Newbie question: Telescope --> Reducer -> Flattener -> Filter --> Camera ... Distance & backfocus mess ?!? - Forums

Topic starter

Mar 17, 2024

Hello everyone,

I have an Omegon 140/910 and a ZWO ASI6200MC with a 7x 2" filter wheel. Theoretically it works somehow, but have major Problems No.2 (see below).
Also I try to get the combination of my Pro Reducer 0.74x for 140/910 Triplet APO working together with my Omegon Field Flattener 2’'. And it's all a big mess...

Happy to hear your comments.

So all in all I have two comprehension problems, although the basic logic of the backfocus is clear to me - I thought so.

Problem Assembly Case 1:
140/910 —> Reducer —> 20mm distance piece —> ZWO EFW —> directly screwed on ZWO6200MC (so without plate)

Here I see dark corners on the photos -, actually circled.
So I assume there is vignette from the filters. Can I solve this issue by increasing my 55mm backfocus to something else? (also I tried a lot already)

Problem Assembly Case 2:
140/910 —> Reducer —> (I tried probably 1000 different distance pieces) —> FLATTENER —> (I tried 1000 different distance pieces) —> ZWO EFW —> directly screwed on ZWO6200MC (so without plate)

Additional, general Questions:

A.)
Does the flattener also need a certain backfocus distance from the telescope? (e.g. 55mm)
And then also again a certain backfocus distance from the flattener to the sensor? (e.g.113mm)

B.)
Also I found the shape of the distance piece that comes with the flattener strangely shaped inside (see pictures)

But can’t even get into focus at all. I actually had to minimze the focus on the telescop to 0 to barely imagine anything - not focused at all. So it seems that it's even getting worse the more I increase backfocus distance....

(The following product Info came with the flattener:

"Our expert comment:
The flattener is suitable for all modern ED refractors
The following applies:
-- The shorter the focal length, the longer the distance must be to compensate for the stronger field curvature.
-- The longer the focal length, the shorter the distance must be, because the field curvature becomes weaker.
The telescope focal length is important for determining the correct distance:
-- Focal length 560-590mm: 116mm
-- Focal length 600-690mm: 113mm
-- Focal length 700-800mm: 111mm
-- Focal length from 800mm: 108mm
The set includes a sleeve combination for a distance of 53mm. This consists of a 15mm long M48 - M48 extension (2" outer diameter) and a 38mm long adapter 2"/M48 to T2. To calculate the correct additional sleeves for your telescope, you need the flange focal distance of your camera and the thickness of your T2 ring."

Joerg

Topic starter

Mar 17, 2024

Here is a schematic view of my setup including the distances.
I read somewhere that a reducer might not be necessary when using a full format sensor like the 6200mc ?

Johannes Maximilian Möslein

Mar 17, 2024

Hi Joerg,

I don't have any experience with flatteners, but can help with other points in your post.
A telescope doesn't have backfocus. Only optical elements like reducers or correctors have backfocus.

Concerning the vignetting, the filters should not cause that big of a vignette, I assume it comes from this distance piece:

That's not 2" in width, right?

Well Written Respectful Concise

Geert Vandenbulcke

Mar 17, 2024 · Edited: Mar 17, 2024

You make it very complicated. Don't use this reducer + this flattener together. The flattener is intended to flatten the field of the telescope at normal focal length (910 mm), so use only the flattener and find out the best distance to the sensor. The reducer shown is a simple one, it would be better to use a dedicated reducer-flattener for your telescope, such as https://www.apm-telescopes.net/en/omegon-pro-reducer-074x-for-140910-triplet-apo .

Helpful Concise

Joerg

Topic starter

Mar 17, 2024

Hey Geert, thanks a lot for this hint.
I could make things work somehow. But find it strange.

What I did now:

1. Removed the reducer completely (but wondering then, why I have it at all. What to do with it now?)

2. Put the Flattener directly in front of the filterwheel. But by that I ignore the backfocus of 55mm to the sensor. I did it, because that was the only way I could get rid of my vignette.

3. I placed a lot of distance between the telescope and the flattener. Roughly as the manual of the flattener tells so. 108mm

I have a focus now, but found everything sttrange still because I messed up the backfocus to the sensor itself (between flattener and sensor)

Doesn't matter?

Joerg

Topic starter

Mar 17, 2024

Johannes Maximilian Möslein:
Hi Joerg,

I don't have any experience with flatteners, but can help with other points in your post.
A telescope doesn't have backfocus. Only optical elements like reducers or correctors have backfocus.

Concerning the vignetting, the filters should not cause that big of a vignette, I assume it comes from this distance piece:

That's not 2" in width, right?

Yes it's 2" width. And I could get rid of the vignette by placing the flattener right in front of the filter wheel - more or less ignoring the 55mm backfocus to the sensor..?! Confused....

