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ASI294 correct spacing for 10"SCT R6.3

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#1 kevinrodgers22

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Posted 28 November 2021 - 09:05 PM

I just bought a ZWO ASI294 for my Meade F/10 LX50, I'm planning on using a 6.3 reducer. I have an older style Meade OAG that came with the scope. I really have no idea what additional accessories or spacers I'll need to get going. It seems like most folks need spacers when using a reducer w/ their SCT's. 

 

I'm pretty sure the ZWO comes with adapters to mate with the OAG. I'm pretty sure I'll need a T-adapter, I found this one listed on CN thats compatible with my scope.

 

Is there anything else I'll need to get going, do I have any compatibility issues thus far? 

 

I haven't settled on a guide camera yet, any recommendations would also be helpful..budget leaning towards the QHY5L-II- M

 

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#2 Alien Observatory

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Posted 28 November 2021 - 10:16 PM

Rethinking, You will need 105mm from the rear lens of the reducer to the 294 sensor including the OAG.  Install 0.63 reducer, Meade T adapter, OAG (thick ring) and make a measurement for any extensions needed.  If the measurement is greater than 105mm then you will need a shorted Meade Style T adapter.  Not sure what else to recommend before you make a measurement of the set up on the telescope.  Pat Utah smile.gif

 

https://agenaastro.c...d-adapters.html


Edited by Alien Observatory, 28 November 2021 - 10:17 PM.


#3 kevinrodgers22

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Posted 28 November 2021 - 11:18 PM

Rethinking, You will need 105mm from the rear lens of the reducer to the 294 sensor including the OAG.  Install 0.63 reducer, Meade T adapter, OAG (thick ring) and make a measurement for any extensions needed.  If the measurement is greater than 105mm then you will need a shorted Meade Style T adapter.  Not sure what else to recommend before you make a measurement of the set up on the telescope.  Pat Utah smile.gif

 

https://agenaastro.c...ad-adapters.htm

So you think the Meade T Adapter in my original post might be a good starting point and then just kind of tweak it in until I reach that 105mm? That sounds pretty straight forward. 

 

What did you mean by "thick ring"? 

 

Thanks for walking me though all this...

 

As usual, thanks..you've pretty much walked me though this whole process. 



#4 dcweaver

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Posted 29 November 2021 - 12:34 AM

Do you intend to send guide signals to correct the mount's RA tracking.  It's not needed for EAA and will complicate your setup.

 

The OAG's purpose is to provide a connection for a separate guide camera that picks off part of the light cone and sends it to the guide camera.  Those images are processed by guiding software that sends signals to the RA tracking motor to speed it up or slow it down.  This is what is referred to as "closed loop" tracking.  It is common for long exposure astrophotography where you have to track an object to pixel level accuracy for 5 minute or longer exposures.  Long exposures were driven by CCD technology that had very high read noise compared to today's CMOS cameras.  For EAA, and Zwo cameras in particular, CMOS technology is used.  It has much lower read noise.  This enables stacking of many short sub exposures to build up an image that has a total integration time comparable to a single long exposure.  Since sub exposures are measured in seconds or tens of seconds, guiding is not needed.  The image doesn't shift in the frame enough to warrant guiding.

 

Get the T-adapter.  It provides 50 mm of the 105 mm spacing for a 0.63 reducer.  Once you have that, Zwo cameras come with spacers that provide the remaining 55 mm of backfocus.  55 mm backfocus comes from the T-mount system used by most DSLR cameras.

 

The final optical train will look like this:  Scope's SCT threads > 0.63 reducer > T-adapter > Zwo spacers > Zwo camera

 

You can look on the Zwo website for how to use the spacers.  There is a lot of material on how to get the remaining 55 mm of backfocus after you install the reducer and T-adapter.


Edited by dcweaver, 29 November 2021 - 12:41 AM.

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#5 Alien Observatory

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Posted 29 November 2021 - 12:51 AM

So you think the Meade T Adapter in my original post might be a good starting point and then just kind of tweak it in until I reach that 105mm? That sounds pretty straight forward. 

 

What did you mean by "thick ring"? 

