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Minimum Sensor Size and Minimum FOV for Platesolving

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#1 steveincolo

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Posted 27 September 2021 - 02:18 PM

There are many posts recommending against, for example, a 224-based camera due to the difficulty when using a small sensor to locate and center objects.  Based on my limited experience, a setup that does a platesolve after a goto and adjusts accordingly takes care of this.  On my ASIAIR Pro, the minimum FOV for a platesolve is 0.2 degrees.  My smallest sensor, a 178MM, and longest focal length, a C6 at f/10 just barely meet this.  But they work.  (With a reducer, the 224 might work, too.). 

 

I believe other software, such KStars, also has the capability to do platesolves as part of a goto.  For anyone who's using this feature in their software: are the minimum FOV requirements for platesolving similar?  I'm considering adding a C8, and at f/10 with the 178MM the ASIAIR Pro platesolve probably won't work.  



#2 DuncanM

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Posted 27 September 2021 - 03:00 PM

There are many posts recommending against, for example, a 224-based camera due to the difficulty when using a small sensor to locate and center objects.  Based on my limited experience, a setup that does a platesolve after a goto and adjusts accordingly takes care of this.  On my ASIAIR Pro, the minimum FOV for a platesolve is 0.2 degrees.  My smallest sensor, a 178MM, and longest focal length, a C6 at f/10 just barely meet this.  But they work.  (With a reducer, the 224 might work, too.). 

 

I believe other software, such KStars, also has the capability to do platesolves as part of a goto.  For anyone who's using this feature in their software: are the minimum FOV requirements for platesolving similar?  I'm considering adding a C8, and at f/10 with the 178MM the ASIAIR Pro platesolve probably won't work.  

The minimum FOV required for platesolving is determined by the star catalog used, camera FOV and exposure time, and the density of stars and their magnitude in any given sky location. 


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#3 DJL

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Posted 27 September 2021 - 03:04 PM

I have 2 methods of working with ASIAIR Pro, a C6 and ASI 462MC for Jupiter and Saturn:

 

1. Get a 5mm spacer for the 462. It now has the same back space as my ASI 071MC. Goto with the AAP and 071. When there, unthread the 071 and replace with the 462.

 

2. Use a guide scope and guide camera riding on the C6 to get to the target. Verify target is centered using a diagonal and eyepiece. If target is not in the frame, use a RACI and / or defocus until the donut appears and then move towards it using the AAP's arrow controls (but not the press to go button, that needs plate solve.) Look at target visually and show friends and family. Pull out the eyepiece and replace with 462.

 

Method 1 lets me use my filter wheel, so I can use my UV/IR filter, but I don't see how to fit in a Barlow. Using a 2x Barlow is supposedly marginal in Bortle 6 but I think I prefer it.

 

Method 2 lets me use the Barlow, but I can't figure out how to fit my 2" UV/IR filter, and don't want to take it out of the EFW.


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#4 steveincolo

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Posted 27 September 2021 - 03:04 PM

The minimum FOV required for platesolving is determined by the star catalog used, camera FOV and exposure time, and the density of stars and their magnitude in any given sky location. 

Thanks!  Do you have any experience being successful with smaller FOVs, perhaps using more detailed star catalogs and longer exposures?  



#5 DuncanM

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Posted 27 September 2021 - 03:25 PM

Thanks!  Do you have any experience being successful with smaller FOVs, perhaps using more detailed star catalogs and longer exposures?  

In the dim and distant past I used the USNOA2 star catalog (8 GB of data) and could plate solve just about any .15 deg random field with a 60sec exposure on 16in SCT at F5 using a cookbook camera. 

 

The problem is that as the FoV gets smaller, you need longer exposures and a very deep and extensive stellar database to ensure consistent success. One way around this is to use a secondary camera attached to a widefield guidescope and use that for platesolving instead of the narrow field camera in the main OTA.

 

An ASI120MM mini attached to a 50mm ~F4 guidescope will provide enough stars, even in a ~3sec exposure, that every random field will be platesolved quickly and accurately even using a compact stellar database. Just make sure that the guidescope and it's camera is aligned to the main OTA and it's camera.


Edited by DuncanM, 27 September 2021 - 03:28 PM.

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#6 steveincolo

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Posted 27 September 2021 - 03:44 PM

In the dim and distant past I used the USNOA2 star catalog (8 GB of data) and could plate solve just about any .15 deg random field with a 60sec exposure on 16in SCT at F5 using a cookbook camera. 

 

The problem is that as the FoV gets smaller, you need longer exposures and a very deep and extensive stellar database to ensure consistent success. One way around this is to use a secondary camera attached to a widefield guidescope and use that for platesolving instead of the narrow field camera in the main OTA.

