Guide scope 70mm F4

[Tommohawk]

Hi all. 

Ive been using an ST80 as a guide scope quite happily for some time but am trying to make things more compact for travel. I'll probably use a 300mm camera lens for imaging.

I dont want to reduce to 60, so what would be ideal is 70mm. The Orion multi looks perfect - except I cant be doing with the adjustment rings being so close together. Maybe I could use the supplied ring for the back, and use a larger ring around the front of the scope? But then the foot of the supplied ring doesnt fit flat on a vixen bar - maybe I'd have to use a ring which could adjust down to the smaller diameter.

So - any ideas on how I could use the Orion offering - or an alternative 70mm or 75mm F4?

[ChrisLX200]

You don't need adjustable rings, just bolt the thing down as tightly as you can. You will always get a suitable guidestar in any random FOV with an aperture that large.

ChrisH

[Thalestris24]

If you're imaging at 300mm focal length then a standard 50mm guidescope would be fine and easy to handle, or a 60mm one, if you must!

Louise

[SonnyE]

Maybe I look at things a bit simply, but my thoughts are that a guide scope is to get you close, then you zero in from there with the lens.

But maybe I'm doing things backwards.

[Tommohawk]

21 minutes ago, ChrisLX200 said:

You don't need adjustable rings, just bolt the thing down as tightly as you can. You will always get a suitable guidestar in any random FOV with an aperture that large.

ChrisH

Fair point re the FOV. But have u seen the mount rings on the Orion multi? They're so close together its hard to believe it would be stable.

 

22 minutes ago, Thalestris24 said:

If you're imaging at 300mm focal length then a standard 50mm guidescope would be fine and easy to handle, or a 60mm one, if you must!

Louise

Well sometimes I havent been spoiled for choice even with the 80mm - I guess I always go for a monster SNR, so maybe I could do with a 60mm. Just trying to play safe!

From a mounting point of view the camera lens has no foot holder thingy for a finder/guider, so I'll use a side by side dual mount which I already have. So a standard vixen mountable guidescope would be fine. I think theres a converter from finder mount to vixen, so maybe I'll get that.

26 minutes ago, SonnyE said:

Maybe I look at things a bit simply, but my thoughts are that a guide scope is to get you close, then you zero in from there with the lens.

But maybe I'm doing things backwards.

Yes but I'm using it for a guider rather than a finder so I need a decent bright image.

 

[SonnyE]

1 minute ago, Tommohawk said:

Fair point re the FOV. But have u seen the mount rings on the Orion multi? They're so close together its hard to believe it would be stable.

 

Well sometimes I havent been spoiled for choice even with the 80mm - I guess I always go for a monster SNR, so maybe I could do with a 60mm. Just trying to play safe!

From a mounting point of view the camera lens has no foot holder thingy for a finder/guider, so I'll use a side by side dual mount which I already have. So a standard vixen mountable guidescope would be fine. I think theres a converter from finder mount to vixen, so maybe I'll get that.

Yes but I'm using it for a guider rather than a finder so I need a decent bright image.

 

That's what I needed to hear. I wasn't sure if you meant for Autoguiding or guide scope use.

I use a 50 mm guide scope and can get very tight autoguiding from it. To my simplistic thinking, it only takes one star for my autoguider to grab and stay on.

A friend of mine also uses a 70mm Orion guide scope, but I have often wondered why. Another uses a 60mm on an 11" Ritchey-Chretien telescope.

Do you need a larger guide scope for a telescope over 1500 mm?

[Tommohawk]

Something odd going on with the quotes here - when I try and quote SonnyE I get my prior comment as a quote!!

Anyhow, 

<<

Do you need a larger guide scope for a telescope over 1500 mm?

>>

I wouldnt have thought the size of the guidescope relates to the EFL of the imaging scope, but Louise's comment below seems to suggest otherwise

<<

If you're imaging at 300mm focal length then a standard 50mm guidescope would be fine and easy to handle, or a 60mm one, if you must!

