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First time EAA - Focal reducer problem

EAA Equipment Imaging SCT Astrophotography
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#1 MaxAnders

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Posted 03 October 2022 - 11:18 AM

I was able to get my EAA rig imaging the very first night out, but I did have an unexpected issue.  My focal reducer is not acting as I would expect it to.  When using SharpCap to plate solve, it gave me a focal ratio of f/7.2.  That might not seem like much of a difference from f/6.3, but I could really notice it when comparing what I saw on my screen to the expected view with astronomy.tools FOA calculator.  I used the back focal length for the Celestron FR (105mm) but I realize now that might be incorrect.  There are several CN conversations that my Antares focal reducer has a back focal distance closer to 85mm.  But from my understanding, having too much back focus should have made the effective reduction of my system even shorter than the listed x0.63 .  Can you see any issue with my set up that would cause this?  I want the full f/6.3.  If this is as good as it gets, I might have to get a GSO x0.5 reducer.

 

Image train

  • Celestron C5 spotting scope - f/10 1250mm FL
  • Antares 0.63 reducer
  • Celestron SCT T-adapter (50mm)
  • ZWO T2-M48 extender (16.5mm)
  • ZWO filter drawer w/ IR-UV cut filter (21mm)
  • ZWO ASI533MC Pro (17.5mm)

 

Thanks,

Andrew

 



#2 Axident

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Posted 03 October 2022 - 11:38 AM

If you don't have the 105 mm distance between the reducer and the chip the the f ratio might not be correct. Sometimes it's a few mm more or less. You will also suffer by stars in the edge and corners not being round. This shape of the stars is easier to see with the bigger chips. The closer to the center of FOV the less visible it is.
Some really don't care and is even stacking reducers. If your FOV is small then the visibility is not that affected.
In the end then you must be satisfied with the results you see, more than the numbers must perfect. EAA is not perfect anyway and was never meant to be. But when that has bee said the I personally like my stars to be perfect round in the corners doesn't matter the on the fly processing is not perfect. It simply look wrong with a star which is not round in my eyes.

Edited by Axident, 03 October 2022 - 06:05 PM.


#3 steveincolo

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Posted 03 October 2022 - 11:44 AM

With my C6 and the Celestron 0.63x reducer at the recommended 105mm back spacing, I get about 0.71x actual reduction.  So your result is not surprising. 
 

I suspect the GSO reducer with the 533MC will be a little disappointing.  


Edited by steveincolo, 03 October 2022 - 11:57 AM.

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#4 mjritchie

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Posted 03 October 2022 - 12:00 PM

I have my C5 with about 100mm of back spacing, and I get around .7x reduction as well. For me it isn't really an issue though. 

 

If you decide to use a .5 reducer along with the .63, be prepared for aberrations. the only time I used that combo was with a 290 sensor, and it was small enough to be serviceable with that. A 533 sensor is 8x larger than a 290, so.....yeah. Just a warning.


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#5 Alien Observatory

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Posted 03 October 2022 - 12:17 PM

Last night I put a 0.63X FR on my C6.  To get 0.63 reduction it took 125 mm of spacing to achieve something close to 0.63 reduction and it was within the focuser travel with no issues.

Pat Utah smile.gif


Edited by Alien Observatory, 03 October 2022 - 12:23 PM.

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#6 MaxAnders

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Posted 03 October 2022 - 12:27 PM

I was planning on adding more back space to see where that would put me.  I have an unused 21mm extension that came with my camera that I can place in the train.  That would put me right about at the 126mm where you achieved x0.63.  Worth a shot I guess.



#7 azcubs76

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Posted 03 October 2022 - 05:03 PM

I have the Antares reducer on my C8 and get exactly 0.63 reduction at 105mm backspacing. I used to have the Celestron and it performed identically in comparison so the reducer itself is probably fine.

