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focal reducer for a newtonian telescope?

EAA Moon Reflector
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#1 kongqk

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Posted 22 September 2021 - 10:52 PM

I am quite new here and just received my ASI385MC last week, and I used it in the last two nights with my Z130 OTA (f5) on GTi mount. I had great view of the moon, and Jupiter showing on the screen. But I found I can not have the full moon to fit in the view as the sensor is small. I am wondering if I can use a focal reducer to help it, I tested on the FOV tool website, it seems if I can use a 0.5X focal reducer, I can fit the whole moon. But this means it will turn my focal ratio to f2.5, does this work? Does anyone have experience before I purchase one? Thanks! 



#2 daveco2

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Posted 22 September 2021 - 11:06 PM

Or you could make a mosaic of 2 or 4 panels and get a higher res, complete picture.


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#3 alphatripleplus

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Posted 23 September 2021 - 07:08 AM

You are likely to have severe aberrations away from the centre of the FOV, at anywhere close to f/2.5 using a cheap 0.5x  reducer. Fast Newtonians often utilize a coma corrector, which is usually not a cheap item.


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#4 JKowtko

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Posted 23 September 2021 - 04:43 PM

If you want to try it out, SVBony sells a 0.5x for around $20 (get the $20 one, not the $10 one) ...

 

I tried it with my 224mc on an old Meade DS-2090 and it worked okay.  

 

I didn't spend too much time with it so I don't know how great the picture really was, but it did reduce the image and I didn't get any vignetting ... so for twenty bucks I have no regrets :)

 

-- John


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#5 kongqk

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Posted 24 September 2021 - 10:17 PM

Thanks all for the comments, sorry for the late reply, somehow I didn't get any notification about the replies. 

 

Anyway, I figured it will have very bad aberrations. Thanks John for the suggestion, I just ordered the SVbony reducer to test it out, let's see how it works ^_^



#6 kongqk

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Posted 24 September 2021 - 10:19 PM

Or you could make a mosaic of 2 or 4 panels and get a higher res, complete picture.

Sorry, I don't quite understand how can you do this in real time. We only have one camera, so even we open a few more panels, how can they show different views in the panels? 



#7 daveco2

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Posted 25 September 2021 - 12:02 AM

Sorry, I don't quite understand how can you do this in real time. We only have one camera, so even we open a few more panels, how can they show different views in the panels? 

You need to take shot of the panels separately with the same camera.  The problem with reducers is that they also reduce the size of the image circle, not to mention the other problems with aberrations, tuning back focus - and cost.

The payoff is that mosaics give you a high res image that is more amenable to cropping and printing.  The downside is that it takes N times longer to get the image, where N is the number of panels.

In any case, to make panels, you could go to Telescopius, telescope simulator, to plan how many panels, percentage overlap, and their RA/DEC coordinates.  

 

https://telescopius....scope-simulator


Edited by daveco2, 25 September 2021 - 12:05 AM.

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#8 alphatripleplus

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Posted 25 September 2021 - 08:21 AM

You need to take shot of the panels separately with the same camera.  The problem with reducers is that they also reduce the size of the image circle, not to mention the other problems with aberrations, tuning back focus - and cost.

The payoff is that mosaics give you a high res image that is more amenable to cropping and printing.  The downside is that it takes N times longer to get the image, where N is the number of panels.

In any case, to make panels, you could go to Telescopius, telescope simulator, to plan how many panels, percentage overlap, and their RA/DEC coordinates.  

 

https://telescopius....scope-simulator

I'm not aware of any EAA software that will construct mosaics in real time either. Most mosaics are constructed as part of post-processing done at a later time, which is beyond the scope of the EAA Forum (no post-processing).

 

Of course, if someone has a script  (maybe in SharpCap 4.0) that can be run on-the-fly in real-time to build a paneled mosaic that can be viewed as it builds while actually doing captures with EAA software, that would make this accessible to EAAers. Maybe it will happen sometime, but I wouldn't count on it.


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#9 Tiago Ferreira

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Posted 25 September 2021 - 04:33 PM

if it's only for the moon it's ok. for the rest only if you cut the borders. Be aware of backfocus. if you can't get focus place it near the sensor and get less reduction factor.
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#10 kongqk

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Posted 27 September 2021 - 11:16 PM

You need to take shot of the panels separately with the same camera.  The problem with reducers is that they also reduce the size of the image circle, not to mention the other problems with aberrations, tuning back focus - and cost.

The payoff is that mosaics give you a high res image that is more amenable to cropping and printing.  The downside is that it takes N times longer to get the image, where N is the number of panels.

In any case, to make panels, you could go to Telescopius, telescope simulator, to plan how many panels, percentage overlap, and their RA/DEC coordinates.  

 

https://telescopius....scope-simulator

Thanks for the clarification, this is pretty cool, and I will give it a try sometime. 




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