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ASI 294 + 20" f/3.8 dob + ServoCat drive... Will it work?

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#1 Cotts

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Posted 17 May 2021 - 02:07 PM

I'm just peeking in the door of the EAA room this spring..... Currently learning SharpCap with a 65mm APO on a GEM. Camera is an  ASI224....I have been able to have successful live stacks even though the field of view of this combo is only 8.6' x 6.5'......

 

I have a 20-inch NewMoon Dob on the way - f/3.8 with the paracorr...  I've seen enough good reports on the ASI 294 camera to be ready to order one....

 

I found the field of view parameters for my 'future' scope/camera combo as follows....

 

Field 34.2' x 23.3' with image scale of 0.495" per pixel..   There is no end of objects out there that will fit in that field......

 

The BIG questions: 

 

1) Is the Servo-Cat drive accurate/steady enough for live stacking?  It is, after all, designed for visual observing with big dobs, not imaging.. Are any of you using this drive for EAA? 

 

2) Does the live stacking of SharpCap take care of the small-but-not-zero field rotation which will come from a driven Alt-Az mount...  (I can mitigate the rotation by choosing targets in certain parts of the sky where the rotation is minimal...)

 

Thanks

 

Dave



#2 bips3453

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Posted 17 May 2021 - 02:16 PM

Congratulations on getting a light bucket of a scope!

 

 

1) I can't comment about that drive, but decent Alt-Az mounts work fine for EAA. 

 

2) Yes, Sharcap can take care of field rotation by using alignment. 



#3 Ptarmigan

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Posted 17 May 2021 - 07:38 PM

People have used large Dobsonians with EAA.



#4 FrankG

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Posted 18 May 2021 - 07:03 AM

Hi Dave,

     Dobs make very good EAA platforms. I have a friend uses various ZWO cameras with great success on a 30" f3 Starstructure dob with ServoCAT. I have been using everything from the old analog Mallincams to an ASI294MC Pro on my 15" Starstructure f4.5 scope for 10 years (and before that on a 15" Obsession). I only have the ServoCat Jr. drive and I can get exposures up to 25 seconds (depending on where in sky) for stacking on Sharpcap. I use a reducer with my cameras to get things down somewhere around f2-3 but not necessary. You might keep in mind that your truss poles could require very slight trimming to get necessary in-focus - the scope I have has adjustable location (in-out) for the UTA. Sharpcap does not take care of field rotation but I simply crop out the borders of saved images. It's not imaging -- I have never bothered with darks or flats since I primarily just do live viewing but I have images posted in various places on here if you want to track them down.

Frank


Edited by FrankG, 18 May 2021 - 07:38 AM.

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#5 Cotts

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Posted 18 May 2021 - 07:37 AM

Congratulations on getting a light bucket of a scope!

 

 

1) I can't comment about that drive, but decent Alt-Az mounts work fine for EAA. 

 

2) Yes, Sharcap can take care of field rotation by using alignment. 

 

 

Hi Dave,

     I have been using everything from the old analog Mallincams to an ASI294MC Pro on my 15" Starstructure f4.5 scope for 10 years (and before that on a 15" Obsession). I only have the ServoCat Jr. drive and I can get exposures up to 25 seconds (depending on where in sky) for stacking on Sharpcap. Dobs make very good EAA platforms. I use a reducer with my cameras to get things down somewhere around f2-3 but not necessary. You might keep in mind that your truss poles could require very slight trimming to get necessary in-focus - the scope I have has adjustable location (in-out) for the UTA. Sharpcap will not take care of field rotation but I simply crop out the borders of saved images. It's not imaging -- I have never bothered with darks or flats since I primarily just do live viewing but I have image posts in various places on here if you want to track them down.

Frank

I'm reminded of the comedian Steven Wright here when he said, "I bought a humidifier and a dehumidifier and put them in the same room to fight it out..."

 

Dave
 


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#6 alphatripleplus

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Posted 18 May 2021 - 07:58 AM

 

2) Does the live stacking of SharpCap take care of the small-but-not-zero field rotation which will come from a driven Alt-Az mount...  (I can mitigate the rotation by choosing targets in certain parts of the sky where the rotation is minimal...)

 

 

Hopefully, I'm not adding to the confusion on field rotation here:

 

As you point out there is field rotation using an Alt-Az mount. What SharpCap cannot do is compensate for field rotation that occurs during a single sub-exposure, so you are correct that you will have a limit on the length of a sub, depending on where the scope is pointed.

 

What SharpCap can do, is rotate and align subs taken one after another so that there is no apparent field rotation between, say the first sub, and the 20th sub in the live view stack for stars that are visible in both frames.

 

However, there will be areas of non-overlap between the first and 20th sub, as the field slowly rotates. So some areas near the edge of the first sub won't appear in the 20th sub, as they will have rotated out of the field of view. The end result in the composite live view stack are these "triangular" non overlap areas that you may want to crop out.

