From oag to guidescope

[Robindonne]

Ok i have to admit you were all right about the benefits of a guidescope over an oag.   Especially when using on an un-comacorrected newton with an asi120 as guidecam.   Ive started to swap my setup and ended with a big mountingplate and 125 mm guiderings on the newton.  

Lets say weight is no problem, should i use a small (71mm) refractor(already have it), or make a guidescope of my 9x50 finderscope(have one extra for finderuse), or go for a 60 mm guidescope like an altair or some sort of?  

If my oag would have worked with a better camera and comacorrected view, then the finderscope-update would be enough i guess?   Just dont want to spend €40,- for an 9x50 to t-mount adapter when the result is bad enough to, in the end, buy a €100,- guidescope that is build for that job.  

If the rebuild finderscope is adviced, it will end up in the guiderings.  So no problems with the standard “2 screws/1 spring” housing.  
 

its all finding its final destination on a eq6r. If a cem70 is not spoiling my skywatcher sale.  

Edited June 2, 2020 by Robindonne

[uhb1966]

Hi Robin,

I'm no pro but i can tell you the following: i am using an 9x50 guidescope with an orion SSAG and i have never had any problem in finding a suitable guide star for PHD2. I do believe that that both the 9x50 and the 71mm should be absolutely fine - no need to splash out extra money IMHO. One small disadvantage in not using an OAG is that if your guide star is too far away form the point your primary is pointing at, and your imaging close to the pole, there might be considerable field rotation. So make sure your finderscope points more or less at the same point as your primary

 

[Freddie]

I can’t imagine why everyone would suggest a guide scope would be better than an OAG on a Newt.

[Robindonne]

25 minutes ago, Freddie said:

I can’t imagine why everyone would suggest a guide scope would be better than an OAG on a Newt.

No your right.  From all the cons that come with an oag, the reflectors were kind of 50/50.  Due to mirror movements it would be a good choise to go for an oag. But overall the oag would be second choise. Thats what the advices told me.   And despite all the advices i bought an oag and found out.   But i use the cheapest cam, no coma correction and have 38 years of inexperience in me😬.  
 

as an amateur-amateur i will probably have more fun and useful hours when swapping the oag for a guidescope when i use the newton.   When that setup is running i can spend some time playing with the oag on a c8 for example.   Only thing i dont know for sure is/was to make my finder a guider or go for seperate guidescope.   Or even use a small zenithstar for guiding that newton

Edited June 2, 2020 by Robindonne

[Robindonne]

1 hour ago, uhb1966 said:

Hi Robin,

I'm no pro but i can tell you the following: i am using an 9x50 guidescope with an orion SSAG and i have never had any problem in finding a suitable guide star for PHD2. I do believe that that both the 9x50 and the 71mm should be absolutely fine - no need to splash out extra money IMHO. One small disadvantage in not using an OAG is that if your guide star is too far away form the point your primary is pointing at, and your imaging close to the pole, there might be considerable field rotation. So make sure your finderscope points more or less at the same point as your primary

 

Thx.   Thats what i was hoping for

[wimvb]

As for which guidescope to use:

1. The guidescope must be firmly connected (bolted) to the main scope. Otherwise you will have an additional source of flexure. 

2. The pixel scale of your guide solution shouldn't differ too much from your imaging solution. You should aim for achieving a guide rms that's about half your imaging pixel scale (arcsecs/pixel). 

3. More aperture means more light on the guide cam, so easier to find a star, and better signal to noise ratio. 

If you already have a scope you can use, why not start with that?

[Robindonne]

16 hours ago, wimvb said:

As for which guidescope to use:

1. The guidescope must be firmly connected (bolted) to the main scope. Otherwise you will have an additional source of flexure. 

2. The pixel scale of your guide solution shouldn't differ too much from your imaging solution. You should aim for achieving a guide rms that's about half your imaging pixel scale (arcsecs/pixel). 

3. More aperture means more light on the guide cam, so easier to find a star, and better signal to noise ratio. 

If you already have a scope you can use, why not start with that?

Hm ok.   Thats something i wasnt aware of.  Its probably not explained in two sentences so i better google for the way of calculating the pixel scale.
Im using a dslr.   And from what I remember it says imaging in the somewhere region of 5000x3500 pixels.   With an 8” newtonian.  
My asi120 has a resolution of 1280x960.  When i compare resolution then i would end up using a 50 mm guidescope with the asi120.  But thats only based on the diameter of the newton.  I dont know if fov is involved in this calculation. If not then Im gonna try guiding with that 71 mm refractor.  

[SimM]

You might try calculating the ratios of focal length and resolution. There is a formula for it.

