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0.5x focal reducer + ASI224MC

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#1 Hordrid

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Posted 26 September 2018 - 09:05 PM

Hi guys,

 

Only recently started getting into astro-imaging (infact it's been less than 12 months since I've owned a telescope!), so a lot of what I've written below might sound nonsensical smile.gif . I've really had fun with the ZWO ASI224MC and wanted to get wider views of the moon and DSOs as well. I bought a 1.25" 0.5x reducer and then I realized that it puts the focus completely out of whack. The backfocus distance of the camera is 12.5mm. The focal reducers information is here:

 

Focal length (estimated) = 101 - 103mm (3.98 - 4.06")

Total item length (excluding external thread) = 10mm (0.39")

the "working distance" of this specific reducer is estimated to be 51mm, i.e., this reducer will provide a reduction of 0.5x when the center of its lens assembly is placed 51mm from the focal plane of the eyepiece or imaging device.

I still don't completely understand but does the above mean that the distance between the focal reducer and the CCD sensor needs to be ~51mm? I see the number 55mm crop up a lot while Googling, what does that relate to?

 

Currently the distance between the camera CCD sensor the focal reducer lens is broken down like this: 12.5mm (camera sensor to the top of 2" ring) + length of the 1.25" nosepiece (I've not measured it yet, but I think it's around 30mm) + 10mm (length of the focal reducer). No spacers are involved at all right now and I'm nowhere near achieving focus. When I play around with it, it feels like to get focus I need to make the focusser travel inwards, more than it's physically capable of doing! Is there any solution to this? 

 

Should I have gone for a 2" focal reducer instead?

 

Sorry for the many questions - but I'm really confused on how to go about troubleshooting this.

 

Thanks

 



#2 mvas

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Posted 26 September 2018 - 09:11 PM

Which 0.5x Focal Reducer are you referring to ?

Post a link to the exact F/R


Edited by mvas, 26 September 2018 - 09:11 PM.


#3 Hordrid

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Posted 26 September 2018 - 09:25 PM

Oh yea, sorry. it's the GSO - https://agenaastro.c...al-reducer.html



#4 jmorales21

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Posted 26 September 2018 - 09:43 PM

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Ezquimo 31m 160framesSQR

I use my focal reducer at the end of the extension tube that came with the camera and although I have some coma, it's not terrible.

The focal reducer does indeed change the focus completely. But this is expected as it happens with all glass you introduce into the optical train. Barlows work in the same way. Do not expect them to be parfocal.


Edited by jmorales21, 26 September 2018 - 09:47 PM.

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#5 Adun

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Posted 26 September 2018 - 10:09 PM

The focal length of the reducer is the distance at which light that entered the reducer "from infinity" (meaning: parallel light rays), is forced (by the reducer) to converge to a single dot.

 

This image shows the "firestarter" method of measuring focal distance of reducers using the Sun as the source of parallel light rays:

 

post-20602-0-19512900-1488346400_thumb.j

 

 

So if you place the camera at the full focal distance of the reducer (103mm in the case of yours), you'd not get any image at all, just a bright dot.

 

In the middle of the focal distance (~51mm for yours), the image will seemed scaled to half (0.5x) it's original size.

 

From this middle point, if you move the camera a little closer to the reducer, and the reduction will be less strong. 0.6x, 0.7x .... until right next to the reducer there will barely be any reduction.

 

Move the other way, and the reduction gets stronger, 0.4x, 0.3x, 0.2x, until at the extreme you get again the dot.

 

So in reality, you get to choose how much you want to "zoom out" with your camera, by varying the distances.

 

From 0mm to 51mm you'll get 0.9x, 0.8x, 0.7x, ... And ~0.5x at 51mm.

 

Further away you might be able to get a little more (0.4x perhaps) but the off axis aberrations of your scope (coma and/or field curvature) will creep in and ruin the image.

 

And another limiting factor is that when you add a reducer, it also moves the "focal plane," (the position at which focus is reached) inside your telescope. This is why some newtonians may not reach focus with a reducer in place.

