image scale???

[northcanadian]

Hi again all:

I'm using a 9.25sct and ASI 120mm with filters.  For Jupiter I will probably continue with the 2x Barlow.  I have a Ctayford style focuser on the visual back. If I attach a flip mirror to this, plus the 2x barlow then the camera, how much more would the image scale be increased with these accessories in the imaging train?

Thanks for any and all suggestions (as always)

Cheers

Roger

[jambouk]

The flip mirror sounds like the only addition, as from my reading of what you've said, you already use the 2x barlow. The flip mirror won't noticeably alter the inage scale, but you'll have to move the focus a turn or two to achieve focus.

James

[JamesF]

To be honest it's hard to say in the general case.

The effective focal length of an SCT varies with the position of the focal plane, so if you just had a camera in the visual back and then added the flip mirror you'd find that the image you get when the flip mirror is in place is larger.  The barlow makes the effect more noticeable, and the result will vary depending on which barlow you're using and how exactly the optical train is configured.

There was some discussion of this a few weeks back, but I can't recall whether we actually ended up with any figures for how much the effective focal length changes based on how far the focal plane is from the back of the OTA, or if it was just the distance from the back of the OTA at which you achieve the stated focal length.  I'll see if I can find the thread.

By way of an illustration, if I put my crop-sensor DSLR straight onto the visual back of my 127 Mak (not an SCT obviously, but the same thing happens with Maks), the image of the Moon pretty much fills the sensor.  If I put a 50mm extension between the camera and the visual back, the image is far too big to fit on the sensor.

With all that out of the way, I also have a 120MM that I use with a filter wheel (a Xagyl 5125, so quite thin) and a Revelation R&P focuser on my C9.25.  I also wanted to use it with a flip mirror and bought the Vixen model.  I found that with a 2x Tele Vue barlow I was getting a focal ratio of somewhere in the region of f/27 which was far higher than I wanted, so I stopped using it.

James

[JamesF]

Here's the thread: http://stargazerslounge.com/topic/237247-saturn-with-asi174mm/

James

[jambouk]

James, so are you saying, using the Mak 127 example, that the extra 50mm adds significantly more to the apparent focal length than you would expect if the 127 went from 1500mm to 1550mm?

When i got my flip mirror, which probably adds an extra 150mm onto the back of the C11 (2700mm) even when using the 2.5x powermate after the flip mirror (making focal length go from 6750mm to 7125mm) i couldn't see much (if any) difference in the size of Jupiter on the laptop screen, though appreciate it will have been different.

James

[jambouk]

That thread is very interesting.

[JamesF]

I discovered the behaviour entirely by accident to be honest, so I didn't really investigate as much as perhaps I would do now.  I'd put the DSLR on the visual back of the Mak and got a nice solar image, but the camera made the focuser knob a bit awkward to get to, so I put a 50mm extension in.  I was really surprised when what fitted nicely on the sensor before was now way too big.

The effective focal length of an optical system containing multiple optical components varies depending on their individual focal lengths and the distance between them.  I can't remember the exact relationship at the moment though.  I have a feeling it's some sort of reciprocal law.  What it does mean though is that if you move the mirrors in a Mak or SCT towards each other (I think) to move the focal plane 50mm further back, you can increase the effective focal length by a fair bit more than 50mm, and that moving from a position of "0" mm to 50mm will not give the same change in effective focal length as going from 50mm to 100mm.

In the end the easiest way to be sure about what effect you're getting is to capture an image and work backwards to the focal length.

James

[northcanadian]

Thank you James and James....guess some experimentation is in order next time out!

much appreciated

Roger

[jambouk]

Merlin66 posted a spreadsheet sometime ago about these sorts of issues. I can't find the thread but will message Merlin66.

This would be an interesting "project" to do, taking an image of something very distant on the terrestrial horizon with various extension tubes an barlows, to work out exactly how much difference it all makes. 

James

[Merlin66]

I use a C11 for spectroscopy and find by changing the back focus position the effective focal length varies (and the f ratio) the same changes need to be taken into account when adding reducers etc.

The attached spreadsheet may be of assistance.

See the notes at the bottom re focal length changes.

reducers_V2a.zip