Or you could put the details into a calculator and compare the arc seconds result for the scope and the guide scope:

https://astronomy.tools/calculators/ccd

With an OAG, your focal lengths are equal and there are no flexure issues. So you only have to compare the resolution (pixels) of the two cameras and the answer is usually about 1:1.

A good result for guiding with a camera is supposed to be 1:3 or 1:5 but 1:1 is even better.

The size of aperture e.g. a 50m or 60mm scope should make a difference but probably won't e.g. use what you have.

Having a slightly longer scope can improve things but 200mm vs 240mm isn't going to make too much of a difference.

The problems start when you do the calculations and then expect a 1:10 difference to also work - but it might (just) be OK.

Off-topic, sorry. I'm looking to use an OAG with an EdgeHD 800 and a 0.7x Reducer - but finding an OAG that will work is proving difficult.

I haven't quite given up on the idea of using a guide camera and a scope e.g. a S/H ED 80 or a cheap achromat - what would work best?

Simon

 

[wimvb]

29 minutes ago, Robindonne said:

Its probably not explained in two sentences

Pixel scale = 206 * pixel size (micrometer) / focal length (mm) arcseconds/pixel. 

Eg 5 um pixels, 1000 mm fl:

Pixel scale = 206*5/1000 =1.03 "/pixel

Edited June 3, 2020 by wimvb

[SyedT]

30 minutes ago, Robindonne said:

Hm ok.   Thats something i wasnt aware of.  Its probably not explained in two sentences so i better google for the way of calculating the pixel scale.
Im using a dslr.   And from what I remember it says imaging in the somewhere region of 5000x3500 pixels.   With an 8” newtonian.  
My asi120 has a resolution of 1280x960.  When i compare resolution then i would end up using a 50 mm guidescope with the asi120.  But thats only based on the diameter of the newton.  I dont know if fov is involved in this calculation. If not then Im gonna try guiding with that 71 mm refractor.  

There's a calculator here: http://astronomy.tools/calculators/guidescope_suitability

Normally people say the ratio should be 1:6 or less between the imaging and guiding scope but I've heard of people succeeding with guiding well beyond these parameters, so I suspect there's a lot to do with your individual setup/seeing conditions etc.

[wimvb]

Phd has an accuracy measured in the tenths of a pixel. But how accurate can the mount move? Besides that, even with longer exposures, it's often the seeing that limits guiding. A larger aperture guide scope will gather more light and give you better/more stars to guide on. With my oag, I can always find a star, but very often that star is so dim, I seem to be guiding on noise. That's with an ASI120MM-S. Hopefully, my newly ordered ASI290 will be more sensitive. I have had good results with a skywatcher ST80 scope and asi120 camera, fixed to a top dovetail on my scope. Now with a longer focal kength reflector, I'm more concerned about mirror movement (flexure), hence the oag. 

[tooth_dr]

2 hours ago, wimvb said:

That's with an ASI120MM-S. Hopefully, my newly ordered ASI290 will be more sensitive.

I just switched from a 120MM to a 290MM for my OAG. The difference is wow! I regularly have my gain down to 40% or less to avoid oversaturating the brighter stars to guide on. The 120MM always like it needed 110% gain, just never quite enough. 

[Robindonne]

3 hours ago, wimvb said:

Pixel scale = 206 * pixel size (micrometer) / focal length (mm) arcseconds/pixel. 

Eg 5 um pixels, 1000 mm fl:

Pixel scale = 206*5/1000 =1.03 "/pixel

You see?! 4!!

[Robindonne]

Thx for all the reply’s.  What i read between the lines is: get an as good as possible guidecam for using an oag, and a more medium cam will work with a more open guider(50mm-80mm).  And second use the oag for the newton.   So another asi order is inevitable 🥳🇨🇳.  I had a related question but I forgot.  Thx so far.  Ill go check the calculators the next two days at least😞

[wimvb]

1 minute ago, Robindonne said:

And second use the oag for the newton.

There is an "imaging with the sw 130pds" thread, with plenty of users that have a guidescope or a finder guider. The 130pds has a focal length of 650 mm. For 1000 mm fl, an oag is probably better. Otoh, seeing in large parts of Europe can be so bad that it hides less than optimal guiding.

Other things to consider:

A large guide scope on top of your imaging scope increases the load on your mount. It can make balancing trickier. 

An oag is a heavier load on your focuser. May cause focuser to sag.

A finder guider collects less light, but has a large field of view. Easy to find stars. But you have to find a way to attach it to your scope. The finder shoe is not up to that task, imo. It can also be harder to focus.

