image train sanity check. - Forums

Topic starter

Jun 20, 2023 · Edited: Jun 20, 2023

Just bought myself a nice second hand TS-Optics GSO 8" Ritchey-Chretien. Unfortunately budget does not stretch far enough for a new camera so I will be using my current ASI183MM. Would the below image train make sense given the use of TS-Optics Optics 2" CCD Reducer 0.67x for RC which according to documentation has 85mm backfocus

Looking for any constructive criticism on this plan before I purchase the OAG.

Thanks in advance

Björn Arnold

Jun 21, 2023

Hi Sean,

Getting the spacing of the reducer to the camera right is probably not the issue, as you can obviously exchange adapters between OAG and reducer. Usually it’s trickier to get the guide camera focused, depending on the design of the OAG. I had to buy a new guide camera (from ZWO) and ditch my Omegon as the sensor couldn’t be brought close enough to the OAG.

Are you sure you actually need the reducer? I also had this scope and my 1“ sensor was sufficient regarding FOV.

Björn

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Sean van Drogen

Topic starter

Jun 21, 2023

Hi Sean,

Getting the spacing of the reducer to the camera right is probably not the issue, as you can obviously exchange adapters between OAG and reducer. Usually it’s trickier to get the guide camera focused, depending on the design of the OAG. I had to buy a new guide camera (from ZWO) and ditch my Omegon as the sensor couldn’t be brought close enough to the OAG.

Are you sure you actually need the reducer? I also had this scope and my 1“ sensor was sufficient regarding FOV.

Björn

Hi Björn, Thanks for the reply and sharing your experience.

Thanks for this I had not even considered not using the reducer as it comes with the scope. The FOV would not really be the main consideration.
Now thinking about it, I think it would be mainly about the benefit of changing F-ratio and resolution. F8 & resolution 0.3"x0.3" per pixel to change to F5.4 & resolution 0.45"x0.45" per pixel. Now my understanding might be flawed but I believe the reduced setup would be more forgiving on my guiding.
It will be mounted on a CEM70.

I have an ZWO ASI174MM mini that I was using on a different setup that I will move, so not to worried about that.

Clear skies,
Sean

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Björn Arnold

Jun 21, 2023 · Edited: Jun 21, 2023

Hi Sean,

Assuming average seeing conditions (say 2-3 arcsecs), you‘re always oversampling with your setup and so I recommend binning. Since you‘ve a CMOS, you could consider binning after imaging.

That being said, your guiding requirements are NOT more strict if you keep your imaging resolution constant. The required guiding precision doesn’t change if you use a 800mm refractor at 1x1 binning or your 1600mm RC at 2x2.

Björn

EDIT: IMHO, reducers should only be used if there is a need and are no other means to increase FOV. Otherwise, don’t use them. For your RC and this sensor size, the imaging quality of the covered field will be absolutely fine

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Sean van Drogen

Topic starter

Jun 21, 2023

Hi Sean,

Assuming average seeing conditions (say 2-3 arcsecs), you‘re always oversampling with your setup and so I recommend binning. Since you‘ve a CMOS, you could consider binning after imaging.

That being said, your guiding requirements are NOT more strict if you keep your imaging resolution constant. The required guiding precision doesn’t change if you use a 800mm refractor at 1x1 binning or your 1600mm RC at 2x2.

Björn

EDIT: IMHO, reducers should only be used if there is a need and are no other means to increase FOV. Otherwise, don’t use them. For your RC and this sensor size, the imaging quality of the covered field will be absolutely fine

Hi Björn,

Will give it a go without the reducer and indeed binning/resampling after the fact seems to be the way to go.
Will put a larger pixelsize camera on the wishlist for next year to get better sampling

Clear Skies,
Sean

Björn Arnold

Jun 21, 2023

Well, general technology trends work against your wish. Pixels usually become smaller and smaller but in your case, your camera has already „extremely“ small pixels.

Omegon sells a camera with 9um pixels but for a new camera, I‘d probably go for an APS-C sized sensor with those 3,7-something micron pixels as for shorter focal lengths, the large pixels probably aren’t what you want.

Björn

Sean van Drogen

Topic starter

Jun 21, 2023

Well, general technology trends work against your wish. Pixels usually become smaller and smaller but in your case, your camera has already „extremely“ small pixels.

