Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

On Auto-guiding and focal length....

  • Please log in to reply
12 replies to this topic

#1 Cotts

Cotts

    Just Wondering

  • *****
  • Moderators
  • topic starter
  • Posts: 9666
  • Joined: 10 Oct 2005
  • Loc: Madoc, Ontario

Posted 20 August 2019 - 09:06 AM

In anticipation of the arrival of my TEC 160..   I seek wisdom.

 

My set-up will have the 160 (f.l. 1120mm) topped by an AstroTech 65mmQuad APO (f.l. 400mm) and an Orion Starshoot Autoguider in a wee little finder scope of about 120mm focal length....  Using PHD guide 2.....AP Mach 1 mount.

 

Is 120mm enough f.l. to guide 1120mm?  

 

What about using the 400mm f.l. to guide the 1120mm? better?  

 

Would it be overkill to guide the 400mm scope with the 1120mm scope? 

 

Is there a rule of thumb for this sort of thing?

 

Dave

 

 

 

 

 

 


  • PirateMike likes this

#2 P_Myers

P_Myers

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 29 Feb 2016

Posted 20 August 2019 - 09:16 AM

http://www.wilmslowa...autoguiding.htm


  • ssantia likes this

#3 P_Myers

P_Myers

    Explorer 1

  • -----
  • Posts: 82
  • Joined: 29 Feb 2016

Posted 20 August 2019 - 09:22 AM

These are the numbers you need:

a=imaging camera pixel size
b=guide camera pixel size (both in microns)

Guide scope focal length (FL) = main telescope FL x b/10a


  • PirateMike likes this

#4 bobzeq25

bobzeq25

    Hubble

  • *****
  • Posts: 16750
  • Joined: 27 Oct 2014

Posted 20 August 2019 - 09:27 AM

It's based on image scale, not focal length.  Comparing main scope and guider, most everyone thinks a 3X ratio is fine.  A majority would think 5X.  Some would go more.

 

I noticed a minor improvement when putting an OAG on my 130 F7 reduced the ratio from 5 to 1.4.  I now only use the OAG, any improvement in guiding is good.  With my shorter focal length scopes I still use a 60mm F4 guidescope, very solidly mounted.


Edited by bobzeq25, 20 August 2019 - 09:32 AM.

  • kingjamez, rgsalinger and PirateMike like this

#5 kingjamez

kingjamez

    Surveyor 1

  • *****
  • Posts: 1867
  • Joined: 03 Oct 2006
  • Loc: Fairfax, VA

Posted 20 August 2019 - 09:47 AM

These are the numbers you need:

a=imaging camera pixel size
b=guide camera pixel size (both in microns)

Guide scope focal length (FL) = main telescope FL x b/10a

That gives a 10:1 ratio between image scale, I'd never go that high. I get nervous at 5x and shoot for 3x if reasonable.

 

-Jim


  • PirateMike and bobzeq25 like this

#6 Ishtim

Ishtim

    Apollo

  • -----
  • Posts: 1346
  • Joined: 10 Nov 2007
  • Loc: N. Alabama, USA

Posted 20 August 2019 - 10:59 AM

Now I'm confused...

If I were to use a self-guided SBIG camera (9u px. main, 7.4u px guide) with Dave's scope (1200mm FL) I'd get 1.66"/px and 1.29"/px using:   Angular resolution  = (px size*206)/FL

Isn't this "backwards"?



#7 jerahian

jerahian

    Viking 1

  • *****
  • Posts: 547
  • Joined: 02 Aug 2018
  • Loc: Maine

Posted 20 August 2019 - 11:05 AM

As bobzeq25 mentioned, it's all about the image scale ratio between your primary and your guider setup.

 

Checkout this calculator:  https://astronomy.to...ope_suitability

 

For the 160 with the 60Da, guided by the 120 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/11.29

For the 160 with the 60Da, guided by the 400 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/3.39

For the 160 with the 224MC, guided by the 120 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/12.94

For the 160 with the 224MC, guided by the 400 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/3.88

 

So, between the two guide scopes, definitely use the 400mm scope for guiding.

 

-Ara



#8 Ishtim

Ishtim

    Apollo

  • -----
  • Posts: 1346
  • Joined: 10 Nov 2007
  • Loc: N. Alabama, USA

Posted 20 August 2019 - 11:51 AM

As bobzeq25 mentioned, it's all about the image scale ratio between your primary and your guider setup.

 

Checkout this calculator:  https://astronomy.to...ope_suitability

 

I plugged in the SBIG main/guider chip pixel sizes numbers of 9um and 7.4um with a 1200mm scope into the calculator and got:  "The imaging / guiding ratio is 1:0.82". 



