spacing requirement for a focal reducer

ASI's Range of Specialised CMOS camera's

Moderator: Sam

Fivel
Posts: 34
Joined: Sat Oct 13, 2018 2:36 am

spacing requirement for a focal reducer

Post by Fivel » Mon Feb 24, 2020 9:46 pm

Hello.
I just bought an Astro Tech (AT) .8 focal reducer/flattener model ATR8 for my scope, and ZWO camera.
I have the AT 80mm/F480mm ED triplet scope and the ZWO1600MC-C camera.
The FR/FF is apparently made by GSO. The manufacturer recommends a spacing (FF rear shoulder to camera sensor) of 55mm.
I tried some images, but that spacing seems to much. The stacked images show a large central bright image circle surrounded by a more normal brightness. Also the stars are showing coma.
Is anyone at your company familiar with this FF/FR and the actual spacing needed for this scope and camera combination?
Thank you for any help with this.

Fivel

Benni
Posts: 52
Joined: Sat Mar 11, 2017 9:52 pm

Re: spacing requirement for a focal reducer

Post by Benni » Tue Feb 25, 2020 12:58 am

I guess you should buy a dedicated flat field scope. And even then, you will see image errors with the ASI.

Unfortunately, most of the telescope/reducer systems were calculated for old noisy sensors with large pixels...
A telescope with a suitable flat field for new cmos cameras will cost money.

For refractors, use quadruplet/quintuplet systems, with a triplet as lens and a one, two or three lens flattener build in.

The flattener is then calculated for the lens in front of it, and that makes some difference.

For newtonian reflectors, there are correctors which can work (GPU corrector for newtonians).

For Schmidt-Cassegrains, i would advise to use an edge hd from Celestron. Unfortunately, the reducer that celestron makes will produce intolerable image errors because it is also old, uses cheap glass and has problems with the spherochromatism from itself and the sct correctors. The native sct unfortunately has only f10.

Rc telescopes have faster aperture than sct work but they need intense collimation.

Similarly, the skywatcher mak newt needs an intense collimation effort. There are people who basically screw it apart and fix some things with the primary and secondary, before collimating with cats eye tools to get better images.

Finally, there are hyperbolic astrographs from takahashi. They appear to work. And then there is the Celestron rasa. It needs a tilter but then it works at f2.2. Sadly, the rasa does not like filter wheels...

Fivel
Posts: 34
Joined: Sat Oct 13, 2018 2:36 am

Re: spacing requirement for a focal reducer

Post by Fivel » Wed Feb 26, 2020 3:13 pm

Hello Benni.
Although I appreciate your effort, to provide me with some information, it did not address my questions.
Since I already have the equipment I listed, I am only interested in the best spacing for the combination.
The AT focal reducer was designed for the scope I have and for today's astro cameras, including the ZWO asi1600.
So the lengthy info you gave me on purchasing and collimating others scopes is not relevant to my situation.
Many other astro imagers have similar setups and get great results.
Is there any other employee, in this company, that can provide me with more helpful spacing recommendations?
The 55mm spacing provided by the focal reducer manufacturer seems to be to long.

Fivel

Support1@ZWO
Posts: 520
Joined: Fri Dec 07, 2018 10:24 am

Re: spacing requirement for a focal reducer

Post by Support1@ZWO » Thu Feb 27, 2020 3:09 am

Hi Fivel,
Could you confirm the back focus and tell us how was the connection?

Fivel
Posts: 34
Joined: Sat Oct 13, 2018 2:36 am

Re: spacing requirement for a focal reducer

Post by Fivel » Thu Feb 27, 2020 3:40 pm

Hello Katherine and thank you for your reply.
Please forgive me, but I do not know what specific information you need. Here is what I have.
As for back focus, I mentioned that the manufacturer of the focal reducer (FR) recommends a spacing (Back Focus) of 55mm from the rear shoulder of the FR to the camera sensor. That is far to much.
The connection setup I have is: telescope/ focuser-FR-50mm spacers-ZWO asi 1600 camera (no T ring) connected directly to the spacers.
This gives me a total back spacing of 56.5mm, well within the tolerance of the recommended spacing.

