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

Focal length guidescope vs telescope, rule of thumb?

  • Please log in to reply
13 replies to this topic

#1 Widotje

Widotje

    Vostok 1

  • -----
  • topic starter
  • Posts: 120
  • Joined: 26 Jan 2016
  • Loc: Utrecht

Posted 25 February 2021 - 06:55 AM

Hi folks,

 

As stated in the title, my main question is whether there is a rule of thumb between the focal length of your main telescope and the focal length of your guidescope. For arguments sake, let's say I'm using two similar cameras with the same pixel size. I'm happy to hear your thoughts. An additional question I have is if anybody would like to share his/her experiences with off axis guiding versus putting an additional guidescope on your mount.

 

Happy to hear your thoughts!

 



#2 Sacred Heart

Sacred Heart

    Viking 1

  • *****
  • Posts: 901
  • Joined: 16 Aug 2020

Posted 25 February 2021 - 08:22 AM

  I'm sure there is a rule to go by, do not know what it is exactly.  

 

   Bottom line,  you want the guide scope to detect movement and correct before the main imaging scope sees it.  Whatever that works out to be probably varies on size of your imaging train.

 

I cannot say for sure, but if you can image say 2 minutes unguided without star trails, and your guide scope does a correction every 30 - 60 seconds you should be OK. What I mean by correction is the whole process. expose, download, make corrective movement within - and I would think the sooner the better but do not what to over correct either - those 30 to 60 seconds.

 

    Easier said than done and in the end,  when it is all said and done - there was more said than done.

 

       Joe


  • Widotje, c2m2t and Sandy Swede like this

#3 bokemon

bokemon

    Messenger

  • *****
  • Posts: 413
  • Joined: 28 Jul 2020
  • Loc: Silicon Valley, California

Posted 25 February 2021 - 08:35 AM

Yes, the rule of thumb is that you can use a guide scope with 3x less focal length than your main scope.  This is with PHD2 as the guiding program, because it knows how to average several pixels to obtain an averaged center position.   It probably got even better now that it has multi star guiding.


  • Widotje likes this

#4 AhBok

AhBok

    Mercury-Atlas

  • *****
  • Posts: 2,974
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 25 February 2021 - 10:23 AM

The focal length ratio was common when people guided manually. With auto guiding and sub-pixel guiding, the ratio of image scale is far more important. You must take into account the cameras used for guiding and imaging. Compute the image scale off your guider and imager at their respective focal lengths and make sure the ratio does not exceed about 5:1.
  • Widotje likes this

#5 sg6

sg6

    Voyager 1

  • *****
  • Posts: 10,179
  • Joined: 14 Feb 2010
  • Loc: Norfolk, UK.

Posted 25 February 2021 - 10:53 AM

Don't know if it is a general opinion or just that a small guide scope looks more "correct" on the imaging scope. I know someone that had an 80mm and 100mm ED refractors. After some trials they determined the 80 produced the better images, so they imaged with the 80 and guided with the 100.

 

And yes the final images were good.

 

I suppose there is an arguement that they should be the same, then the guide star movement matches the imaging target movement. Just an odd thought that literally crept into my head.


  • Widotje likes this

#6 AhBok

AhBok

    Mercury-Atlas

  • *****
  • Posts: 2,974
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 25 February 2021 - 11:13 AM

I’ve seen lots of guys with refractors mounted on their SCTs. When imaging, the SCT is the guide scope for the imaging refractor.
  • Widotje likes this

#7 Widotje

Widotje

    Vostok 1

  • -----
  • topic starter
  • Posts: 120
  • Joined: 26 Jan 2016
  • Loc: Utrecht

Posted 01 March 2021 - 04:43 PM

The focal length ratio was common when people guided manually. With auto guiding and sub-pixel guiding, the ratio of image scale is far more important. You must take into account the cameras used for guiding and imaging. Compute the image scale off your guider and imager at their respective focal lengths and make sure the ratio does not exceed about 5:1.

Thanks, very useful! Hope I understand this correctly for my gear, could you confirm this is correct?:

 

I currently have a the 50mm orion guidescope, it has a focal length of 162mm. In combination with the asi120MM my I got 4.77" per pixel.

My first scope is an 80/480mm  APO, I used it with the asi1600MM pro, this combi gives me 1.65" per pixel - so that would be less then 1:5. Is that interpretation correct?

I also have an Edge HD 8", with a reducer, the focal length is about 1440mm, with the asi1600mm pro, I'm getting .55" per pixel. So now I'm getting 1:8.6, so the difference here is more than 1:5 and I should consider a guide scope with a longer focal length, that will get me to about 2.5" per pixel...or a camera change...(but the asi120 has a pretty small pixel size already). So my new guidescope should have a focal length of about 310mm...

