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Focal Reduction Question - Refractor

EAA refractor
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#1 gfstallin

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Posted 10 February 2019 - 08:04 PM

Hey folks, 

 

I have a very basic question here. Looking through various EAA topics, I see folks who are taking native F/6 and F/7 refractors down to below F/3. Are folks using refractor field flatteners/reducers combined with threaded 1.25" 0.5X reducers from companies like Mallincam to achieve this type of reduction? 

 

I'm wondering whether I can make my Orion ED80 into a good EAA package, or whether I should...start over with a faster Newtonian with a little more aperture for lower integration times. 

 

George



#2 OleCuss

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Posted 10 February 2019 - 09:55 PM

You can do good work with your current scopes.  Bigger means more light-gathering and therefore more light presented to the sensor so with 8", 9.25", and 11" OTAs you are in pretty good shape already.

 

The problem I see is that you might be intending to use the ASI224 sensor.  When you combine long focal length with a tiny sensor you end up with a tiny FOV.  That tiny FOV makes find and tracking targets much more challenging - and bigger targets simply won't fit into your FOV.  So I'd be using a focal reducer in order to get a better FOV and/or getting a camera with a bigger sensor.

 

What I see when people stack reducers is increasing aberration.  Doesn't seem to bother some folk in the slightest but it would bother me and I'm simply not going to do that.

 

For that Orion 80ED I'd probably be looking for a standard 0.7-0.8x reducer/flattener.  For the 8" and 9.25" SCTs I'd probably be looking to Optec for their 0.33x reducer.  I'm not sure I'd try a standard reducer for the C11 but would be considering a HyperStar for that use.

 

But again, I'd be thinking about getting the bigger sensor if the current plan is to use the ASI224.  That IMX224 is a great sensor, but it is so small. . .  But that's just me - and you might be delighted with it.


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#3 gfstallin

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Posted 10 February 2019 - 10:21 PM

You can do good work with your current scopes.  Bigger means more light-gathering and therefore more light presented to the sensor so with 8", 9.25", and 11" OTAs you are in pretty good shape already.

 

The problem I see is that you might be intending to use the ASI224 sensor.  When you combine long focal length with a tiny sensor you end up with a tiny FOV.  That tiny FOV makes find and tracking targets much more challenging - and bigger targets simply won't fit into your FOV.  So I'd be using a focal reducer in order to get a better FOV and/or getting a camera with a bigger sensor.

 

What I see when people stack reducers is increasing aberration.  Doesn't seem to bother some folk in the slightest but it would bother me and I'm simply not going to do that.

 

For that Orion 80ED I'd probably be looking for a standard 0.7-0.8x reducer/flattener.  For the 8" and 9.25" SCTs I'd probably be looking to Optec for their 0.33x reducer.  I'm not sure I'd try a standard reducer for the C11 but would be considering a HyperStar for that use.

 

But again, I'd be thinking about getting the bigger sensor if the current plan is to use the ASI224.  That IMX224 is a great sensor, but it is so small. . .  But that's just me - and you might be delighted with it.

OleCuss, 

 

Thank you. So I've dabbled in EAA before, once using a generic 0.5 reducer with the ASI224 on a C8. Despite hearing that the sensor was too small to see much in the way of aberrations, they were quite evident to me and really distracting. I did not like the view at all. So, without hijacking my own topic, I guess the suggestion is for larger pixels. Hmm...thinking1.gif

 

George



#4 OleCuss

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Posted 11 February 2019 - 05:21 AM

I would not be arguing so much for bigger pixels as for a larger sensor.

 

However, bigger pixels can have some advantages.  Given the same technology, a bigger pixel does tend to give better SNR and better dynamic range.

 

The bigger sensor means you have a bigger FOV (as long as your OTA's image circle is big enough to fully illuminate it).  Finding and tracking a target is considerably easier and you can image some of the larger targets.

 

There can be drawbacks. . .  The IMX224 and IMX385 have an amazing SNR1 and you are not likely to equal that with anything except maybe a monochrome IMX183 or possibly something like the monochrome QHY163 (we're not sure how well they compare because Sony has not done the SNR1 ratings for them).

 

However the IMX294 has very nearly the same SNR1 as do the IMX224 and the IMX385 and has very good dynamic range in a much more generously sized package.

 

With the Orion 80ED I'd expect quite significant under-sampling with the IMX294 (you're likely under-sampling even with the IMX224) camera but for OAP it really may not matter.  For the SCTs the IMX294 cameras would not have a problem with under-sampling - you may have over-sampling but it has so little noise that the over-sampling shouldn't be a significant issue.



#5 ChrisFC

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Posted 18 February 2019 - 05:33 PM

I used SCT reducers on my 80mm refractor. Both a Meade 0.33 and Celestron 0.63 - just cause I still had them after selling my c8. It was fine with a small sensor like the 224. I worked at about f3.5 from the scopes native f5.9. yes there was a bit of coma etc etc, but so what ...

Made for a nice light portable setup with my modded eq5


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