>>

Its a good point - does imaging scope affect choice of guide scope??

And a quick follow-up - I thought the ST80 is popular as a guide scope. How come, if a 50mm is all thats required? 

Edited November 21, 2016 by Tommohawk

[Tommohawk]

Sorry - slight bump. Anyone have an answer as to the popularity of ST80 as guide scope if 50mm is adequate?

And also does the imaging FL govern the choice of guidescope? Louise's reply seems to imply that, no?

On 11/21/2016 at 17:45, Thalestris24 said:

If you're imaging at 300mm focal length then a standard 50mm guidescope would be fine and easy to handle, or a 60mm one, if you must!

Louise

 

[SonnyE]

I'm still stuck on the one star lock on idea. In my admittedly limited experience, it seems my guiding locks on one star, then rides the night skies holding onto that one star until cloud, human, tree, or horizon alters it.

So I am having difficulty coming to terms with why I would need very many more stars to confuse the issue. ie: a different guide scope. To be fair, I am only using an 80mm telescope. So don't see a reason to consider something alike or larger than my imaging telescope.

I have lately noticed something during my alignment procedure. When doing my alignment, if I have my mount slew to an alignment star, then go away for a while, when I return minutes or an hour later, I have found my mount has been tracking that star in my absence. And I'm wondering why it does without having been set to guide. It appears as if the mount is guiding itself, and appears to be doing a pretty good job of it. I still continue at that point and finish my alignment, then go on with my evenings viewing/imaging. But have been becoming more and more curious about that.

But it wouldn't be the first time I was looking upon and idiot peering at me from a mirror....

 

A friend was imaging through a 12" Vixen SCT his son had given him. He felt the 50 mm (162 mm FL) guide scope was inadequate. He did offer this LINK in the discussion. I offered my simple-mindedness. His image was good, not up to his typical standards, but good. He shelved the Vixen telescope for now.

Edited November 26, 2016 by SonnyE
Add a link...

[graham56]

this my take on my mod to orion multi finder, mounted on astro tech  90mm guide rings

[graham56]

here,s the guide scope mounted on the telescope via skywatcher guide scope bracket ,this is given you an idea how you can modify the 70mm

orion multi finder

[Jessun]

There's a lot written on SGL about guiding, and the relation between guide scope and main scope. I can't remember the ins and outs of it of the top of my head but might be worth searching for.

The ST80 is popular as a first scope, so rather than let it collect dust as people upgrade to the next scope - one that's better for imaging - it makes for a decent guide scope so that's were you find many.

Remember though that as you increase the size of the guide scope the focal length goes up (for same f-ratio) so your guide camera displays a smaller FOV. This can be problematic as you get fewer starts to pick from.

A 50mm finder at f4 is often plenty enough.

Next trick is to keep things as solid as possible. As long as the guide scope points roughly where the imaging scope is aiming, you're good to go, so bolt it down if you can. Adjustable rings introduces six new problems - each screw! Keep whatever is on the saddle as compact as possible, think wind...

/Jesper

 

[Tommohawk]

On 11/28/2016 at 20:48, graham56 said:

here,s the guide scope mounted on the telescope via skywatcher guide scope bracket ,this is given you an idea how you can modify the 70mm

orion multi finder

 

Hey thanks for that and sorry for slow reply. Thats a nice mod - but is yours a 60mm? The 70mm has a conic case so make it tricky to mount in that way.

On 11/29/2016 at 22:34, Jessun said:

There's a lot written on SGL about guiding, and the relation between guide scope and main scope. I can't remember the ins and outs of it of the top of my head but might be worth searching for.

The ST80 is popular as a first scope, so rather than let it collect dust as people upgrade to the next scope - one that's better for imaging - it makes for a decent guide scope so that's were you find many.

Remember though that as you increase the size of the guide scope the focal length goes up (for same f-ratio) so your guide camera displays a smaller FOV. This can be problematic as you get fewer starts to pick from.

A 50mm finder at f4 is often plenty enough.