Joerg

Topic starter

Mar 17, 2024

Geert Vandenbulcke:
You make it very complicated. Don't use this reducer + this flattener together. The flattener is intended to flatten the field of the telescope at normal focal length (910 mm), so use only the flattener and find out the best distance to the sensor. The reducer shown is a simple one, it would be better to use a dedicated reducer-flattener for your telescope, such as https://www.apm-telescopes.net/en/omegon-pro-reducer-074x-for-140910-triplet-apo .

Tim Ray

Mar 17, 2024

It is 17.5 mm from the front of camera to the sensor. I agree, use a combination reducer/flattener vs 2 discrete devices. Or at this point use only the flattener and see how it goes. Most combination reducer/flatteners need 55mm of distance to the front of the camera sensor… much easier to achieve. all you would need is your camera/filter wheel and a provided 16.5mm extension tube that came with your camera.

CS Tim

Concise

Tim Ray

Mar 17, 2024

This pic shows a Sky-Watcher .85x reducer flattener with 55mm of back focus. This is achieved with a 16.5mm ext. tube, Fliter drawer (Same thickness as your filter wheel) and a 2600mmPro camera. A much shorter light path. The length of extention tubes shown earlier in this thread is probably the reason (or at least contributing) to the excessive vignetting that is visible in your sample image. If you can't achieve focus, insert extention tubes between focuser and front (non camera side) of reducer.

CS Tim

Helpful Concise

Geert Vandenbulcke

Mar 17, 2024

If I understand correctly, the flattener is suitable for a range of different refractors with different focal lengths. Back focus distance depends then on the refractor it is used on, so the back focus figures given are an estimate and it needs experimenting to get the best back focus distance. It can be less or more than specified. Once you have a distance between flattener and sensor that gives good star images in the center and at the edges/corners, then the goal is achieved.
I only read now that you also have the reducer specified for your telescope (Pro Reducer 0.74x for 140/910 Triplet APO), it is not intended to be used together with the flattener. Use only the flattener OR use only the reducer if you want a shorter focal length = larger field of view. A good designed reducer should also flatten the field.

Geert

Helpful Insightful Respectful

Joerg

Topic starter

Mar 19, 2024

I somehow finally made it to focus separately.

The only thing that seems to stay is the vignetting - although I am using 2" components on a full format ZWO ASI6200 Sensor.

Any ideas to get rid of the vignette? e.g. different Reducer or Flattener?

The light picture is with the flattener.
The darker picture is with the reducer.

Johannes Maximilian Möslein

Mar 19, 2024

The 6200 has a full frame sensor, so some vignetting can be expected. (Are you still using that bottleneck of an imaging train?)
Proper calibration using flats should correct that without issues.

Well Written Concise

Joerg

Topic starter

Mar 19, 2024

Johannes Maximilian Möslein:
The 6200 has a full frame sensor, so some vignetting can be expected. (Are you still using that bottleneck of an imaging train?)
Proper calibration using flats should correct that without issues.

Okay , thanks. Gonna try the flat shots and see what happens.

Question: What is meant by „image train“? 🤔🫢

Tim Ray

Mar 19, 2024

Glad to see you have achieved focus. One of the negatives about using a reducer / flattener (Mostly the reducer) is that it shrinks the "Circle of illumination" of your telescope by the same factor as it reduces the effective focal ratio. I am going make a fictious telescope for an example.

A telescope with a FL of 1000mm designed for imaging with a image circle of 50mm. Once again, the image circle is the diameter of the fully illuminated image, without vignetting at the camera sensor. Your telescope has an image circle that is smaller than your sensor, the reason why you see vignetting on your image.

If I insert a .8x reducer into the mix. My effective focal length for the fictitious telescope is now 800mm. The image circle is also reduced by .8x and is now 46mm instead of 50mm.

Try a photo without anything except the Filter Wheel and camera and see if the vignetting is reduced. With that config you will have the maximum image circle available and hopefully no vignetting. Imaging with full frame sensors have this issue. I have a Canon 6D EOS camera, I have used it on all my scopes to see if I wanted the 6200. In the end, I stayed away from Full Frame Sensors. My scopes just don't have the image circles to support the upgrade.

CS Tim

Helpful

Tim Ray

Mar 19, 2024

The image train is slang for every component used. Basically the OTA (Optical tube assembly - the tube), the focuser, extension tubes, filter wheels, filter drawers, OAG's (Off axis guiding accessory used in place of a separate guide scope), the camera.

You really need to move away from 48mm extension tubes and move to a larger diameter. Look a your focuser, its large diameter focus tube is used so it does not introduce vignetting. Vignetting I feel you introduced by using extension tubes and other 48/42mm accessories. Another reason I did not make the jump to Full Frame and stayed with the 2600's.

CS Tim

Helpful