 

I'm struggling with this...a bigger guidescope also means adding more weight to the rig...I have an EQ6R-pro so I should be fine....

 

Cheers!



#8 Rasfahan

Rasfahan

    Viking 1

  • -----
  • Posts: 504
  • Joined: 12 May 2020
  • Loc: Hessen, Germany

Posted 01 March 2021 - 05:13 PM

That sounds about right. But for the EdgeHD you will certainly want an OAG instead of a guidescope. The ASI120mm will not work terribly well on that but will suffice for starters (from my experience at 1600mm). Dont be afraid of ugly stars in the OAG.


  • Widotje likes this

#9 AhBok

AhBok

    Mercury-Atlas

  • *****
  • Posts: 2,974
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 01 March 2021 - 05:14 PM

Your math seems correct. I have an EQ6R Pro. I mount my 6” RC on it with my 72mm Apo on top of it. I use a 30mm guide scope on the Apo. When using the Apo, I guide with the 30mm guide scope. When using the RC, I guide with the Apo. My mount handles the load with no difficulty.

That said, for the Edge, you might really want to consider something like the ZWO OAG. You can then use the ASI120, OAG combo for imaging with your 80mm as well. I know that is what I would do.
  • Widotje likes this

#10 Widotje

Widotje

    Vostok 1

  • -----
  • topic starter
  • Posts: 120
  • Joined: 26 Jan 2016
  • Loc: Utrecht

Posted 02 March 2021 - 05:16 PM

Your math seems correct. I have an EQ6R Pro. I mount my 6” RC on it with my 72mm Apo on top of it. I use a 30mm guide scope on the Apo. When using the Apo, I guide with the 30mm guide scope. When using the RC, I guide with the Apo. My mount handles the load with no difficulty.

That said, for the Edge, you might really want to consider something like the ZWO OAG. You can then use the ASI120, OAG combo for imaging with your 80mm as well. I know that is what I would do.

OK, so you don't experience any difficulties in balancing the mount with two scopes on top of each other? That gives me hope...Ah, if I'd have an APO and the SCT, I'm sure I couldn't resist trying to image with both scopes at once, and still use the smaller guidescope :p.  



#11 AhBok

AhBok

    Mercury-Atlas

  • *****
  • Posts: 2,974
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 02 March 2021 - 07:10 PM

I had to add another counterweight, but my mount handles up to about 36 lbs with no difficulty and I get excellent guiding (.6 arcsec typical and .38-.50 when seeing is very good). Also, you can guide an SCT with a guide scope as long as you anticipate that from about +\- 5 degrees of zenith, you will experience mirror flop. This is one reason many prefer an OAG for SCTs. I was, however, successful imaging with a C9.25 by stopping my imaging run a half hour before the zenith and resuming about a half hour after the meridian flip. The downside is losing an hours worth of prime imaging time. But—give it a try. Yours may not be as bad and you will learn a lot dealing with it!

 

 

Attached Thumbnails

  • 3C667D4D-09DF-413D-A258-332727FA4F38.jpeg


#12 AhBok

AhBok

    Mercury-Atlas

  • *****
  • Posts: 2,974
  • Joined: 02 Dec 2010
  • Loc: Lakeland, TN

Posted 02 March 2021 - 07:27 PM

I should add that balancing RA only involves the amount and position of the counterweights. Balancing DEC only involves positioning the refractor by sliding it along an accessory rail mounted on top of the larger OTA. The weight of the 30mm guide scope is minuscule so does not require anything special. Because of its feather light weight, flexure is no issue and a finder mount works perfectly fine.

#13 pweiler

pweiler

    Vostok 1

  • *****
  • Posts: 107
  • Joined: 20 Oct 2020
  • Loc: Chicago, IL, USA

Posted 02 March 2021 - 08:52 PM

OK, so you don't experience any difficulties in balancing the mount with two scopes on top of each other? That gives me hope...Ah, if I'd have an APO and the SCT, I'm sure I couldn't resist trying to image with both scopes at once, and still use the smaller guidescope :p.


The guide scope can help balance the primary scope especially with AP or Heavy EPs on the back of the primary/larger scope.

#14 SonnyE

SonnyE

    Mercury-Atlas

  • -----
  • Posts: 2,984
  • Joined: 01 Nov 2015
  • Loc: Cali for ni a

Posted 02 March 2021 - 11:27 PM

It seems to me when I was wrestling with this I came across a 5 to 1 ratio.

Simply put, if the main scope was 500 mm, the guide scope to be 100 mm in focal lengths.

 

In reality, what I use is a 480 mm FL main telescope, and 162 mm FL guide scope.

Or 2.962 to 1 ratio.

 

An 80mm Main Refractor, 50 mm Guide Scope.

 

And it works good for me, for DSO Nebula imaging.




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