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Autoguider and Guide Scope


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Hi 

Can I ask about a autoguider. I have a skywatcher 150 pds 750 focal length. Is there anything I need to know for a autoguider when it comes to focal length of the telescope? For example could I use a Orion 50mm mini with mine? Or do I need a larger guide scope. Just don't want to spend money and buy the wrong one! 

Thanks! 

Gerry

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Guest Tuomo

I think rule of a thumb is something like: guide scope focal lenght should be about third of imaging scope FL. I think guide scope FL is about 160mm...

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I initially used an Orion 50mm (162mm focal len) guide scope on my 200P-DS 1000mm focal length

Saw somewhere suggesting ratio of  focal lengths of the main tube to guide should be around 3:1 or thereabouts.

I now use a StarTraveller 80mm F5 (400mm fl) for guide scope.

 

 

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9 minutes ago, iapa said:

I initially used an Orion 50mm (162mm focal len) guide scope on my 200P-DS 1000mm focal length

Saw somewhere suggesting ratio of  focal lengths of the main tube to guide should be around 3:1 or thereabouts.

I now use a StarTraveller 80mm F5 (400mm fl) for guide scope.

 

 

Do you notice much difference in the guiding accuracy? I'm thinking about upgrading my guide scope but since I image with a short tube frac (star 71) I'm unsure I'll notice any improvement 

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54 minutes ago, Tuomo said:

I think rule of a thumb is something like: guide scope focal lenght should be about third of imaging scope FL. I think guide scope FL is about 160mm...

So that being the case then the Orion is not ideal. So I'm looking for something around 250mm FL then? For the camera? Any suggestions? What works well for people?

 

Thanks

Gerry

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Just found another note I have

the ratio of pixel scale for each camera/tube < 4

the pixel scale is

pixel size (in micrometers)/focal length (mm) * 206.3

so, in my case using the Orion with a QHY6,and a canon 600d with the 200P-DS

6.5/162*206.3 = 8.2 arcsec / pixel for guider

4.3/900*206.3 = 0.99

ratio come out as 8.4 so, not to good.

Replace the Orion with the Startravel 80 and the ration come out to 3.4

or, replace the QHY6 with a ToupTek Mono (1280*960, 3.75mm pixel size) the ration is 4.84

 

If we use the Canon 600d, on your 150P-DS, and the Orion with the ToupTek camera

the ration is 3.63, which is likely to be manageable.

You've not mentioned which cameras you are using, or planning to use.

I've a spreadsheet I set up to a do a whole range of cal cautions for different gear - worth the effort.

Sorry,I keep scraps, my handwriting is abysmal, so rarely note URLs etc.

Edited by iapa
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Guest Tuomo
22 minutes ago, Gerry Casa Christiana said:

So that being the case then the Orion is not ideal. So I'm looking for something around 250mm FL then? For the camera? Any suggestions? What works well for people?

 

Thanks

Gerry

Imo you are fine with mini guide scope. Check it and update if needed. Many guide with finder/guider scope. I think modern guide cams works fine with smaller scopes

 

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  • 3 weeks later...
On 25/03/2017 at 11:59, iapa said:

Just found another note I have

the ratio of pixel scale for each camera/tube < 4

the pixel scale is

pixel size (in micrometers)/focal length (mm) * 206.3

so, in my case using the Orion with a QHY6,and a canon 600d with the 200P-DS

6.5/162*206.3 = 8.2 arcsec / pixel for guider

4.3/900*206.3 = 0.99

ratio come out as 8.4 so, not to good.

Replace the Orion with the Startravel 80 and the ration come out to 3.4

or, replace the QHY6 with a ToupTek Mono (1280*960, 3.75mm pixel size) the ration is 4.84

 

If we use the Canon 600d, on your 150P-DS, and the Orion with the ToupTek camera

the ration is 3.63, which is likely to be manageable.

You've not mentioned which cameras you are using, or planning to use.

I've a spreadsheet I set up to a do a whole range of cal cautions for different gear - worth the effort.

Sorry,I keep scraps, my handwriting is abysmal, so rarely note URLs etc.

Hi I still haven't bought a autoguider! I have a Skywatcher 150pds and Canon 550d. So I want the best option for this really. You lost me on the technical stuff but if you could explain I'll work it out. 

Cheers

 

Gerry

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The ratio based simply on focal length is a rough starting point. However, a guiding setup works by monitoring pixels on the guide camera and will try and guide to this. The movement between the guide camera's and the imaging camera's images is based on focal length and pixel size - this is measured in arcseconds per pixel (aps).