[knobby]

2 hours ago, tooth_dr said:

I just switched from a 120MM to a 290MM for my OAG. The difference is wow! I regularly have my gain down to 40% or less to avoid oversaturating the brighter stars to guide on. The 120MM always like it needed 110% gain, just never quite enough. 

Is the 290 really that much better ? Interesting...

120 qe - 75% 290 qe 'about 80%'

Edited June 3, 2020 by knobby

[uhb1966]

4 hours ago, wimvb said:

Phd has an accuracy measured in the tenths of a pixel. But how accurate can the mount move? Besides that, even with longer exposures, it's often the seeing that limits guiding. A larger aperture guide scope will gather more light and give you better/more stars to guide on. With my oag, I can always find a star, but very often that star is so dim, I seem to be guiding on noise. That's with an ASI120MM-S. Hopefully, my newly ordered ASI290 will be more sensitive. I have had good results with a skywatcher ST80 scope and asi120 camera, fixed to a top dovetail on my scope. Now with a longer focal kength reflector, I'm more concerned about mirror movement (flexure), hence the oag. 

I share Wim's view, it's usually seeing that limits guiding (and imaging ). FWIW i have calculated my ratio and it's 1:3 - and i do believe i'm limited by seeing, my mount's precision and, last but not least, my skillset 😬

Edited June 3, 2020 by uhb1966
typo

[tooth_dr]

11 hours ago, knobby said:

Is the 290 really that much better ? Interesting...

120 qe - 75% 290 qe 'about 80%'

Yes, in my practical side by side experience it’s a lot better. I didnt make it up!

Edited June 4, 2020 by tooth_dr

[wimvb]

12 hours ago, knobby said:

Is the 290 really that much better ? Interesting...

120 qe - 75% 290 qe 'about 80%'

It's not just quantum efficiency, but also sensitivity. A photon generates an electron (+ hole) = qe

The electron creates a voltage on a "capacitor" = sensitivity. The larger that voltage, the more sensitive the camera. 

[knobby]

2 hours ago, tooth_dr said:

Yes, in my practical side by side experience it’s a lot better. I didnt make it up!

Cheers Tooth_dr, reaaly didn't think you'd made it up 🙂 just good to see real world experience vs figures, of course now I need to update my Gpcam mono to a 290 🙂 

[tooth_dr]

9 minutes ago, knobby said:

Cheers Tooth_dr, reaaly didn't think you'd made it up 🙂 just good to see real world experience vs figures, of course now I need to update my Gpcam mono to a 290 🙂 

The 290MM is expensive, and on paper it is hard to justify based on figures alone.  But in reality it works really well as an OAG.  I also use it for solar imaging, so I felt more justified in the cost.

[iapa]

Agreed, hard to justify on paper, but, a bonus from work last year meant I had budget - so, I've been guiding with the ASI174MM, "that like proper sensitive man" to use the vernacular , or however the kids speak these days 

• pier mounted CGEM-DX,
• C8, SCT + 0.63 reducer/flattener, ASI1600, EFW LRGBHa
• f5 StarTravel 80, ASI174

guiding was about 1.1" RMS

 

[Starwiz]

On 02/06/2020 at 20:38, Freddie said:

I can’t imagine why everyone would suggest a guide scope would be better than an OAG on a Newt.

I agree.  I went from guide scope to OAG on my Newt in April.  Before that I used to throw away loads of frames and now it's hardly any.

I'm not using the ideal guide camera either (ASI120mc), but it works.  The stars are elogated due to the coma, but PHD2 handles it fine and my tracking is much better than it ever was with a guide scope.

John

[alacant]

On 02/06/2020 at 19:46, Robindonne said:

benefits of a guidescope

Hi

Ok, if you're going with the guide scope, maybe best to fit stronger primary mirror springs and set the mirror in its cell with silicone sealant [1]. As long a dovetail rail as you can and with the top of the rings tied with either a second rail or rigid box section aluminium.

HTH

[1] a blob at each point of contact then allowing the mirror to settle under its own mass whilst the silicone cures.

[iapa]

1 hour ago, iapa said:

Agreed, hard to justify on paper, but, a bonus from work last year meant I had budget - so, I've been guiding with the ASI174MM, "that like proper sensitive man" to use the vernacular , or however the kids speak these days 

• pier mounted CGEM-DX,
• C8, SCT + 0.63 reducer/flattener, ASI1600, EFW LRGBHa
• f5 StarTravel 80, ASI174

guiding was about 1.1" RMS

 

as an FYI, the Dalèkъ Obs UK is undergoing a regeneration, and this SCT will be fitted with an OAG for comparison purposes over the next few months

 

 

 

Edited June 4, 2020 by iapa