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

Advice needed on a guide scope/camera

  • Please log in to reply
10 replies to this topic

#1 drice

drice

    Lift Off

  • *****
  • topic starter
  • Posts: 10
  • Joined: 23 Sep 2014

Posted 21 September 2021 - 07:55 PM

Hello, I have a Takahashi Epsilon 180 scope and a Takahashi EM200 mount. Currently I am shooting with a Nikon D800 but plan to move to a ZWO 2600 later. 

 

I would like to get a guide scope to increase the sharpness of my images. What would be an appropriate guide scope/camera combo for my 500mm focal length scope? I have read that you should match them but I don't know the guidelines on that. Appreciate any info on the subject, thanks.   -Derryl



#2 drd715

drd715

    Viking 1

  • *****
  • Posts: 972
  • Joined: 07 Jan 2015
  • Loc: Fort Lauderdale

Posted 21 September 2021 - 08:25 PM

Asi290mm

Sent from my SM-N950U using Tapatalk
  • drice likes this

#3 jerahian

jerahian

    Surveyor 1

  • *****
  • Moderators
  • Posts: 1,523
  • Joined: 02 Aug 2018
  • Loc: Maine

Posted 21 September 2021 - 08:42 PM

Hello, I have a Takahashi Epsilon 180 scope and a Takahashi EM200 mount. Currently I am shooting with a Nikon D800 but plan to move to a ZWO 2600 later. 

 

I would like to get a guide scope to increase the sharpness of my images. What would be an appropriate guide scope/camera combo for my 500mm focal length scope? I have read that you should match them but I don't know the guidelines on that. Appreciate any info on the subject, thanks.   -Derryl

 

Yes, the ASI290MM Mini is a great guide cam.  You can put it in an OAG or slap it on an inexpensive 50-60mm guide scope if you need an extra weight to help balance your Epsilon.



#4 mikewayne3

mikewayne3

    Surveyor 1

  • -----
  • Posts: 1,899
  • Joined: 02 Feb 2009
  • Loc: San Diego CA

Posted 21 September 2021 - 08:54 PM

Well when I image with my 6" newt i use an 8X50 and a DSI for guiding and when I use my AT72ED I piggy back it on my newt and thein use the 6" newt as my guide scope

Mike



#5 TelescopeGreg

TelescopeGreg

    Soyuz

  • -----
  • Posts: 3,521
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 21 September 2021 - 08:59 PM

They have to "match" with a factor of about 5, i.e. the ratio of pixel scale between the imaging and guiding system shouldn't be more than about 5:1.  Pixel scale is 206 * pixel size / focal length.  Calculate it for both, and see what the ratio is.


  • drice likes this

#6 mikewayne3

mikewayne3

    Surveyor 1

  • -----
  • Posts: 1,899
  • Joined: 02 Feb 2009
  • Loc: San Diego CA

Posted 21 September 2021 - 09:27 PM

I guess I'm just lucky

I never think about that I just Push Here Dummy 1 and it works fine

but I think TelescopeGreg makes a good point 

 

Mike



#7 tosjduenfs

tosjduenfs

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 03 Jan 2017

Posted 21 September 2021 - 09:46 PM

They have to "match" with a factor of about 5, i.e. the ratio of pixel scale between the imaging and guiding system shouldn't be more than about 5:1.  Pixel scale is 206 * pixel size / focal length.  Calculate it for both, and see what the ratio is.

Genuine question incoming:

 

I've seen a few rules of thumb about this; anywhere from 1:3 to 1:5 but I've never been clear on the explanation. 

 

I've had great success with the QHY mini guide scope and QHY5L-II camera.  The guidescope is 130mm FL.  Whether I put it on a small widefield lens or my 700mm FL APO the mount still guides around 0.5" RMS.    With the advent of multistar guiding and the ability for PHD to calculate star mass on a subpixel level does it really matter?  With such a small and lightweight guiding setup differential flexure is much easier to overcome. Would my guiding get better with a longer FL guide scope?  


Edited by tosjduenfs, 21 September 2021 - 09:48 PM.


#8 TelescopeGreg

TelescopeGreg

    Soyuz

  • -----
  • Posts: 3,521
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 21 September 2021 - 10:52 PM

Genuine question incoming:

 

I've seen a few rules of thumb about this; anywhere from 1:3 to 1:5 but I've never been clear on the explanation. 

 

I've had great success with the QHY mini guide scope and QHY5L-II camera.  The guidescope is 130mm FL.  Whether I put it on a small widefield lens or my 700mm FL APO the mount still guides around 0.5" RMS.    With the advent of multistar guiding and the ability for PHD to calculate star mass on a subpixel level does it really matter?  With such a small and lightweight guiding setup differential flexure is much easier to overcome. Would my guiding get better with a longer FL guide scope?  