Omegon sells a camera with 9um pixels but for a new camera, I‘d probably go for an APS-C sized sensor with those 3,7-something micron pixels as for shorter focal lengths, the large pixels probably aren’t what you want.

Björn

Yes a version of IMX571 is on the wishlist , but with new filters and new filterwheel that is not in the budget anymore

not soon anyway.

Sean

Joe Linington

Jun 21, 2023

Hi Sean,

Assuming average seeing conditions (say 2-3 arcsecs), you‘re always oversampling with your setup and so I recommend binning. Since you‘ve a CMOS, you could consider binning after imaging.

That being said, your guiding requirements are NOT more strict if you keep your imaging resolution constant. The required guiding precision doesn’t change if you use a 800mm refractor at 1x1 binning or your 1600mm RC at 2x2.

Björn

EDIT: IMHO, reducers should only be used if there is a need and are no other means to increase FOV. Otherwise, don’t use them. For your RC and this sensor size, the imaging quality of the covered field will be absolutely fine

I am curious why you dislike reducers? Every scope I own has a reducer on it, it's basically a requirement for me. The speed advantage of a 0.67 reducer is not insignificant. f/8 to f/5.36 cuts your required imaging time more than in half while also reducing the amount that he is oversampled, and yes a reduction in focal length from 1600mm to 1072mm will make guiding slightly easier.

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Björn Arnold

Jun 21, 2023 · Edited: Jun 21, 2023

Joe Linington:
...

I am curious why you dislike reducers? Every scope I own has a reducer on it, it's basically a requirement for me. The speed advantage of a 0.67 reducer is not insignificant. f/8 to f/5.36 cuts your required imaging time more than in half while also reducing the amount that he is oversampled, and yes a reduction in focal length from 1600mm to 1072mm will make guiding slightly easier.

The only advantage that I see is that they increase FOV if you need it and have not other means to reduce the effective focal length.

The other things you mention can easily be done by binning and avoid some possible drawbacks:
1. The RC that Sean mentions provides a very good imaging quality over the sensor size he has. Therefore, additional image correction, like a reducer/flattener combination may provide, would be unnecessary and if it's a reducer only, it won't improve image quality significantly (some reducers help reducing field curvature a bit).
2. and adding to 1.: Contrary to improving the imaging quality of the scope, it's even possible to deteriorate the image quality by either having issues finding the proper spacing and if the reducer is of questionable quality, it may simply make image quality worse, not matter what the back spacing is.
3. Reducers add vignetting which can create problems for the OAG as the OAG is by definition off-axis an depending on how far he has to be off the axis, the light gathering capability might be significantly reduced, despite the shorter EFL.
4. More glass, more loss of light.
5. Glass close to the camera and filter may produce halos.

Björn

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Joe Linington

Jun 21, 2023

Joe Linington:
...

I am curious why you dislike reducers? Every scope I own has a reducer on it, it's basically a requirement for me. The speed advantage of a 0.67 reducer is not insignificant. f/8 to f/5.36 cuts your required imaging time more than in half while also reducing the amount that he is oversampled, and yes a reduction in focal length from 1600mm to 1072mm will make guiding slightly easier.

The only advantage that I see is that they increase FOV if you need it and have not other means to reduce the effective focal length.

The other things you mention can easily be done by binning and avoid some possible drawbacks:
1. The RC that Sean mentions provides a very good imaging quality over the sensor size he has. Therefore, additional image correction, like a reducer/flattener combination may provide, would be unnecessary and if it's a reducer only, it won't improve image quality significantly (some reducers help reducing field curvature a bit).
2. and adding to 1.: Contrary to improving the imaging quality of the scope, it's even possible to deteriorate the image quality by either having issues finding the proper spacing and if the reducer is of questionable quality, it may simply make image quality worse, not matter what the back spacing is.
3. Reducers add vignetting which can create problems for the OAG as the OAG is by definition off-axis an depending on how far he has to be off the axis, the light gathering capability might be significantly reduced, despite the shorter EFL.
4. More glass, more loss of light.
5. Glass close to the camera and filter may produce halos.

Björn

Thank you. Unfortunately I don't have any scope that will function well without a flattener or reducer so some of those arguments don't apply to my case.

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