Hello all, I have read that BF with a TV PM does not matter. But what about the BF from the back of the scope to the TV?I have a C11 and the BF is 152mm. I am using a Crayford style focuser. With the focuser midrange, should I I add enough spacers before the TV to get 152mm from the back of the scope? Thanks!Art
Back Focus w/ Televue Powermate 2.5
Not sure what you mean. If you change the back focus of the SCT you end up with a slightly larger (or smaller) focal length. OTOH, a telecentric barlow lens (I haven't checked whether the 2.5x PM is telecentric) will keep the magnification factor constant regardless of the distance between its hard stop and the sensor/eye.
@andrea tasselli the PowerMates are telecentric.
Do you have a classic C11 SCT or an EdgeHD 11"? (I'm asking because a lot of people call an EdgeHD X" a CX which is incorrect)
If it's an EdgeHD, you should try to be close the the BWD as the imaging quality deteriorates much quicker with the EdgeHDs as for the classic SCT. If you have a classic SCT, you have much more freedom.
In either case, it holds what @andrea tasselli had said: the effective focal length of the scope changes with BWD and so the overall magnification changes.
Hope that helps.
Björn
PS: Here's a chart from TeleVue that shows the magnification versus BWD for the PowerMates. It's not perfectly constant but compared to a classical barlow the curve's slope is very low.
Do you have a classic C11 SCT or an EdgeHD 11"? (I'm asking because a lot of people call an EdgeHD X" a CX which is incorrect)
If it's an EdgeHD, you should try to be close the the BWD as the imaging quality deteriorates much quicker with the EdgeHDs as for the classic SCT. If you have a classic SCT, you have much more freedom.
In either case, it holds what @andrea tasselli had said: the effective focal length of the scope changes with BWD and so the overall magnification changes.
Hope that helps.
Björn
PS: Here's a chart from TeleVue that shows the magnification versus BWD for the PowerMates. It's not perfectly constant but compared to a classical barlow the curve's slope is very low.
Helpful Engaging
andrea tasselli:
Not sure what you mean. If you change the back focus of the SCT you end up with a slightly larger (or smaller) focal length. OTOH, a telecentric barlow lens (I haven't checked whether the 2.5x PM is telecentric) will keep the magnification factor constant regardless of the distance between its hard stop and the sensor/eye.
andrea tasselli:
Not sure what you mean. If you change the back focus of the SCT you end up with a slightly larger (or smaller) focal length. OTOH, a telecentric barlow lens (I haven't checked whether the 2.5x PM is telecentric) will keep the magnification factor constant regardless of the distance between its hard stop and the sensor/eye.
For example, if I want to use my ASI183mm with my .63 focal reducer I have to maintain a back focus of 90.3mm from the FR.
If I want to use my 183 w/o the FR my back focus would be 152mm from the back of the scope (Classic C11). If I insert a TV PM, does this change things? Do I need to worry about BF at all?
Björn:
@andrea tasselli the PowerMates are telecentric.
Do you have a classic C11 SCT or an EdgeHD 11"? (I'm asking because a lot of people call an EdgeHD X" a CX which is incorrect)
If it's an EdgeHD, you should try to be close the the BWD as the imaging quality deteriorates much quicker with the EdgeHDs as for the classic SCT. If you have a classic SCT, you have much more freedom.
In either case, it holds what @andrea tasselli had said: the effective focal length of the scope changes with BWD and so the overall magnification changes.
Hope that helps.
Björn
PS: Here's a chart from TeleVue that shows the magnification versus BWD for the PowerMates. It's not perfectly constant but compared to a classical barlow the curve's slope is very low.
Thank you for the chart! I See that it is important for me to have the sensor as close to the PM as possible for max magnification.
My concern is the distance from the back of the scope to the beginning of the PM. I have a Classic C11 with a Crayford focuser. Should I not concern myself with this distance? Or, follow the 152mm back focus spec? My setup will be SCT--> Focuser --> PM --> ASI224MC
Well written Respectful
Björn:
@andrea tasselli the PowerMates are telecentric.
The 5x PM isn't. I know because I've tested it. The 2" 4x is telecentric. Also some of APM and Baader ones are telecentric too.
Art Trail:andrea tasselli:
Not sure what you mean. If you change the back focus of the SCT you end up with a slightly larger (or smaller) focal length. OTOH, a telecentric barlow lens (I haven't checked whether the 2.5x PM is telecentric) will keep the magnification factor constant regardless of the distance between its hard stop and the sensor/eye.andrea tasselli:
Not sure what you mean. If you change the back focus of the SCT you end up with a slightly larger (or smaller) focal length. OTOH, a telecentric barlow lens (I haven't checked whether the 2.5x PM is telecentric) will keep the magnification factor constant regardless of the distance between its hard stop and the sensor/eye.
For example, if I want to use my ASI183mm with my .63 focal reducer I have to maintain a back focus of 90.3mm from the FR.
If I want to use my 183 w/o the FR my back focus would be 152mm from the back of the scope (Classic C11). If I insert a TV PM, does this change things? Do I need to worry about BF at all?
For a classic SCT (non Edge) you keep the best BFL (152mm) and add the PM. It will shift (likely if it is anything like mine) the focus somewhat out but that shouldn't be a serious concern for your SCT. Otherwise you add spacers between the camera and the PM so that the focusing doesn't change much in terms of BFL.
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