As with any rule of thumb, I think of the 1:3 or 1:5 or whatever ratio as a high level measure of how "numb" the guider is to errors.  I've read some that say PHD2 can make useful estimates of the guide star's centroid with even a 1:10 ratio, but with my mount (AVX) I doubt that would be sufficient.  If the mount is really good and your seeing is good, the guider isn't going to be working all that hard.  You can certainly push the ratio and just pick up any residual periodic error.  I'm at 1:5, and the guider is running borderline frantic since the mount has an internal 11 second periodicity from the RA pinion gear that PEC can't fix.  On a good night, my RA RMS guiding is about 3x what the Dec is.  Bin the camera 2x2 and I can just make it...

 

So try guiding a 2m focal length SCT with a shorty guide scope, and, no, it doesn't work.  Ratio?  Beyond 1:10...  Sanity check.


  • OldManSky likes this

#9 tosjduenfs

tosjduenfs

    Explorer 1

  • -----
  • Posts: 83
  • Joined: 03 Jan 2017

Posted 22 September 2021 - 01:55 AM

As with any rule of thumb, I think of the 1:3 or 1:5 or whatever ratio as a high level measure of how "numb" the guider is to errors.  I've read some that say PHD2 can make useful estimates of the guide star's centroid with even a 1:10 ratio, but with my mount (AVX) I doubt that would be sufficient.  If the mount is really good and your seeing is good, the guider isn't going to be working all that hard.  You can certainly push the ratio and just pick up any residual periodic error.  I'm at 1:5, and the guider is running borderline frantic since the mount has an internal 11 second periodicity from the RA pinion gear that PEC can't fix.  On a good night, my RA RMS guiding is about 3x what the Dec is.  Bin the camera 2x2 and I can just make it...

 

So try guiding a 2m focal length SCT with a shorty guide scope, and, no, it doesn't work.  Ratio?  Beyond 1:10...  Sanity check.

I guess I'm wondering, if the mechanical limit of my mount is ~0.5" RMS and that can be acheived with a 130mm FL guidescope, what is the benefit from using a longer FL?  

 

Although with a longer FL I wonder if 0.5" is the limit? Not that I need that accuracy.  I've gone down as low as 0.3" to 0.4" on exceptional nights. 

 

I suppose there must be a limit on the subpixel resolution the guiding software can achieve so if the mount is up to the task then a longer focal length would be necessary.


Edited by tosjduenfs, 22 September 2021 - 02:00 AM.


#10 drice

drice

    Lift Off

  • *****
  • topic starter
  • Posts: 10
  • Joined: 23 Sep 2014

Posted 22 September 2021 - 10:01 AM

Thanks TelescopeGreg, that is exactly the info I was looking for.  -Derryl



#11 TelescopeGreg

TelescopeGreg

    Soyuz

  • -----
  • Posts: 3,521
  • Joined: 16 Jul 2018
  • Loc: Auburn, California, USA

Posted 22 September 2021 - 02:53 PM

I guess I'm wondering, if the mechanical limit of my mount is ~0.5" RMS and that can be acheived with a 130mm FL guidescope, what is the benefit from using a longer FL?  

 

Although with a longer FL I wonder if 0.5" is the limit? Not that I need that accuracy.  I've gone down as low as 0.3" to 0.4" on exceptional nights. 

 

I suppose there must be a limit on the subpixel resolution the guiding software can achieve so if the mount is up to the task then a longer focal length would be necessary.

The rule of thumb is all I've ever seen in determining how effective a guider will be, and that's only dependent on the relative pixel scales of the imager and guider.  Partly it's saying that if your imaging scale is "x", you need better than 5*x (or 3x or whatever) to keep your stars sharp.  But I suppose on the other side, it could also be saying you don't need to be any better than that, so save your money?  The OAG fans would probably disagree...

 

So, an interesting question.  Let's think about this.  What does the manufacturer mean by the "mechanical limit" of the mount?  A single number is practically meaningless unless one knows the conditions under which it is measured.  As you say, the mount and the particulars of the night both contribute to it.  Probably more so, what you have positioned on top of the mount, in terms of weight and balance.  All those things only seem to make things worse, so I'm guessing they're talking about the best case?  I searched, and can't find such a spec for my AVX.  Probably not a surprise.

 

Then I'm curious how one can achieve any particular RMS with any particular guidescope and camera.  If indeed one is accurately measuring the sky to sufficient accuracy, sure, there's probably no benefit in measuring it more accurately.  But a 130mm focal length and a camera with, say, 3.75u pixels, has a pixel scale of 5.94 arc-seconds.  You'd be pushing the software to measure down to 0.5".  That said, my guider arrangement has a pixel scale of 4.3", and according to PHD2 I'm getting an RMS of 0.5" in Dec and 1.5" in RA, but I have no idea how one translates (predicts) one set of numbers to the other.  There's a lot of mechanical stuff in between. 

 

{shrug}

 

I just go with the rule of thumb and then optimize the stuff around it - balance, compact arrangement of accessories, etc.


  • tosjduenfs likes this


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