 

So the answer is expect the triangular artifacts of field rotation, but the stars themselves should not be trailed due to field rotation. Clear as mud, right?  lol.gif


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#7 FrankG

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Posted 18 May 2021 - 08:17 AM

Hopefully, I'm not adding to the confusion on field rotation here:

 

As you point out there is field rotation using an Alt-Az mount. What SharpCap cannot do is compensate for field rotation that occurs during a single sub-exposure, so you are correct that you will have a limit on the length of a sub, depending on where the scope is pointed.

 

What SharpCap can do, is rotate and align subs taken one after another so that there is no apparent field rotation between, say the first sub, and the 20th sub in the live view stack for stars that are visible in both frames.

 

However, there will be areas of non-overlap between the first and 20th sub, as the field slowly rotates. So some areas near the edge of the first sub won't appear in the 20th sub, as they will have rotated out of the field of view. The end result in the composite live view stack are these "triangular" non overlap areas that you may want to crop out.

 

So the answer is expect the triangular artifacts of field rotation, but the stars themselves should not be trailed due to field rotation. Clear as mud, right?  lol.gif

What Errol said! smile.gif See image of Horsehead IC434 below

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Edited by FrankG, 18 May 2021 - 08:57 AM.

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#8 MunichAtNight

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Posted 18 May 2021 - 08:51 AM

Hello!

 

I think photos helps more than long explanations. Here some examples how it will looks like. They were done with a little bit smaller Newton than yours! smirk.gif

 

M101 Stack 232frames 1746s WithDisplayStretch
 
 
M97 - Owl nebulae - Stack 343frames 1585s

 

 

M51 ALL Stack 378frames 1444s
 
Stacks were done with a ASI 183MC with a Skywatcher f/5 130/650.Have fun and I am excited to see your photos from a 20" Newton.

 

MunichAtNight - Ewald



#9 alphatripleplus

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Posted 18 May 2021 - 08:53 AM

They say a picture is worth a thousand words....so maybe I should write some more.lol.gif



#10 bips3453

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Posted 18 May 2021 - 09:29 AM

I'm reminded of the comedian Steven Wright here when he said, "I bought a humidifier and a dehumidifier and put them in the same room to fight it out..."

 

Dave
 

Sorry, my comment was missing information about rotation withing single frame (can be eliminated by using shorter exposures, since you have 20" f/3.8 light bucket, it probably will not be an issue :-) )

and about dark edges due to field rotation across frames and alignment. 



#11 Cotts

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Posted 18 May 2021 - 05:12 PM

Thanks for the clarifications, fellow travellers.

 

I was referring to field rotation from frame to frame over a long-ish period of 5 or 10 minutes or more, not the rotation within one frame..

 

I'm the one who caused the confusion...

 

Sorted now.

 

Dave



#12 alphatripleplus

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Posted 18 May 2021 - 05:19 PM

Excellent. If you use that monster Dob for EAA, you may get some amazing views.



#13 mklosterman1

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Posted 23 May 2021 - 11:10 PM

Hi Dave,

     Dobs make very good EAA platforms. I have a friend uses various ZWO cameras with great success on a 30" f3 Starstructure dob with ServoCAT. I have been using everything from the old analog Mallincams to an ASI294MC Pro on my 15" Starstructure f4.5 scope for 10 years (and before that on a 15" Obsession). I only have the ServoCat Jr. drive and I can get exposures up to 25 seconds (depending on where in sky) for stacking on Sharpcap. I use a reducer with my cameras to get things down somewhere around f2-3 but not necessary. You might keep in mind that your truss poles could require very slight trimming to get necessary in-focus - the scope I have has adjustable location (in-out) for the UTA. Sharpcap does not take care of field rotation but I simply crop out the borders of saved images. It's not imaging -- I have never bothered with darks or flats since I primarily just do live viewing but I have images posted in various places on here if you want to track them down.

Frank

Hi Frank,

We”re almost neighbors (I live up just above Culpeper). I have am 18” f4.3 dob with servocat and Argo Navis and just purchased my first astrocam which also is a zwo asi294mc pro. I downloaded sharpcap and bought the license and will need to learn how to really get going. Would love to meet somewhere wherever you observe and get some tips sometime if possible.

 

Mike


Edited by mklosterman1, 23 May 2021 - 11:11 PM.


#14 FrankG

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Posted 25 May 2021 - 04:31 AM

Hi Frank,

We”re almost neighbors (I live up just above Culpeper). I have am 18” f4.3 dob with servocat and Argo Navis and just purchased my first astrocam which also is a zwo asi294mc pro. I downloaded sharpcap and bought the license and will need to learn how to really get going. Would love to meet somewhere wherever you observe and get some tips sometime if possible.

 

Mike

Mike, be glad t help but I almost never plan ahead -- too many variables (moon, weather, work, etc.). A group of us have been meeting near Galax, Va. for years to do EAA. We were there this month and may go back again in June. Will keep you posted. The 294 and Sharpcap are the way to go for that dob.

Frank



#15 Tiago Ferreira

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Posted 25 May 2021 - 04:53 AM

 The 294 and Sharpcap are the way to go for that dob.

Frank

+1

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