Fivel

Chuckwagon
Posts: 29
Joined: Tue Dec 04, 2018 3:52 pm

Re: spacing requirement for a focal reducer

Post by Chuckwagon » Thu Feb 27, 2020 11:59 pm

"The connection setup I have is: telescope/ focuser-FR-50mm spacers-ZWO asi 1600 camera (no T ring) connected directly to the spacers.
This gives me a total back spacing of 56.5mm, well within the tolerance of the recommended spacing."

Are you saying that you have tried the spacing (56.6mm) that you have, and the reduction was not what was advertised, as in you did not get a .8 reduction? (The apparent field of view from the 1600+AT80 of about 95 arcmin x 126 arcmin did not go to about 120 x 158.) If so, what amount of reduction did you get? If it was too much reduction, then you need to shorten your spacing, and if it was too little reduction you need to increase the spacing. It may take some trial and error to get the spacing correct to get the exact .8 reduction advertised.

As for the uneven illumination, a vignetted field is to be expected when using a reducer in most circumstances. If the AT80 does not provide a fully illuminated (no vignetting) circle of at least 34mm diameter, then the reducer is going to show that when it "shrinks" the circle. Also, the largest illuminated circle the reducer can create will be its reduction factor (.8) times the smallest clear aperture in the imaging train from the reducer forward. Usually, it would be the reducer's own clear aperture that would limit the size of the circle. But if you have a filter in front of the reducer somewhere, or an especially narrow tube, or whatever, that will limit the size. Even if the reducer is the only restriction, the circle isn't likely to be evenly illuminated. So the closer your sensor size gets to the size of the illuminated circle, the more vignetting you'll see. Use flat frames to account for the uneven illumination.

Reducers can be a pain the the backside. Not all makes of reducer will work equally well with every make of scope. Generally it's best to use a reducer specifically designed for a given scope. So even if you are getting the correct reduction factor, you may not be getting field correction that works well with your scope, and the images will suffer. If you check your reduction factor, and it's close to .8, and the images are poor, you may need to get a different FF for your scope.

Fivel
Posts: 34
Joined: Sat Oct 13, 2018 2:36 am

Re: spacing requirement for a focal reducer

Post by Fivel » Fri Feb 28, 2020 3:49 pm

Hello chuckwagon and thank you for your detailed reply.
I did submit the image to nova.astrometry.net and the FOV came back as 768X1031 arcseconds.
This (if correct) seems to be way to large for the recommended 55mm spacing.
The FR/FF is the one designed for my scope.
I will take your ideas, and start with no FR and submit that. It should be a good base, to compare Fr/spacer combinations for the amount of FL reduction.
Then I will compare submissions with the FR /no spacing, and then various spacers.
Another reply, from a different forum suggested I just use the spacing of a typical (?) T ring adapter.
The one I have is about 15mm (it doesn't have the correct threads to connect my camera though). It seems reasonable that it should get me closer to the 0.8 reduction ( I have a 15mm spacer to use).
Of course, that will be one of my astrometry submissions for comparison.
It also looks like I may need to purchase some more spacers, to get more options.
Thanks again for your great info.
Fivel

Chuckwagon
Posts: 29
Joined: Tue Dec 04, 2018 3:52 pm

Re: spacing requirement for a focal reducer

Post by Chuckwagon » Fri Feb 28, 2020 8:35 pm

Hmm, the field of view reported by Astrometry seems to be far too small for that camera. My calculations, for a focal length of 480mm with 3.8 micron pixels and a resolution of 4656x3520, yield a field of view of 95.7 x 126.6 arc-minutes. (5,742 x 7596 arc-seconds) The reducer should make those numbers larger, not smaller. 768 x 1031 arc-sec is about what FOV you'd get with a 3500mm FL scope. :)

