BAADER RCC COMA CORRECTOR , BACKFOCUS AND NO FOCUS

I think the RCC is not the problem, but the focus point of your newtonian is. I would guess that it is too low and/or your focuser base height is too high and that’s why it can’t focus properly. Your focus point sits somewhere in the focus drawtube. Be careful, the RCC is no barlowed coma corrector, so it does not push out the focus point like some other correctors (Paracorr or TS GPU)

I had a similiar problem with my 8 inch newtonian and the only solution is to get a focuser that is lower in height, so that my setup is closer to the focus point AND has enough headroom for a focus curve.

Most mass produced newtonians have a focus point 130-140mm above the tube which is not enough if you have a rigid focuser installed. Only the generation 2 newtonians from GSO are finally built with a much higher focus point (mine reaches focus ~160mm above the tube).

I use my camera (17,5mm) + filter wheel (20mm) + oag (17,5mm) + Baader heavy duty rotator (15mm) + M54/M48 Adapter (~7mm) with my RCC and still need M48 spacers to reach the correctors backfocus. When I ordered my newtonian I knew I would need a low profile focuser and ordered a custom one to be installed. Now I could even use some hefty reducers like the 0.73x one and still have enough focus travel for a focus curve.

You can also move the primary closer to the secondary to push out the focus point, but then you will need a bigger secondary to prevent additional vignetting. Thats not a workable and fast solution imo.

Edit: A big advantage of a higher focus point is also, that the RCC does not protrude in the light path of the primary. (Less obstruction) This happens with most of the newtonians that are available, too.

Jandro.rm · Aug 12, 2025, 11:17 AM

Tobiasz:
I think the RCC is not the problem, but the focus point of your newtonian is. I would guess that it is too low and/or your focuser base height is too high and that’s why it can’t focus properly. Your focus point sits somewhere in the focus drawtube.

I had a similiar problem with my 8 inch newtonian and the only solution is to get a focuser that is lower in height, so that my setup is closer to the focus point AND has enough headroom for a focus curve.

Most mass produced newtonians have a focus point 130-140mm above the tube which is not enough if you have a rigid focuser installed. Only the generation 2 newtonians from GSO are finally built with a much higher focus point (mine reaches focus ~160mm above the tube).

I use my camera (17,5mm) + filter wheel (20mm) + oag (17,5mm) + Baader heavy duty rotator (15mm) + M54/M48 Adapter (~7mm) with my RCC and still need M48 spacers to reach the correctors backfocus. When I ordered my newtonian I knew I would need a low profile focuser and ordered a custom one to be installed. Now I could even use some hefty reducers like the 0.73x one and still have enough focus travel for a focus curve.

You can also move the primary closer to the secondary to push out the focus point, but then you will need a bigger secondary to prevent additional vignetting. Thats not a workable and fast solution imo.

Yes i think that is the problem. By the way what kind of focuser do you use? maybe i can looking for another focuser with low base

I think one of the lowest & more affordable focusers on the market is the UNC G2 focuser from TS (UNCN2-G2). I think TS does not produce them themselves so you might find the focuser with the same design from other vendors, too. Depending on the focuser base you need to install on the tube, the focuser can be as low as 50mm. It is REALLY low. But installing an EAF on it sucks, I would recommend 3D printing a flange instead of using the only screw hole it provides.

This focuser with the Generation 2 Newtonians from GSO is a match made in heaven.

Jandro.rm · Aug 12, 2025, 11:32 AM

Tobiasz:

Jandro.rm · Aug 12, 2025, 11:17 AM

Tobiasz:
I think the RCC is not the problem, but the focus point of your newtonian is. I would guess that it is too low and/or your focuser base height is too high and that’s why it can’t focus properly. Your focus point sits somewhere in the focus drawtube.

I had a similiar problem with my 8 inch newtonian and the only solution is to get a focuser that is lower in height, so that my setup is closer to the focus point AND has enough headroom for a focus curve.

Most mass produced newtonians have a focus point 130-140mm above the tube which is not enough if you have a rigid focuser installed. Only the generation 2 newtonians from GSO are finally built with a much higher focus point (mine reaches focus ~160mm above the tube).

I use my camera (17,5mm) + filter wheel (20mm) + oag (17,5mm) + Baader heavy duty rotator (15mm) + M54/M48 Adapter (~7mm) with my RCC and still need M48 spacers to reach the correctors backfocus. When I ordered my newtonian I knew I would need a low profile focuser and ordered a custom one to be installed. Now I could even use some hefty reducers like the 0.73x one and still have enough focus travel for a focus curve.