 

Thanks for walking me though all this...

 

As usual, thanks..you've pretty much walked me though this whole process. 

Yes to Q 1.  Thicker is the Wider SCT Thread on the OAG, when you try to thread the Meade T Adapter on the 0.63 reducer to the SCT Thread Meade OAG it will become obvious which end is correct on the OAG.  The 105 mm is not exact, but the closer to 105mm the closer it will be to a 0.63 reduction.   Pat Utah smile.gif

 

 

p.s.  Telescope_0.63 reducer_Meade T Adapter_Meade OAG_T Spacer_Cam....My guess is that the Meade T Adapter is way too long to get the 105 mm back spacing.

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Edited by Alien Observatory, 29 November 2021 - 02:05 AM.

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#6 dcweaver

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Posted 29 November 2021 - 01:15 AM

In addition to trying to talk you out of guiding (at least initially), I will also answer the question directly if you are really set on doing it.  It's like Pat said:

 

Scope's SCT threads > OAG > some length spacer > remaining 55 mm from Zwo spacers and camera

(This assumes the OAG provides less than 50 mm spacing)

 

Guiding is fun, but it costs more (second camera) and I hate to see you get lost in the details of PhD2 if you haven't done that before.  With your long focal length, you'll have your hands full with basic alignment, pointing, gain, exposure, and stacking.


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#7 GazingOli

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Posted 29 November 2021 - 02:20 AM

just get an extension tube set like this: 

https://www.bresser....n-Tube-Set.html

 

it comes with an adaptor for SCT and will fit to the reducer on one side and the camera on the other.

 

That is the easiest way to get started.

 

CS.Oli


Edited by GazingOli, 29 November 2021 - 02:21 AM.

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#8 alphatripleplus

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Posted 29 November 2021 - 08:20 AM

One thing to keep in mind is that the recommended 105mm spacing from reducer to sensor should give you close to f/6.3  with  the least aberrations, but you may want to vary it a little to see if it is optimal and how aberrations vary with spacing. For EAA, many people will  also vary the spacing for more or less focal reduction.   


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#9 Noah4x4

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Posted 29 November 2021 - 09:58 AM

One thing to keep in mind is that the recommended 105mm spacing from reducer to sensor should give you close to f/6.3  with  the least aberrations, but you may want to vary it a little to see if it is optimal and how aberrations vary with spacing. For EAA, many people will  also vary the spacing for more or less focal reduction.


+1 for this.

My Evolution doesnt offer enough rear end clearance for 105mm backfocus behind the x0.63 focal reducer in a straight line for a cooled camera. Hence I use a 2" diagonal. I reckon that is nearer 120mm and f/5.2. No problem finding focus with an SCT. Some folk suggest that will increase aberations at the edge of the field, but I have not noticed any adverse impact, but then I have no aspirations of winning Astrophotographer of the year (if I did I wouldn't be using an Alt-Az!).

#10 kevinrodgers22

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Posted 29 November 2021 - 11:25 AM

Yes to Q 1.  Thicker is the Wider SCT Thread on the OAG, when you try to thread the Meade T Adapter on the 0.63 reducer to the SCT Thread Meade OAG it will become obvious which end is correct on the OAG.  The 105 mm is not exact, but the closer to 105mm the closer it will be to a 0.63 reduction.   Pat Utah smile.gif

 

 

p.s.  Telescope_0.63 reducer_Meade T Adapter_Meade OAG_T Spacer_Cam....My guess is that the Meade T Adapter is way too long to get the 105 mm back spacing.

I couldn't find the exact specs for the OAG but after closely eyeballing it, I think its right around 2" (50mm). Based off that it looks like you are right about the T Adapter being way too big. I think I need something half that size, somewhere around .75"-1.00". Would you agree with this? 

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#11 Alien Observatory

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Posted 29 November 2021 - 11:46 AM

I couldn't find the exact specs for the OAG but after closely eyeballing it, I think its right around 2" (50mm). Based off that it looks like you are right about the T Adapter being way too big. I think I need something half that size, somewhere around .75"-1.00". Would you agree with this? 