Was your image cropped? If your image was cropped, then the Astrometry results would be skewed. What is the resolution of the image you submitted? A full resolution image, no crop, should be 4656x3520. (That's the resolution specified by the data sheet for the 1600, though I realize that sometimes final images may be a few pixels less. So it should be close to that.) Did you happen to note the image scale (Arc-seconds per pixel) reported by Astrometry? With a resolution of 4656x3520, your image scale with a 1600 and a 480mm FL should be about 1.63 arc-sec/pixel. The .8 reducer should effectively give you around 384mm FL, which would change the image scale to 2.04 arc-sec/pixel. If the image scale reported by Astrometry was larger than 1.63, then something is very wrong. (Like the reducer is on backwards and magnifying instead of reducing. :) I don't know if they do that, I've never tried it, but the point is, something is wrong.)

I think you're on the right track, so follow your plan and get an image with no reducer and then work from there. The good thing about testing is you don't need great images. Just take an image that's long enough for stars to clearly show and you're good. No processing needed. Use full resolution, no binning or cropping. M45 is a good target for tests like this. Even at full res and 480mm it will fit (barely) in the FOV. And the relatively bright stars are easy to get in short exposures.

Good luck.

Fivel
Posts: 34
Joined: Sat Oct 13, 2018 2:36 am

Re: spacing requirement for a focal reducer

Post by Fivel » Sat Feb 29, 2020 3:45 pm

Hi again chuckwagon. My friend and I did some spacing test last night.
The only test I did not do since I had (temporarily) misplaced the 1-¼" focuser adapter, was the "no reducer" test.
I do not yet have an adapter, to connect the camera directly to the telescope focuser (2" nosepiece to female T42 thread).
That will be my next purchase.
We did try three different spacings on the same M45 image.
Here is the strange situation.
Although we did need to refocus each time, astrometry came back with the exact same FOV, (each looked the same before submitting) for each spacing change. We could not determine why there was no change of focal reduction.
On the positive side, the FOV is decently wide. M45 looked promising as a future imaging object.
We swung over to the Horsehead/Flame field and got excellent results.
Here is a link to the astometry job. http://nova.astrometry.net/user_images/ ... #annotated
And here is the actual image we submitter (no crop or processing).
28 Feb 2020-M45_RGB_24_1x1_180s_ -30C_185_001.jpg
Since I finally found the 1-¼" adapter, I will (next clear night) submit an image without the reducer, to see what I get.

Fivel

Chuckwagon
Posts: 29
Joined: Tue Dec 04, 2018 3:52 pm

Re: spacing requirement for a focal reducer

Post by Chuckwagon » Sat Feb 29, 2020 8:22 pm

Well, at least you're making progress. The FOV seems to be more in line with what that camera and scope should see. Astrometry indicates 123.6 x 93.6 arc-min. But it also shows the image scale to be 3.43 arc-sec/pixel, which is probably because it shows the image as being 2160 x 1633 pixels. Is that the full resolution image size from that camera? I thought the 1600MC had a max size of 4656x3520. You are using a 1600 based camera, right? When I checked, all of the 1600s that ZWO lists have the same chip size and resolution of 4656x3520.

Odd that there wasn't a change in the FOV as you changed spacing. The reducer should be reasonably tolerant of slight spacing changes from a performance standpoint, but there should be a little difference between them in FOV. Maybe something else is causing a problem here, like the image acquisition process. If the camera is supposed to have a resolution of 4656x3520, but you are only getting 2160 x 1633 images, maybe the capture program is somehow scaling to 2160 x 1633, and that's monkeying up the process.

In any case, at least the FOV is more in line, though not as large as I'd expect. There appears to only be about .976 reduction instead of closer to .8. How much spacing was between the reducer and the image sensor? AstroTech says 55mm is optimal. So with your camera, if you measure from the front surface of the camera housing to the shoulder of the reducer, there should be 48.5mm of spacers. I have attached a diagram to make it easier to visualize. Assuming AstroTech's advice is correct, ( :) never can tell) use this spacing as a start and see what it gets you.

Optimal Spacing.jpg

Post Reply