 

And another limitation is aperture: if the angle is too steep, a 1.25" reducer at the end of a 224 camera's nosepiece, could cut the cone of light a little. In those cases, yes, a 2" reducer would be better, but then again it must first be determined whether said 2" reducer can indeed go inside the focuser deep enough for you to reach focus. That will depend on your particular  telescope, which you did not specify


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#6 Hordrid

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Posted 27 September 2018 - 12:18 AM

Thank you all, those are really good and helpful explanations. It seems I hadn't differentiated between focal lengths of the various optical equipment and the focal plane of the telescope to properly understand what is going on. My telescope is the Skywatcher 12" Dobsonian Collapsible GoTo.

 

Specifically how does the focal reducer change the focal plane? If I work that out, it might help me establish if focus can indeed be reached at all. Given that it's a collapsible, another suggestion given to me was to 'collapse' it a bit to see if focus can be achieved that way.


Edited by Hordrid, 27 September 2018 - 12:26 AM.


#7 Scott Mitchell

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Posted 27 September 2018 - 10:41 AM

Adun, that's the best explanation I have ever read about how focal reducers work! Thanks!

 

The focal length of the reducer is the distance at which light that entered the reducer "from infinity" (meaning: parallel light rays), is forced (by the reducer) to converge to a single dot.

 

This image shows the "firestarter" method of measuring focal distance of reducers using the Sun as the source of parallel light rays:

 

post-20602-0-19512900-1488346400_thumb.j

 

 

So if you place the camera at the full focal distance of the reducer (103mm in the case of yours), you'd not get any image at all, just a bright dot.

 

In the middle of the focal distance (~51mm for yours), the image will seemed scaled to half (0.5x) it's original size.

 

From this middle point, if you move the camera a little closer to the reducer, and the reduction will be less strong. 0.6x, 0.7x .... until right next to the reducer there will barely be any reduction.

 

Move the other way, and the reduction gets stronger, 0.4x, 0.3x, 0.2x, until at the extreme you get again the dot.

 

So in reality, you get to choose how much you want to "zoom out" with your camera, by varying the distances.

 

From 0mm to 51mm you'll get 0.9x, 0.8x, 0.7x, ... And ~0.5x at 51mm.

 

Further away you might be able to get a little more (0.4x perhaps) but the off axis aberrations of your scope (coma and/or field curvature) will creep in and ruin the image.

 

And another limiting factor is that when you add a reducer, it also moves the "focal plane," (the position at which focus is reached) inside your telescope. This is why some newtonians may not reach focus with a reducer in place.

 

And another limitation is aperture: if the angle is too steep, a 1.25" reducer at the end of a 224 camera's nosepiece, could cut the cone of light a little. In those cases, yes, a 2" reducer would be better, but then again it must first be determined whether said 2" reducer can indeed go inside the focuser deep enough for you to reach focus. That will depend on your particular  telescope, which you did not specify


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#8 Adun

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Posted 27 September 2018 - 12:06 PM

My telescope is the Skywatcher 12" Dobsonian Collapsible GoTo.

 

The good news is that Skywatcher collapsible dobs have plenty of in-focus (50mm extra by removing the 2" and 1.25" eyepiece adapters), and if that is not enough: you can use the binoviewer position of the trusses to push the focal plane out of the focuser.

 

That would certainly work, but the focal plane will be so further out of the focuser you're likely to need long extenders to be able to place the camera so further away (I've done it). And your telescope will be working at a slightly smaller aperture (because the secondary mirror, being moved closer to the primary, will miss some of the primary's light and not reflect it to the camera)

 

Specifically how does the focal reducer change the focal plane? If I work that out, it might help me establish if focus can indeed be reached at all. 

 

To calculate how much "in focus" in mm would be required, you can try this calculator. It's for refractors and SCTs, but it can give you an idea.


Edited by Adun, 28 September 2018 - 09:46 AM.

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#9 Hordrid

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Posted 27 September 2018 - 09:35 PM

Thanks once again Adun, I really appreciate your time here. I'd like to bother you with one last set questions.

 

The good news is that Skywatcher collapsible dobs have plenty of in-focus (50mm extra by not removing the 2" and 1.25" eyepiece adapters), 

I didn't understand completely - could you please elaborate? Specifically around removing the eyepiece adaptors - I don't have a choice there do I? I'll have to use them whenever I use an EP or camera? And does in-focus of 50mm mean I have 50mm to play with in the focusser?

