Coma Corrector Back focus distance

[woodblock]

I have a SW 200PDS and I bought it with a matching SW Coma Corrector lens. I've been using it with my DSLR for ages and the setup works well. But I'm now looking at changing to a CMOS camera still using the coma corrector and the coma corrector back focus distance has come into 'focus'. When I started with it I didn't even think about it I just connected it all up and it worked. I tried without the coma corrector and I could see straight away that the coma corrector is doing it's job. The back focus distance is given as 55mm.

 

So I have a few questions about this Back Focus distance.

 

1. When measuring the distance do you measure from the rear surface of the coma corrector lens to the sensor?

2. How critical is this distance?

3. What happens if the distance is wrong? Was I just very lucky that when I connected it all up the distance was correct?

 

Cheers

Steve

 

[paul]

The backfocus in this instance is the distance between the Coma Corrector flange and the camera sensor.  Your camera manufacturer specifies how back from the camera front the sensor is. That distance plus all the adapters between the camera front and the Coma corrector flange need to add up to 55mm.

 

The larger the sensor the more critical it is to get the backfocus correct. The distortion you see from an incorrect spacing is worse the further you go from the centre of the image. Its why manufacturers often provide a pack of very thin spacers allowing you to adjust spacing in small steps like 0.1, 02, 0.5 mm. Such small differences can make a big change in star shapes. 

Try to get as close to 55mm as possible. Try it out and check the images for distortion at the edges. Because of tolerances in manufacture, you often have to make it a bit more or a bit less than a measured 55 to get better shaped stars at the edge of the frame. Depending on your setup s it may not be possible to get distortion free everywhere.

 

(fyi: My DLSR camara had a mark on the body  ( a circle with a line through it) that indicates how far from the camera front its sensor is. If I measured the distance between that mark and the Coma Corrector flange it would be 55mm.)

[woodblock]

Just a quick question. My scope has a nominal focal length 1000mm. The stated factor for the coma corrector is 0.9 which should give an overall focal length of 900mm. But when I plate solve an image it invariably gives a focal length of 920mm. Does that change things and does it matter?  I guess I should take a picture without the CC and plate solve to see what it gives for the focal length and that should tell me if the difference is in the scope of the CC. I've not done that.

 

[Padraic M]

The 55mm distance for most coma correctors is designed for a 10mm T-ring and 45mm DSLR distance from the front flange of the camera lens attachment to the sensor surface inside the camera (marked by the circle/line mark on the camera).

If you are looking at a ZWO CMOS astro camera, many of them come with the spacers required to make up 55mm back-focus. 

The webpage for the specific camera you buy on the ZWO website gives the details - this is the page for the 533MC Pro, which is the same for the 1600MM Pro.

 

[Padraic M]

3 minutes ago, woodblock said:

Just a quick question. My scope has a nominal focal length 1000mm. The stated factor for the coma corrector is 0.9 which should give an overall focal length of 900mm. But when I plate solve an image it invariably gives a focal length of 920mm. Does that change things and does it matter?  I guess I should take a picture without the CC and plate solve to see what it gives for the focal length and that should tell me if the difference is in the scope of the CC. I've not done that.

 

If we assume that the coma corrector design is good and it is providing 0.9x reduction, then your back-spacing is out. You could try changing the distance slightly in both directions until it's spot on at 900mm. However, the faster your scope, the more critical this becomes. At f/5, 20mm is unlikely to make a difference.

[edit] What I mean is it shouldn't impact on actual star quality. If the stars in your images look good at 920mm focal length, then there's no need to worry about it.

Edited April 1, 2022 by Padraic M

[woodblock]

On 4/1/2022 at 10:24 PM, paul said:

The backfocus in this instance is the distance between the Coma Corrector flange and the camera sensor.  Your camera manufacturer specifies how back from the camera front the sensor is. That distance plus all the adapters between the camera front and the Coma corrector flange need to add up to 55mm.

 

 

Hi Paul, just to check. Looking at my coma corrector the back surface of the lens is actually set back from the flange face. I just want to confirm that you measure from the flange face rather than the back of the lens.  Is this an industry standard?

 

I've just received my new camera and a pile of spacers so I'm making sure I've got it right.

 

Thanks

Steve

 

[paul]

Hi Steve, yes the flange is the right place to measure from. This is a sort of a convention for all coma correctors. 

 

 

 

I have two CC, th

[woodblock]

Thanks Paul, I'm excited about getting my new camera up and running. Just need a clear night or two.

Steve

 

[Geert]

i also have a Quattro 150/600 with coma corrector and works well with my DSLR. Now i upgraded to a baader 2" clicklock and i cant get in focus anymore. that Baader is twice the with off the stock connection ring. Would it work with astro camera? or without coma corrector and my DSLR?