You can also move the primary closer to the secondary to push out the focus point, but then you will need a bigger secondary to prevent additional vignetting. Thats not a workable and fast solution imo.

Yes i think that is the problem. By the way what kind of focuser do you use? maybe i can looking for another focuser with low base

I think one of the lowest & more affordable focusers on the market is the UNC G2 focuser from TS (UNCN2-G2). I think TS does not produce them themselves so you might find the focuser with the same design from other vendors, too. Depending on the focuser base you need to install on the tube, the focuser can be as low as 50mm. It is REALLY low. But installing an EAF on it sucks, I would recommend 3D printing a flange instead of using the only screw hole it provides.

This focuser with the Generation 2 Newtonians from GSO is a match made in heaven.

It must be check the holes for the focuser. Maybe with other marks don´t fit the skywatcher tube holes

I think TS offers the corresponding base for it to fit for Skywatcher Newtonians. Just contact and ask them if you can make this focuser work with your newtonian and how. They are very eager to help, even Wolfgang himself (the owner of TS) helped me setting up my new newtonian with this focuser :-)

Jandro.rm · Aug 12, 2025, 11:40 AM

Tobiasz:

Jandro.rm · Aug 12, 2025, 11:32 AM

Tobiasz:

Jandro.rm · Aug 12, 2025, 11:17 AM

Tobiasz:
I think the RCC is not the problem, but the focus point of your newtonian is. I would guess that it is too low and/or your focuser base height is too high and that’s why it can’t focus properly. Your focus point sits somewhere in the focus drawtube.

I had a similiar problem with my 8 inch newtonian and the only solution is to get a focuser that is lower in height, so that my setup is closer to the focus point AND has enough headroom for a focus curve.

Most mass produced newtonians have a focus point 130-140mm above the tube which is not enough if you have a rigid focuser installed. Only the generation 2 newtonians from GSO are finally built with a much higher focus point (mine reaches focus ~160mm above the tube).

I use my camera (17,5mm) + filter wheel (20mm) + oag (17,5mm) + Baader heavy duty rotator (15mm) + M54/M48 Adapter (~7mm) with my RCC and still need M48 spacers to reach the correctors backfocus. When I ordered my newtonian I knew I would need a low profile focuser and ordered a custom one to be installed. Now I could even use some hefty reducers like the 0.73x one and still have enough focus travel for a focus curve.

You can also move the primary closer to the secondary to push out the focus point, but then you will need a bigger secondary to prevent additional vignetting. Thats not a workable and fast solution imo.

Yes i think that is the problem. By the way what kind of focuser do you use? maybe i can looking for another focuser with low base

I think one of the lowest & more affordable focusers on the market is the UNC G2 focuser from TS (UNCN2-G2). I think TS does not produce them themselves so you might find the focuser with the same design from other vendors, too. Depending on the focuser base you need to install on the tube, the focuser can be as low as 50mm. It is REALLY low. But installing an EAF on it sucks, I would recommend 3D printing a flange instead of using the only screw hole it provides.

This focuser with the Generation 2 Newtonians from GSO is a match made in heaven.

It must be check the holes for the focuser. Maybe with other marks don´t fit the skywatcher tube holes

I think TS offers the corresponding base for it to fit for Skywatcher Newtonians. Just contact and ask them if you can make this focuser work with your newtonian and how. They are very eager to help, even Wolfgang himself (the owner of TS) helped me setting up my new newtonian with this focuser :-)

Great, thanks Tobias

You’re welcome and good luck!

Sam Badcock:
The backfocus is 91.5mm from the last piece of glass to the camera sensor so you will have heaps of space for an OAG and CAA!!

I have the same scope with an upgraded focused from AliExpress with the Sky Watcher Coma Corrector which has a backfocus requirement of 55mm!!

For me my image train is -

Coma Corrector —> ZWO OAG-L —> ZWO EFW 7x36mm —> ZWO ASI2600mm …….. that gives me the 55mm required back focus in my situation!! All pushed hard up to the focuser tube as well

Not sure what you mean by you can’t have an OAG or CAA??

By the way. Does that scope work with the skywatcher coma corrector? i mean, is the f4 coma corrector or the f5?