 

I'll also play with the different positions of the trusses to see what works. Does the scope require to be re-collimated when I do that? 

 

Cheers again!



#10 Adun

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Posted 28 September 2018 - 10:42 AM

I didn't understand completely - could you please elaborate? Specifically around removing the eyepiece adaptors - I don't have a choice there do I? I'll have to use them whenever I use an EP or camera?

 
You do have a choice. That adapter is there to give you the option of using it or not.
 
This is what the focuser of my skywatcher dob looks like:
 
2" to 1.25" adapter with helical fine focusing and brass ring.

 
The 2" to 1.25" adapter comes with the scope, and it's ~50mm long.
For 2" eyepieces a similar extender of the same length comes with the scope (it's just 2" wide).
 
The scope tube length was designed so that eyepieces (be them 2" or 1.25") need this adapter to reach focus. The focal plane is located somewhere near the edge of the adapter. Eyepieces usually expect the focal plane to be somewhere near the eyepiece's "shoulder". This means that for normal, visual use, you do need the adapter, otherwise naked eyepieces can't reach focus.
 
However: 
 
These adapters are held by 2 screws. You could place anything that's 2" instead of the adapters: an eyepiece, a coma corrector, etc. So for example:

  • My GSO coma corrector, which does need some additional in-focus, works in place of (replacing) the 2" extension.
  • A 2" focal reducer, screwed to the end of some M48 spacer rings, and those screwed to a 2" eyepiece can work with the correct spacing. I have tried this with my 20mm Meade UWA and it did reach focus.
  • A 2" focal reducer, screwed to the end of a shortish (<50mm) 2" extension tube, ending on a zero profile 2" to 1.25" adapter, would allow you to insert the 1.25" nosepiece of a camera, and slightly vary the distance from the camera's sensor to the focal reducer. This works best with "bullet shaped" cameras like the ASI290 Mini or my RT224.
  • Finally, as I have shown in this thread, by replacing the 2"_to_1.25" adapter with the ($35) non-rotating helical fine focusing adapter featured in that thread, you get very fine focusing for high power/planetary without needing the dual-speed-focuser upgrade.

So, you do have a choice. You only need the original adaptors to use the telescope on "vanilla" observing conditions with regular eyepieces.
 
But when you need 50mm more in-focus (as can happen for using some binoviewers, some coma correctors, some focal reducers or the helical adapter), you can gain those 50mm of in focus by just removing the 2" adapter and inserting your (2") coma corrector, binoviewer or whatnot.
 
People who own other newtonians often don't have this luxury, and have to resort to "pushing the primary mirror closer to the secondary" (using the collimation screws, or sometimes moving the whole cell) in order to get their gizmos closer to the primary mirror.
 
Better yet, when 50mm of in-focus is not enough, then there's the binoviewer position of the trusses, which pushes the focal plane by as much as the trusses are collapsed (I think 300mm) which is huge in terms of focal distance, but as far as I know is needed by some big binoviewers that have really long internal light paths.
 
The skywatcher collapsible is the most "focus friendly" newtonian there is. Take advantage of it.
 
For EAA, while it's easier with a bullet sized camera, you can probably get some focal reduction using a 2" reducer, a kit of M48 spacers (cheaper on Amazon) and an M48 to T adapter.
 
Be wary that these SW dobsonians are already sub F5, so you don't really want full 0.5x focal reduction, for you'd then be at F2.5 and that's serious coma territory. I think 0.7x reduction (resulting in ~F3.5 depending on your scope) would be much more effective, and this means spacing the reducer closer to the camera sensor.

 

I'll also play with the different positions of the trusses to see what works. Does the scope require to be re-collimated when I do that?


In theory yes, and the more aggressive your reduction, the truer this will be.

 

In the case of my particular SW dob, it seems requires re-collimation every time I set it up anyway.


Edited by Adun, 28 September 2018 - 10:50 AM.

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#11 Hordrid

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Posted 29 September 2018 - 03:07 AM

Wow, that was a brilliant post - thank you! I feel I understand what I have much better now, having read your detailed explanations and have quite a few things to try.

 

The helical focusser also looks great - for that price, I will definitely give that a go in the future. 


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