Jump to content

sgl_imaging_challenge_banner_31.thumb.jpg.b7a41d6a0fa4e315f57ea3e240acf140.jpg

Focal reducer question (sorry)


Recommended Posts

I know some form of this question has been asked before, but I cannot find it worded/answered the way I need it.

I have a 150mm x 750mm f/5? reflector tube and a 150mm x 1370mm f/9 ritchey chretien. The ritchey chretien came with a .75 reducer. What is the benifit of the reducer? I know it changes the scope to f/6? What is the point of the f/9 setup vs the f/6? Why not just buy a faster scope in the first place? I bought it because it was primarily a deep sky scope that would take better deep sky images, although I havent had the chance to use it much, and havent used the reducer. What would ne the difference in images?

Edited by Ozone
Forgot something
Link to post
Share on other sites

Larger area in the sky due to shorter FL as said - provided that you use proper sensor size (to avoid aberrations and vignetting that may be present with larger sensors).

F/9 with reducer vs F/6 native in 6" scope is down to type of instrument that you want to work with.

Realistic options at F/6 in this aperture size are RC and Newtonian (yes you can look for 6" F/6 apo but that's going to cost a bit more? :D ). RC will be more compact (easier to mount and guide), probably have better definition over larger field - but that depends on properties of focal reducer and coma corrector used, rather than scope native optics (RC is better there - when used without any additional optics). It will also have better stray light protection (due to design and baffling).

RC will be more versatile compared to Newtonian - you can use it at native FL, or reduced, while with newtonian you need coma corrector at native for any sensible field size, and will need barlow (with coma correction) to get to FL of RC.

Link to post
Share on other sites

Reducing the focal length also decreases your exposure time by effectively compressing the available photons into a smaller image circle.  If you were imaging at full image circle natively, you won't be able to do that with the focal reducer in place.  That would be the advantage of having a scope natively at the lower focal ratio.  It would allow for shorter exposures while maintaining a larger image circle.  The downside is the increased expense involved in creating a shorter focal length instrument since faster mirrors have steeper curves and are less forgiving of any figuring errors.  You'll also have a larger central obstruction which can lead to lower contrast.

Link to post
Share on other sites

Check here http://astronomy.tools/calculators/field_of_view/

Simples....

With F9, you will be in a very nice position to image planets or close ups of the Moon as you will not need Barlow.

For Nebulas or Galaxies, F9 will cause several problems: tiny field of view (most nebulas will not fit in), longer exposures and better/more sensitive guiding, - Focal Reducer will reduce these problems :)

Link to post
Share on other sites
5 hours ago, Ozone said:

Thanks! The calculator doesnt show a example image though. Is it supposed to, Roland?

It should do... 

1. Select the imaging mode tab

2. Select a target [ie Messier - M31]

3. Select your telescope from the drop-down list (or custom scope and manually enter it - Focal length / aperture)

4. Select your camera from the drop-down list (or custom camera and manually enter it - Resolution / pixel size)

5. If you want to add a barlow or reducer, put that in the barlow option [Binning I usually leave at 1x1, ditto angle]

6. Click "add to view"... and M31 should be displayed (and you get all the calculations for focal ratio, resolution, field of view and Dawes limit)

If you then go back and enter different values for step 3-5 and add that to view, you get 2 boxes showing the difference in field (and you can add more if you want :))

Here's an example of what you get:

image.thumb.png.aa982b0fe068f3fc1bae19b2463113c1.png

 

Link to post
Share on other sites

Andy thanks for posting the link snd desciption. I checked it out and created som3 very useful pictures of varios telescopw vs camera vs focal reducer solution for M31 and M45  which are both a bit big to get in with my current set up.ie Evostar 80 ED, Starwave 0.8 flatner focal reducer and theAltair 183c hypercam . I was trying to find out if the  Evostar 80 runs out of infocus with a 0.6 focal reducer on instead. Still searching for this one though. ?

astronomy_tools_fov-1.png

Link to post
Share on other sites
3 minutes ago, CARAVAN65MAN said:

Andy thanks for posting the link snd desciption. I checked it out and created som3 very useful pictures of varios telescopw vs camera vs focal reducer solution for M31 and M45  which are both a bit big to get in with my current set up.ie Evostar 80 ED, Starwave 0.8 flatner focal reducer and theAltair 183c hypercam . I was trying to find out if the  Evostar 80 runs out of infocus with a 0.6 focal reducer on instead. Still searching for this one though. ?

astronomy_tools_fov-1.png

Don't miss out on your targets while you are looking into x0.6 reducer - use mosaic technique (shoot target out of two halves or use four pieces and stitch them up in processing).

Link to post
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now
  • Recently Browsing   0 members

    No registered users viewing this page.

×
×
  • Create New...

Important Information

We have placed cookies on your device to help make this website better. You can adjust your cookie settings, otherwise we'll assume you're okay to continue. By using this site, you agree to our Terms of Use.