Looks close enough to give it a try.  Pat Utah :)



#12 kevinrodgers22

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Posted 30 November 2021 - 03:46 PM

In addition to trying to talk you out of guiding (at least initially), I will also answer the question directly if you are really set on doing it.  It's like Pat said:

 

Scope's SCT threads > OAG > some length spacer > remaining 55 mm from Zwo spacers and camera

(This assumes the OAG provides less than 50 mm spacing)

 

Guiding is fun, but it costs more (second camera) and I hate to see you get lost in the details of PhD2 if you haven't done that before.  With your long focal length, you'll have your hands full with basic alignment, pointing, gain, exposure, and stacking.

You're probably right, I will likely move to a flip mirror once I really get going...I figure I ought to give the OAG a shot first. I'll wait to get a guide camera until I figure some things out.  

 

If I do decide to use the OAG to auto guide, wouldn't it be better if I put the spacers before the OAG? That way both the guide camera and ZWO are closer to the 105mm mark? 

 

I found my OAG and measured it yesterday, it's actually closer to 3" (76mm) in length. If the ZWO camera's sensor is recessed 17mm, I only need to come up with another 12mm of spacing to reach the 105mm. I definitely won't need a T-adapter...Is 93mm close enough you think?



#13 Alien Observatory

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Posted 30 November 2021 - 03:53 PM

Getting an OAG working throws a little more complexity to the spacers.  The sensor in the OAG must be at the same distance as the OAG mirror is to the Main Sensor (294).  It is common to use a "mini" camera, small body with a 1.25 extension, so that it may move up or down a small amount to come into focus at the same time as the main sensor.  It may take a couple of iterations to get it correct.  Get the main cam working on a bright target before attempting the OAG.  Pat Utah :)


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#14 Borodog

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Posted 30 November 2021 - 05:40 PM

Um, which f/6.3 reducer? It's only the Celestron version that has a 105 mm back spacing spec.



#15 kevinrodgers22

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Posted 30 November 2021 - 06:30 PM

Um, which f/6.3 reducer? It's only the Celestron version that has a 105 mm back spacing spec.

I lucked out, thats the one...



#16 dcweaver

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Posted 30 November 2021 - 09:17 PM

93 mm is definitely close enough to start with.  As long as you stay somewhere on Pat's curve, you should be fine.  You can dial in the spacing to get the maximize the "corrector" part of the "reducer-corrector" later.

 

Yeah, it might be better to put the spacer(s) in front.  OAG do require the distance between the pickoff mirror/prism to each sensor be the same.  It will take some artistry to get the 105 mm and maintain the OAG spacing constraint.

 

Fortunately, the 105 mm on the reducer-corrector is not a hard-and-fast requirement.  It can vary several mm without messing things up too bad.  That gives you more room to play and get the spacing right in each leg of the optical path.


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#17 dcweaver

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Posted 30 November 2021 - 09:28 PM

Um, which f/6.3 reducer? It's only the Celestron version that has a 105 mm back spacing spec.

That's good to know.  Is the difference between the Meade and Celestron brands, or between some of the other offerings from various distributors.  With the epidemic of product re-branding from the same manufacturers (GSO, Synta, JOC), I didn't realize there were different specs on any of these.


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#18 Alien Observatory

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Posted 01 December 2021 - 12:55 AM

93 mm is definitely close enough to start with.  As long as you stay somewhere on Pat's curve, you should be fine.  You can dial in the spacing to get the maximize the "corrector" part of the "reducer-corrector" later.

 

Yeah, it might be better to put the spacer(s) in front.  OAG do require the distance between the pickoff mirror/prism to each sensor be the same.  It will take some artistry to get the 105 mm and maintain the OAG spacing constraint.

 

Fortunately, the 105 mm on the reducer-corrector is not a hard-and-fast requirement.  It can vary several mm without messing things up too bad.  That gives you more room to play and get the spacing right in each leg of the optical path.

A "Typical" OAG spacing to the sensors is about 50mm, but each brand / vendor is unique in its spacing requirements.  Pat Utah smile.gif


Edited by Alien Observatory, 01 December 2021 - 12:56 AM.



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