 

An ASI120MM mini attached to a 50mm ~F4 guidescope will provide enough stars, even in a ~3sec exposure, that every random field will be platesolved quickly and accurately even using a compact stellar database. Just make sure that the guidescope and it's camera is aligned to the main OTA and it's camera.

Thanks again!  I guess then I should be pretty happy with the (claimed) ASIAIR Pro 0.2 degree limitation, and go to a guidescope if I really need something narrower.   


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#7 steveincolo

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Posted 27 September 2021 - 03:47 PM

I have 2 methods of working with ASIAIR Pro, a C6 and ASI 462MC for Jupiter and Saturn:

 

1. Get a 5mm spacer for the 462. It now has the same back space as my ASI 071MC. Goto with the AAP and 071. When there, unthread the 071 and replace with the 462.

 

2. Use a guide scope and guide camera riding on the C6 to get to the target. Verify target is centered using a diagonal and eyepiece. If target is not in the frame, use a RACI and / or defocus until the donut appears and then move towards it using the AAP's arrow controls (but not the press to go button, that needs plate solve.) Look at target visually and show friends and family. Pull out the eyepiece and replace with 462.

 

Method 1 lets me use my filter wheel, so I can use my UV/IR filter, but I don't see how to fit in a Barlow. Using a 2x Barlow is supposedly marginal in Bortle 6 but I think I prefer it.

 

Method 2 lets me use the Barlow, but I can't figure out how to fit my 2" UV/IR filter, and don't want to take it out of the EFW.

Clever, especially your spacer hack!  I'm trying to automate as much as possible and avoid swapping, though, due to my fumble-fingers.  Likely to lose a camera or eyepiece over the edge of my deck.  It's enough risk for me to be using a filter drawer!


Edited by steveincolo, 27 September 2021 - 03:49 PM.

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#8 DJL

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Posted 27 September 2021 - 06:33 PM

In the dim and distant past I used the USNOA2 star catalog (8 GB of data) and could plate solve just about any .15 deg random field with a 60sec exposure on 16in SCT at F5 using a cookbook camera. 

 

The problem is that as the FoV gets smaller, you need longer exposures and a very deep and extensive stellar database to ensure consistent success. One way around this is to use a secondary camera attached to a widefield guidescope and use that for platesolving instead of the narrow field camera in the main OTA.

 

An ASI120MM mini attached to a 50mm ~F4 guidescope will provide enough stars, even in a ~3sec exposure, that every random field will be platesolved quickly and accurately even using a compact stellar database. Just make sure that the guidescope and it's camera is aligned to the main OTA and it's camera.

My guide scope is a William Optics Uniguide 32, light enough for a DSLR but also work with my GT81IV and my C6. The issue is, it is mounted in a shoe and has no adjustment capability. I have since learned that shims might be a solution. So I got a RACI, and again getting it aligned with the C6 puts it at the edge of its travel where one of the adjustment screws is not doing anything. Nevertheless, I am now managing to get Jupiter in the frame - ungainly but working.


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#9 DJL

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Posted 27 September 2021 - 06:36 PM

Clever, especially your spacer hack!  I'm trying to automate as much as possible and avoid swapping, though, due to my fumble-fingers.  Likely to lose a camera or eyepiece over the edge of my deck.  It's enough risk for me to be using a filter drawer!

From my DSO experience, I like to build my rig indoors, as clean as I can get it, and then take it to the mount. I definitely do not like adding and removing parts outdoors. 

 

I saw this and thought, perfect! A table with lens and camera holders! https://www.amazon.c...uct/B073RRTR2G/


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#10 DuncanM

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Posted 27 September 2021 - 07:51 PM

My guide scope is a William Optics Uniguide 32, light enough for a DSLR but also work with my GT81IV and my C6. The issue is, it is mounted in a shoe and has no adjustment capability. I have since learned that shims might be a solution. So I got a RACI, and again getting it aligned with the C6 puts it at the edge of its travel where one of the adjustment screws is not doing anything. Nevertheless, I am now managing to get Jupiter in the frame - ungainly but working.

I use mounting rings similar to these for my 50mm guidescopes:

 

http://www.scopestuff.com/ss_fring.htm

(FRQ8)

 

As they allow for lots of adjustment to line up with the main camera and 10in SCT OTA. However, they're not used much anymore on my CEM120 as I moved that OTA  to an OAG. My APSC sized camera gives me about a 1deg diagonal field at F7.


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#11 steveincolo

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Posted 28 September 2021 - 05:43 PM

From my DSO experience, I like to build my rig indoors, as clean as I can get it, and then take it to the mount. I definitely do not like adding and removing parts outdoors. 

 

I saw this and thought, perfect! A table with lens and camera holders! https://www.amazon.c...uct/B073RRTR2G/

Yes, perfect, though I’d probably knock it over in the dark. 




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