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Possible to push a .63 reducer to .5 or .4 using rings for small sensors?

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#1 Phil Ramsay

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Posted 14 September 2019 - 09:26 PM


I’m about to jump into EAA but have no practical experience yet, so looking for some insight.

My setup initially will be a Celestron C8 on a alt az mount, a Celestron reducer corrector to bring it to F6.3, and a ZWO ASI 385. I’ll have M42 rings to put focal plane in the correct working distance.
So far pretty generic.

Here’s the question: having read this, https://agenaastro.c...cers-guide.html
I’m wondering if anyone has favorable experience trying to increase the focal reduction by intentionally using a non-standard operating distance.

From the article it sounds like a small sensor camera would be at least somewhat forgiving - but I don’t know what might be reasonable. Any experience out there with this?

Would trying to push down to the range of F5 or even F4 using spacers be possible, or is there just not that much latitude without unacceptable side effects?

Any thoughts or experience?

#2 SkyHunter1

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Posted 15 September 2019 - 12:51 AM

Phil,

On a cassegrain to my experience you can reduce further by spacing more. This is not the case on a refractor which is much more exacting in terms of spacing when using a reducer. 

 

I found the spacing for the reducer which in my case with a Meade LX200 GPS, 85mm was the correct spacing to get .63 reduction if i recall. I was able to reduce to approx 4.5 using 125 or 130mm spacing.

 

In my gallery, the full moon shot was a mosaic of 2 shots as I couldn't get the whole moon, or even upper and lower halves at .63. I reduced to about f/4.5 and created the final image from the 2 halves (upper and lower) taken at the lower focal ratio.

 

I used a zwo294mc for that image so I had a rather large chip and didn't have any issues covering it. I would think that if i had gone any further the image circle wouldn't have covered the chip, but if you're using a smaller chip I think you wont have a problem. 

 

Now I cant give you any details on deep-sky imaging, but the image came out pretty sharp for the lunar shot and I would surmise it would have worked well for deep-sky too.

 

http://www.wilmslowa...formulae.htm#FR

 

This calculator will give you the focal length of your reducer (240mm) and you will need to input the backspacing you intend to use. It will tell you the resulting focal ratio.

 

Hope this helps,

 

Regards,

Skyhunter1



#3 Lorenz0x7BC

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Posted 15 September 2019 - 01:14 AM

I have a very similar rig (C8 alt-az 0.63 reducer ASI224) and tried the same (increasing the distance to get down to F4) but I did not succeed.

First option I tried: using a SCT/T thread adapter behind the 0.63 reducer and then T thread spacers all the way to the camera. Problem here: this setup gets really long and heavy. I ran into severe mount clearance problems.

Second option: using the stock 1.25 inch visual back and star diagonal, then 1.25 inch extenders to increase the distance. Here the problem was that the whole setup got too unstable. The optical axis was not centered anymore and there were ugly reflections.

In my case the Night Owl .4 reducer solved those problems.

But the story would be different with an EQ mount... and others had more luck in combining 0.63 and 0.5 reducers, surely your saw those posts here in the forum.

#4 Phil Ramsay

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Posted 15 September 2019 - 07:59 AM

Thanks very much for the feedback - I really appreciate it.

The calculator is quite helpful, as is the practical experience from everyone.

I’m thinking I’ll probably use a Starizona Night Owl to get to F4 a little later down the line, but was curious what my options were with equipment ‘on hand’ as I’m getting started.

Phil
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#5 biomedchad

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Posted 15 September 2019 - 06:38 PM

If you go with a 224 or a 183 the night owl is perfect. I upgraded to a 294 sensor and it’s useless. I would argue the 183 fits the bill as described by starizona now that I’ve had a chance to play with a larger sized camera. I will be selling mine now.

#6 Stargazer3236

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Posted 20 September 2019 - 02:14 PM

Try using this chart, it may provide some insight for you.

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  • Focal Reduction.jpg



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