#9 bobzeq25

bobzeq25

    Hubble

  • *****
  • Posts: 16750
  • Joined: 27 Oct 2014

Posted 20 August 2019 - 11:52 AM

Now I'm confused...

If I were to use a self-guided SBIG camera (9u px. main, 7.4u px guide) with Dave's scope (1200mm FL) I'd get 1.66"/px and 1.29"/px using:   Angular resolution  = (px size*206)/FL

Isn't this "backwards"?

Here's the easy to remember calculation.  Take the focal length of the scope.  Divide by 200.

 

Divide the pixel size by the result.


  • jerahian likes this

#10 jerahian

jerahian

    Viking 1

  • *****
  • Posts: 547
  • Joined: 02 Aug 2018
  • Loc: Maine

Posted 20 August 2019 - 11:57 AM

Now I'm confused...

If I were to use a self-guided SBIG camera (9u px. main, 7.4u px guide) with Dave's scope (1200mm FL) I'd get 1.66"/px and 1.29"/px using:   Angular resolution  = (px size*206)/FL

Isn't this "backwards"?

 

I plugged in the SBIG main/guider chip pixel sizes numbers of 9um and 7.4um with a 1200mm scope into the calculator and got:  "The imaging / guiding ratio is 1:0.82". 

Well, in your case since it's basically using the same scope for guiding like an OAG, you have a higher resolution for guiding than you do for your primary imaging because, for the same focal length, your guide camera has smaller pixels than your main camera.  So, yes, I guess this would be "backwards" from what most people normally experience.  Another way to look at your example is that you have a situation where your guider provides more accuracy than your primary imaging can use.


Edited by jerahian, 20 August 2019 - 11:58 AM.


#11 Cotts

Cotts

    Just Wondering

  • *****
  • Moderators
  • topic starter
  • Posts: 9666
  • Joined: 10 Oct 2005
  • Loc: Madoc, Ontario

Posted 20 August 2019 - 02:56 PM

As bobzeq25 mentioned, it's all about the image scale ratio between your primary and your guider setup.

 

Checkout this calculator:  https://astronomy.to...ope_suitability

 

For the 160 with the 60Da, guided by the 120 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/11.29

For the 160 with the 60Da, guided by the 400 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/3.39

For the 160 with the 224MC, guided by the 120 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/12.94

For the 160 with the 224MC, guided by the 400 scope with the Orion Starshoot Autoguider, you would have a ratio of 1/3.88

 

So, between the two guide scopes, definitely use the 400mm scope for guiding.

 

-Ara

Thanks for doing all those calculations.  I can put away the 'wee' scope of 120mm f.l.. and use the AstroTech to guide the TEC 160..

 

Ok.

 

But what if i wanted to image something with the 400mm f.l. AstroTech APO (2x3degree field) scope and my Canon 60 Da.  Can I guide it with the 1120mm f.l. TEC 160?  Is there such a thing as having the guide scope focal length too long for the imaging scope?

 

BTW the ASI224 is only used for video lucky imaging to measure visual binary stars at about f/28 or so....  No auto-guiding needed because it's video....

 

Thanks

 

Dave


  • jerahian likes this

#12 kingjamez

kingjamez

    Surveyor 1

  • *****
  • Posts: 1867
  • Joined: 03 Oct 2006
  • Loc: Fairfax, VA

Posted 20 August 2019 - 03:02 PM

No issue at all guiding with your main scope. The focal length doesn't matter, all that really matters is that you aren't dramatically undersampled.

 

-Jim



#13 bobzeq25

bobzeq25

    Hubble

  • *****
  • Posts: 16750
  • Joined: 27 Oct 2014

Posted 20 August 2019 - 04:29 PM

Thanks for doing all those calculations.  I can put away the 'wee' scope of 120mm f.l.. and use the AstroTech to guide the TEC 160..

 

Ok.

 

But what if i wanted to image something with the 400mm f.l. AstroTech APO (2x3degree field) scope and my Canon 60 Da.  Can I guide it with the 1120mm f.l. TEC 160?  Is there such a thing as having the guide scope focal length too long for the imaging scope?

 

BTW the ASI224 is only used for video lucky imaging to measure visual binary stars at about f/28 or so....  No auto-guiding needed because it's video....

 

Thanks

 

Dave

Once again, what you need to look at is image scale.  Since it numerically decreases as focal length increases, you decrease the guider/mainscope image scale ratio, and you simply can't go too small on that.   The slower optics might make it hard to find a good guidestar, but that's an unlikely problem.




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics