Coma error when using reducer

[cwinstone]

Hello, does anyone know if my imaging train looks correct, and if it does, why am i still getting these coma errors?

Could it be incorrect backfocus? Searching the internet makes me think my dslr is 44mm, adding the t ring (even tried a 1mm spacer too) gets me to the required 55mm (assuming that's correct)

I have no idea how to solve this and i feel like I'm just throwing money down the drain fighting this in vien. Help would be much appreciated.

Skywacher Evostar 72ed

Reducer rotator for 72ed (needs this for extra distance to achieve focus, the reducer and adapter alone doesn't allow for enough outwards travel)

Reducer/corrector for ed72

Canon eos 650d

Edited January 10, 2021 by cwinstone

[vlaiv]

according to this diagram:

your camera is too close - try extending distance between camera and flattener to see if you can reduce / remove issue?

[cwinstone]

8 hours ago, vlaiv said:

according to this diagram:

your camera is too close - try extending distance between camera and flattener to see if you can reduce / remove issue?

Would you know how to find out the correct distance? Ive tried many lengths (0.25mm to 30mm) with no luck.

If the reducer needs 55mm and my camera has 44 plus the t ring of 10, even adding a 1mm spacer, that should be correct right? 

 

[cwinstone]

9 hours ago, vlaiv said:

according to this diagram:

your camera is too close - try extending distance between camera and flattener to see if you can reduce / remove issue?

Out of curiosity, if the sensor is too close, wouldnt it be a line instead of a blob? I dont know what its supposed to look like in a real image.

[vlaiv]

3 hours ago, cwinstone said:

Would you know how to find out the correct distance? Ive tried many lengths (0.25mm to 30mm) with no luck.

If the reducer needs 55mm and my camera has 44 plus the t ring of 10, even adding a 1mm spacer, that should be correct right? 

I'm afraid that best method I know is trial and error method.

Start very slowly, like changing distance by 1mm and compare results - if effect is smaller - then you should be going in right direction - if effect is increasing - you are moving away from ideal position.

Variable extensions come in handy in situations like this, but you can try with set of spacers as well.

If I recall correctly Canon T2 ring is actually 11mm - so ideal position is without any spacers, so start there perhaps and see if increasing distance reduces or amplifies effect?

[vlaiv]

2 hours ago, cwinstone said:

Out of curiosity, if the sensor is too close, wouldnt it be a line instead of a blob? I dont know what its supposed to look like in a real image.

Above diagram shows mostly astigmatism due field curvature - but there can be coma as well - or combination of the two.

You could be having a bit of sensor tilt in your setup since coma is stronger on one side.

vs

Right image (right side of your shot) is what it should look like in case of astigmatism - but it can become coma like when there is a bit of tilt. Deal with distance issue first and then look into tilt.

If you can't find proper distance - try removing rotator. In itself, it should not cause any optical issues as it is in front of flattener and does not contain optical elements - but it can cause tilt, so remove that and see if it helps.

[cwinstone]

2 hours ago, vlaiv said:

Above diagram shows mostly astigmatism due field curvature - but there can be coma as well - or combination of the two.

You could be having a bit of sensor tilt in your setup since coma is stronger on one side.

vs

Right image (right side of your shot) is what it should look like in case of astigmatism - but it can become coma like when there is a bit of tilt. Deal with distance issue first and then look into tilt.

If you can't find proper distance - try removing rotator. In itself, it should not cause any optical issues as it is in front of flattener and does not contain optical elements - but it can cause tilt, so remove that and see if it 

Ive also tried a filter tray with the reducers draw tube attachment (needs something to achieve enough outwards travel to focus) and still get the same problem.  Could the reducer be damaged in some way or is it the wrong one?

[cwinstone]

could my draw tube be slightly wonky?

[cwinstone]

I bought a new reducer. Everything is now fixed. There must be something wrong with my first one, it's identical so maybe there's a factory error. Very expensive solution but at least i can start imaging now.

[Lonestar70]

13 minutes ago, cwinstone said:

I bought a new reducer. Everything is now fixed. There must be something wrong with my first one, it's identical so maybe there's a factory error. Very expensive solution but at least i can start imaging now.

It is good to hear that you have resolved the problem, albeit at considerable expense to yourself.

There has been several cases of faulty assembly with this particular reducer and it would seem that you have another one to add to the score.

You have a VERY strong case for returning the original one for a full refund as you can now prove it is faulty, especially if the new one was purchased from the same supplier.

Even if purchased from a different supplier you can still prove it is faulty and not fit for purpose and should be able to get a full refund on its return.

As you say, they are not cheap so certainly worth a complaint and return for full refund.