 

I'm pretty sure the back spacing is different for the larger and smaller SCTs and the 105mm is for the 8" only. I vaguely remember seeing a table here on CN with all the different spacing for the different SCT sizes but can't seem to find it now.



#8 Alien Observatory

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Posted 03 October 2022 - 11:25 PM

I installed another 20 mm spacer tonight as I wanted to frame the Moon in a better overall context.  Total spacing is about 150 mm and the FR is about 0.55 X.  Still had plenty of Focuser Travel to get a good image, with no issues.  I still have a couple of T spacers to try to extend the FR, within the Focus travel of the C6.  Will try another 20  mm Tuesday night.  Pat Utah smile.gif

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#9 alphatripleplus

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Posted 04 October 2022 - 07:32 AM

The 105mm quoted backfocus distance to yield f/6.3 with a Celestron reducer is for the C8 optics. With the C6 or C5, you'll have to experiment.


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#10 nic35

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Posted 04 October 2022 - 08:03 AM

If you use the calculator found here:   http://www.wilmslowa...rmulae.htm#FR_b

 

it shows the computed f ratio for 105mm spacing on a C6 to be f/7.  He makes the point that they are not 0.63 reducers, but are actually called f/6.3 reducers.   

 

It shows an f ratio of 5.5 for a C6 with 150mm spacing, just as Alien O found.

 

Interestingly, his formula says the f/6.3 reducer on a C8 gives an f ratio of 6.7.  I'm not sure how to interpret that !

 

john


Edited by nic35, 04 October 2022 - 08:04 AM.

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#11 Mark Lovik

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Posted 04 October 2022 - 05:48 PM

The SCT back spacing game has been played by many of us over the years. 

  • Go past nominal (more spacers) - get more reduction
  • Shorten up spacing - get less reduction

For smaller sensor cameras (maybe 10-11mm diagonal) we can really change the backspacing and still get reasonable images.

Bigger camera sensors (15mm on up) - deviating too much from the nominal backspacing will show up quickly

 

Some tricks in my toolbox to really play the backfocus game  (note: Edge SCTs have a different game than described here)

 

1. I keep at least a set of M48 and M42 spacers (use depends on the scope and M42 is typically the easiest) with different spacers you can play with - typically in 5mm increments.

2. I have a normal SCT to M42 (t) converter from Celestron.  It is long and convenient for connecting cameras to the reducer (for typical 8" SCT).   It's not so good if you need a short back-focus length - for doing things like using dual reducers.

3. I also have a short SCT to M42 adapter.  This provides me better flexibility when I need a short backfocus.

4. I really like M42 variable spacers.  This is good for making fine spacing changes to tune a system.

5. Have a precision rule or caliper handy.  I like the old manual calipers.  It's handy for many unexpected events.

6. Platesolving images is a good way to evaluate your image train - in particular when adjusting reducers and spacers.


Edited by Mark Lovik, 04 October 2022 - 05:50 PM.

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#12 Alien Observatory

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Posted 05 October 2022 - 08:46 AM

I installed another 10 mm spacer to the FR stack (its getting very long) for a total of 162 mm which equates to F5.  Still have some focus available and no issues.  At F5 I get the Moon framing I wanted,

 

Resolution is 0.66 arc sec / pixel, which results in a small oversample.  I also installed the counterweight (about 2.5 lb total.  The tracking last night was better maybe due to the counterweight and the FR 5 being less sensitive.  I will order another SW counterweight and test the tracking again.

 

Skies were a bit better and I took my first ROI image (1920 x 1080) with the 183 cam.  I am very pleased with the results and it meets my expectations I hade for it.  Pat Utah smile.gif

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#13 MaxAnders

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Posted 07 October 2022 - 11:01 PM

I finally got some clear skies tonight and added 21mm of back spacing to my original setup, for a total of 126mm.  Plate solving determined that my C5 with the 0.63 reducer is now at f/6.7  Not where I wanted to be, but better.


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