Imaging telescope. Focal length and aperture

[5ash]

I am at present imaging with a SW 120ED telescope and wanted to gain more photons more quickly . I have a C9.25" with a meade F6.3 focal reducer and did the calculations on image scale and brightness for both scopes . I found, correcct me if im wrong , them to be Similar for both scopes . The only advantage of the c9.25" being increased resolution. I then looked at the possibility of a small fast imaging newtonian like those offered by GSO and Skywatcher. However with my present setup I use an Orion DL off axis guider with no back focus problems , would I still be able to use one of these with a coma corrector on a photographic optimized Newtonian? I suspect an 8" Schmidt Newtonian OT would better fit the bill , but i dont think they make them anymore.
Regards philip:question:

[glend]

How thick is the OAG? I image with a GSO 8" imaging newt (f5) and use the GSO coma corrector ahead of my DSLR and it works very well. The GSO newt come with a 35mm extension so it can be used for visual use as well. The reason for asking about the OAG thickness is to work out whether a spacer is required between the coma corrector and camera. The coma corrector can be taken apart and just the lense element section used with appropriate T-thread spacer if required. Focal point is best around 80mm from the top lense element of the coma corrector to the sensor. What sort of camera are you using?

I picked the f5 GSO to improve coma management (there isn't any right out to the edges with my setup), and would recommend it over the f4 if your mount can handle the longer tube length (1000 fl ).

I use a guide scope (ASI130 camera) riding on top the tube on a long dovetail bar. So guiding weight is not on the focuser. The GSO focuser holds my coma corrector and camera (Canon 450D) with no problem - just remember to lock the focuser screw once focused. I am not sure how much an OAG and its camera would add to the weight on the focuser. I had considered upgrading to a Moonlight but so far the stock focuser is doing fine.

BTW those scopes are rediculously cheap at Andrews right now.

[Amaranthus]

For the same camera/chip, image scale is determined solely by focal length of the imaging train. Your C9.25 @ f/6.3 would be 1457 mm, whereas your 120ED native would be 900mm. So the C92.5 will give an image scale 61% smaller than the refractor. It will also gather photons faste than the 120ED, because the former is f/6.3 and the latter is f/7.5.

[5ash]

Hi Barry , if the image scale was just one side of the frame , yes it would be about 61% ,but it's the other way round as the image scale is greater with greater focal length .However we are looking at a 2 dimensional surface over which the light is spread to form the image , thus it is a division of the square of the two focal lengths to compare the two image scales. ie (900x900)/ (1457x1457)=0.38 or 38% smaller for the refractor (that's why more fits on the sensor at shorter focal lengths). Then compare the light gathering power of both scopes using the square of the aperture and taking into account the obstruction on the c9.25" (~7cm diameter) . ie (120x120)/ (235.5x235.5)- (70x70) = 0.28 or 28%

Thus the c9.25 captures 72% more light than the refractor . That comes to focus over a greater surface area than the refractor. Thus the advantage of greater light gathering power is to a large extent cancelled out by the same size of the sensor in each case . The extra light is outside the perimeter of the sensor for the c9.25" compared to the refractor.

Hope this makes sense regards philip

[Amaranthus]

I'm not really sure what you were asking then Philip. Measuring image scale in arcsec/pixel, what I said above is correct - for a given chip, the longer FL will give a higher per-pixel resolution. The speed (f/ratio) determines the rate of photon accumulation per unit area (pixel), not the aperture.

But it seems like you were already well equipped to answer your question, so I suspect you were after something else. Could you re-clarify? :shrug:

[5ash]

Not sure whether my understanding of image scale is the same as yours ? In my mind the bigger the scale the bigger the image that is projected on a surface . The longer the focal length the larger the image projected , the bigger the scale. The c9.25" would have a bigger image than the sw120 at prime focus . The sensor is a fixed size , so more of the image produced by the sw120 would fall on it than the c9.25". Hence more of the light captured by the sw120 would fall on the sensor than the light captured by the c9.25"( a lot of which would be lost outside the sensor ).
Philip

[5ash]

Hi glen ,
I think it's less than/ about 80 mm as I've used it with my Schmidt cassegrain and f6.3 reducer that requires 80mm separation from the camera. The camera is a bit weighty with the OAG , probably ~ 1kilo , would the focusser handle that?
Regards philip

[Terry B]

Just to be confusing, this only works for extended objects. For stars that are point sources of light, aperture is more important than f ratio. If the aim is photometry then more aperture means dimmer stars can be measured.