Quote:
Originally Posted by xelasnave
I ordered a RASA 11 inch and a ZWO 2600 mono plus all the filters...probably a very stupid move if I dont get to use them but it was more a "my health is not going to hold me back thing"...anyways... My most recent imaging set up has a 115 mm f5.5 refractor with a ZWO 1600 mono, a very capable unit....now with time on my hands whilst the goods cross the seas I actually started wondering just how fast will this new set up be...what factors will come into play? The larger appeture? the new camera?
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Hi Alex,
Well you've picked all the right variables in terms of what will effect your exposure duration between your old and new setup, but as you're changing the camera as well as the scope there are a few things to consider, in comparing both setups, namely:
- differences in f-ratio
- differences in camera QE
- differences in Camera Sensor size/FOV
- differences in Noise
- differences in telescope optical aberrations
These will all effect the answer as to what constitutes an equivalent exposure between both systems, in terms of capturing a similarly detailed, similarly deep, similarly bright/exposed image. Fortunately in this comparison the focal length of both the f/2.2 RASA and the f/5.5 Scope are very similar 620mm V 632mm, if we take it that the f/5.5 image circle is not vignetted in any way by a focal reducer then the following comparison is possible....
Just focusing on number 1:
differences in f-ratio for now
with everything else unchanged and based only on the stated f-ratios of both systems you should expect
(5.5/2.2)^2 = 6.25 times more light, so that your previously mentioned 60 second exposure (with your f/5.5 system)
would in light gathering terms be equivalent to about a 10 second exposure on your new f/2.2 system.
There's one
minor thing in the fine print that often goes unmentioned in these fast optical systems that have a central obstruction : whilst the stated f/ratio is by definition a true ratio of focal length to objective lens diameter it should be used with some correction for the loss of light obscured by the central obstruction when trying to equate exposures on different systems since the difference in exposure is equal to the ratio of Light collecting area of the two systems
which can only be approximated by the square of the f-ratio as I did above.
If we look more accurately at the ratio of the light collecting area ratio of both these instruments:
RASA f/2.2 with 279mm objective diameter & 114mm central obstruction diameter then
Light collection Area = 3.1416(279/2)^2 - 3.1416(114/2)^2 = 50,929 mm^2
115mm f/5.5 Scope
Light collection Area = 3.1416 (115/2)2 = 10,387 mm^2
Ratio of light collection Area = 50,929/10,387 = 4.9, so that once the central obstruction of the RASA 11 is accounted for the RASA 11 collects
4.90 times more light than the 115mm f/5.5 scope. So that your previously mentioned 60 second exposure (with your f/5.5 system) would in light gathering terms be equivalent to about a 12 second exposure on the RASA 11 "f/2.2" system.
When accounting for the central obstruction
in light gathering terms only one can think of the the RASA 11 as behaving like an f/2.43 system.
Best
JA