Next trick is to keep things as solid as possible. As long as the guide scope points roughly where the imaging scope is aiming, you're good to go, so bolt it down if you can. Adjustable rings introduces six new problems - each screw! Keep whatever is on the saddle as compact as possible, think wind...

/Jesper

 

I guess a number of folk do end up with the ST80 as a spare having upgraded as you say.

I'm trying to avoid having too many bits of kit - ultimately I want to guide my C9.25 and if I can have one guide scope for both that would be good. I thought the 50mm would be too small and mismatched EFL for the C9.25 - though maybe even the 70mm is too short. Not sure about this and hoping for some - terrible pun alert -   guidance.

On 11/26/2016 at 16:12, SonnyE said:

I'm still stuck on the one star lock on idea. In my admittedly limited experience, it seems my guiding locks on one star, then rides the night skies holding onto that one star until cloud, human, tree, or horizon alters it.

So I am having difficulty coming to terms with why I would need very many more stars to confuse the issue. ie: a different guide scope. To be fair, I am only using an 80mm telescope. So don't see a reason to consider something alike or larger than my imaging telescope.

I have lately noticed something during my alignment procedure. When doing my alignment, if I have my mount slew to an alignment star, then go away for a while, when I return minutes or an hour later, I have found my mount has been tracking that star in my absence. And I'm wondering why it does without having been set to guide. It appears as if the mount is guiding itself, and appears to be doing a pretty good job of it. I still continue at that point and finish my alignment, then go on with my evenings viewing/imaging. But have been becoming more and more curious about that.

But it wouldn't be the first time I was looking upon and idiot peering at me from a mirror....

 

A friend was imaging through a 12" Vixen SCT his son had given him. He felt the 50 mm (162 mm FL) guide scope was inadequate. He did offer this LINK in the discussion. I offered my simple-mindedness. His image was good, not up to his typical standards, but good. He shelved the Vixen telescope for now.

Thanks for the link - very interesting and seems to suggest that the convention for guide scope EFL may be a bit off. This certanly helps my cause if I  want a 70mm F4 for all purposes.

Re your alignment procedure, when you slew to a star I imagine the tracking has to continue even if you go away for a while, so when you return if your polar alignment is about right your alignment star should be exactly as it was. 

[SonnyE]

Quote

Thanks for the link - very interesting and seems to suggest that the convention for guide scope EFL may be a bit off. This certanly helps my cause if I  want a 70mm F4 for all purposes.

Re your alignment procedure, when you slew to a star I imagine the tracking has to continue even if you go away for a while, so when you return if your polar alignment is about right your alignment star should be exactly as it was. 

You're welcome, Tom. It would seem that if one was using a telescope with a 3000 mm FL, it would call for a 1000 mm FL guide scope.

Since I have my main telescope (a whopping 80 mm), and my guide scope very closely aligned, I use PHD2's window with its crosshairs as my eyepiece for alignment. My entire visual is based on cameras to my laptop. So when it says Target Acquired, if I have to wonder in the house for anything, when I return the star is still in the PHD2's window.

I find that intriguing because for my use if Stellarium and PHD2 were incorporated into each other it would be ideal for me.

[graham56]

Yes thommohawk and sorry for late reply that is a orion multi finder the one that is conical ie the 70 mm I replaced the conical body with a strait one  ie to sit perfectly in the guide rings,it's a 80mm cap with 40mm centre hole with powder coated rolled white aluminium, sheet metal worker by trade

[Tommohawk]

9 minutes ago, graham56 said:

Yes thommohawk and sorry for late reply that is a orion multi finder the one that is conical ie the 70 mm I replaced the conical body with a strait one  ie to sit perfectly in the guide rings,it's a 80mm cap with 40mm centre hole with powder coated rolled white aluminium, sheet metal worker by trade

Nice job! The conical body looks pretty but it does make secure mounting more tricky - I think the design is flawed. I just wish I could source a basic 70mm F4 somewhere else.