Celestron Focal Reducer

[daveangie0110]

Hi All

Can anyone help me with these questions.

I have a Nexstar 6SE, and i do planetary imaging using a DSLR and a ZWOASI120 Camera, I am thinking of buying a Celestron focal reducer as I want to try to image some DSO, I know i will be limited to 30 sec max exposures

I have the following questions.

If I buy the Celestron focal reducer will I be able to take better images of some DSO ?

Will I be able to do longer exposures ?

Can I stil use my ZWO camera with the focal reducer fitted ?

Can I use my 2 X Barlow with the focal reducer fitted ?

many thanks for your help.

[jambouk]

1. Probably

2. Theoretically, marginally maybe, but I guess not

3. Yes, but this would only be advisable for imaging the moon

4. Yes, but why would you want to.

James

[Tinker1947]

The 6.3f FR requires 105mm from it to the camera sensor, i would think a barlow would make a real mess of this.....

[knobby]

All good answers ... As for the last one , I've been using a reducer and Barlow as I'm finding f10 to small , f20 too big and f14 about right for my back yard

[sgazer]

The celestron f6.3 reducer is excellent. However i have the zwo camera and the sensor is tiny, so it gives s very small fov with focal lengths over say 200mm, making dso imaging difficult with an sct.

Depending on how much you can spend, I'd recommend a dslr (100 quid+), which would be perfect with the reducer.

Despite the specs of the zwo camera, I found it way less sensitive and much more noisy than even a compact camera. I even cooled it to - 5c and still wasn't impressed. Coupled with the small sensor, I gave up using for dsos.

[jambouk]

Yes, i wouldn't bother trying to use the ZWO for DSOs - way too noisey (i tried cooling it too) and the sensor way too small. DSLR all the way for DSOs until you can afford to buy a dedicated cooled CCD... But if spending that sort of money, maybe think about getting a faster scope with a wider field of view for the DSO work.

James

[Scorpius]

Hi All

Can anyone help me with these questions.

I have a Nexstar 6SE, and i do planetary imaging using a DSLR and a ZWOASI120 Camera, I am thinking of buying a Celestron focal reducer as I want to try to image some DSO, I know i will be limited to 30 sec max exposures

I have the following questions.

If I buy the Celestron focal reducer will I be able to take better images of some DSO ?

Will I be able to do longer exposures ?

Can I stil use my ZWO camera with the focal reducer fitted ?

Can I use my 2 X Barlow with the focal reducer fitted ?

many thanks for your help.

I’ve got a Celestron 8” SCT and their f/6.3 focal reducer. I originally got the reducer for DSO’s but didn’t have much luck (suspect improper spacing) so went ahead and purchased a 80mm ED refractor and a monochrome camera (w/ wheel, etc). I’ve had some limited success with that combination but have recently been doing some reading which has got me thinking I need to give the SCT/reducer another try at some point down the road.

As a beginner, and speaking strictly in terms of the Messier objects, what’s becoming clear is the size (angular field) of these objects varies quite a bit. Some like M31 and M42 are huge but others like M1 and M27 are quite small (as they would appear to us if we could actually see them). Presently, I’m trying to wrap my head around another variable in deep sky photography called image scale (sampling). Basically what it boils down to is combining the right focal length telescope (can be tweaked by a Barlow and/or reducer) with the right size sensor having the right size pixels to give you an image scale somewhere between 1.5 – 3 arc seconds per photosite for DSO’s and .5 for planetary. If the image scale is too far off you’ll start to see issues with over or under sampling which apparently can have  a big effect on the final image. Sampling is simply how much of the sky in arc seconds is “seen” by each individual microscopic photosite on the sensor’s surface.

I’ve been working on an Excel spreadsheet, and it’s only good for the Messier objects, but what it does is allow you to input telescope focal length in mm, sensor dimensions in mm and photosite dimensions in microns and then returns the image scale for that setup in arc seconds. It also highlights which of the 110 objects you’ll be able to capture in their entirety based on the specs you enter. I’m currently working on adding an output for eyepiece FOV but based on what I’ve read regarding image scale, I think it will eventually develop into a useful, albeit rudimentary tool, for imaging the Messier objects. My plan is, if I ever get it done - to share it here on the forum in hopes it will be useful to others and to also get some feedback on any errors and possible improvements. Stay tuned but for now I would say the setup you describe would have a small FOV ok for some Messier objects and since it seems like there are a fair number of those smaller objects in the list, an imaging set up consisting of a long focal length SCT with focal reducer combined with the right camera could be useful for imaging some DSO’s.

By the way, the spreadsheet outputs the following for the 6SE with focal reducer and the ZWOASI120. Sensor FOV in degrees is 0.18 x 0.14 (11.1 x 8.3 arc minutes) and the sampling rate is 0.52 which is perfect for planetary but is well below the recommended 1.5 – 3 arc second per pixel recommended for DSO’s. You could theoretically image about 40 of the 110 Messier objects in their entirety (mostly small galaxies and a few star clusters and nebula) however, the sampling rate would not be ideal. Dithering might help with the noise but objects with larger angular fields are going to be beyond its reach. Stellarium, which is a free download, has a nice plug-in that places a box around objects to represent what can be imaged with a certain setup but, unless I missed it, it does not give you the image scale. As for longer exposures, I believe that primarily depends on the tracking capabilities of the mount...