 filter drawer, 20mm of M42 extension tubing, the camera rotator, and an M42 female  to female adapter for the camera, which also has a Baader 1 1/4” UV/IR filter in it. The distance from the back of the back focus adjusting ring to the camera face is 60.5mm. Allowing 6.5mm for the sensor recessed into the camera body, that makes 67mm total. Back focus need 67.9mm so the adjusting ring should be set to 0.9mm.
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Chris avatar
Chris avatar
Thanks for the info in this thread…have close to the same setup and realized I had the wrong back focus and my stars in the corners were a bit oval.  My original miscalculation led to the adjuster being set to 10.4…which was incorrect.

After correction I realize it should be MUCH smaller in my imaging train.

For my imaging train:

Sensor depth = 6.5
Filter wheel = 20
spacers = 21 & 16.5

C = A - B
C= 67.9 - 64
C = 3.9

I'm a bit curious with a MUCH different adjustment how the stars will look tonight.  

Appreciate the details in the thread…short, sweet and to the point!
Hans-Joachim Schmid avatar
Hans-Joachim Schmid avatar
Hello!

When I measure the Flat 61A flattener without the adjustment ring, it has a height of 38mm.
If I screw on the adjustment ring to the scale value of 10, the height of the flattener should be 48mm, but it is actually 50mm.
That is roughly the difference in my system. The recommended height is 67.9mm and I measured 69.5mm.
Should I now trust the scale or my measurements, i.e. set about 2mm less on the scale?


CS jacky_smith
Chris avatar
Chris avatar
Hans-Joachim Schmid:
Hello!

When I measure the Flat 61A flattener without the adjustment ring, it has a height of 38mm.
If I screw on the adjustment ring to the scale value of 10, the height of the flattener should be 48mm, but it is actually 50mm.
That is roughly the difference in my system. The recommended height is 67.9mm and I measured 69.5mm.
Should I now trust the scale or my measurements, i.e. set about 2mm less on the scale?


CS jacky_smith

I would trust the scale....but then verify with a night's worth of data.  Once I adjusted mine, I took some test shots and it was dialed in. I realize this is not what you want to hear (you dont want to get out for a night of imaging, get all setup only to find you have to adjust this flattener...which is tough to do when your imaging train all set up).

But it is trial and error.  Once you get it locked...dont touch it!

Best of luck!
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Chris Ashford avatar
Chris Ashford avatar
I have found that the scale gave me a fairly accurate back focus adjustment. But always start out checking for star distortion at the corners before your first night of imaging after adjusting it. I do remember having to make further adjustments, but not as much as 2mm.

Good Luck!
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Hans-Joachim Schmid avatar
Hans-Joachim Schmid avatar
Thanks for your feedback.

CS jacky_smith