Your imaging scope has a focal length of 750mm and the imaging camera a pixel size of 4.3um.

aps_imaging = pixelsize * 206 / focal_length = 4.3 * 206 / 750 = 1.18 arcseconds per pixel (each pixel of the imaging camera covers 1.18 arcseconds of the sky)

An Orion mini guide scope has a focal length of 162mm. Although the guide camera has not been specified, assuming a QHY5-II Mono with a 3.75um pixel size :

aps_guiding = 3.75 * 206 / 162 = 4.77 arcseconds per pixel

For good guiding, a maximim ratio of around 4:1 between aps_guiding and aps_imaging is often suggested. This is based on modern guiding software being able to guide to sub-pixel accuracy.

The above guiding/imaging setup gives :

aps_guiding / aps_imaging = 4.77 / 1.18 = 4.04

So the above system is at the bounds of the typical match between the guiding and imaging parts of the setup and should be ok to go ahead with (other factors such as mount stability and seeing can make for much bigger variation).

HTH.

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I'd say a 80mm guide scope will do the job very well..and 50mm might be too small ..so somewhere between 60-80mm with be fine..i will be using a 80mm on my sct.. .but the same on my 80ed might be too much...consideration should be given to the mounts capabilities and the resolution of the guide camera..

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I'm in a similar quandary. For my setup the pixel ratios guider:imager come out as follows:

Orion 50mm: 3.7

Skywatcher ST80 1.5 (both rounded up)

How important are these factors compared with:

- risk of flexure (ST80 on the 150PDS rings vs Orion on a finder post on the rings)

- added weight (ST80 is heavier)

- aperture / guide star brightness (80mm aperture @F5 vs 50mm aperture @F3.2)

 

I'm leaning towards the ST80 but the Orion would be lighter and a bit cheaper. Another option may be the Celestron 70mm travel scope, which is a bit cheaper again.

Lastly, does a bit more weight actually add to stability, provided the mount isn't overpowered?

Questions, questions.

Or, just pick any one of them and see how it goes?!

 

 

Edited by Yearofthegoat
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9 hours ago, Yearofthegoat said:

Questions, questions.

Yes, there are many questions when deciding on a guiding setup! Each setup is different, so that means there isn't a single correct answer. Anyway, possible responses as follows:

Pixel ratios guider:imager - Orion suggest their 50mm guide scope is good for scopes up to 1500mm focal length. Of course that doesn't mean a lot on its own, therefore the pixel ratios make for a good comparison. With a very good guiding setup, a ratio of 10:1 could be possible, with 4:1 being conservative. So both of your suggested image scopes look ok.

Flexure - haven't noticed this to be  problem (I'm imaging with the Orion 50mm at 650mm fl), though my EQ2 is way below any required stability requirements for imaging. Assuming you are imaging up to 750mm focal length and everything is well tightened, flexure shouldn't be too big an issue.

Added weight - best to keep weight down so that the mount is in control and not the scope.

Aperture/guide star brightness - depends on the guiding camera. in my case a simple webcam, so a low f ratio means more/brighter stars. If a purpose made CCD camera is used then not so important.

Weight & stability - the mount is trying to keep the scope/guide camera under control, so best to keep scope/guide camera weight down.

With a good solid mount, such as an EQ5, that will track well (assuming appropriate polar alignment) for many minutes, a guiding system only need gently nudge the mount in the correct direction from time to time. Mount periodic error is perhaps the real issue that guiding is trying to deal with as PE can suddenly appear from nowhere and needs to be corrected quickly. 

It would be great to see an 'equation' covering the many parameters of a guiding setup. A PhD thesis perhaps?

Would be good to hear a few more responses on this often discussed topic!

Edited by bobro
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  • 3 weeks later...

Well I've just had a nightmare with Altair Astro just trying to buy one of their guide scopes. 2 weeks nothing! Very disappointed I cancelled the order today so now I need to look for somewhere else to get a decent guide scope. I'm also going to be involved in a observatory build. They lost out there. 

 

So guys and girls. What can I do now? 

Just so you know what I've got. Skywatcher 150pds which is 750 focal length on a Eq6.  I already bought the ASI 120mm so I just need a guide scope!

Shame FLO don't have other options. 

Any ideas will be gratefully received. Maybe they did me a favour if I can get something better. 

Gutted. 

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Hi!

Have you looked at one of these:

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p5632_TS-Optics-50mm-f-6-6-APO-Doublet-Refractor---Day-Night-Scope-and-Tele-Objective.html

I've purchased from Teleskop-Express in the past and delivery is typically five days.

I've been considering upgrading my SW finder/guider on my ED80DS Pro ; AA scopes seem to be constantly out of stock/awaiting delivery. All very frustrating!

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Musing here.... Bare with me....

What is a guide scopes function? To find and lock onto a single star, correct? And to draw the guiding from that single star. Right?

So what does using a larger (and more expensive) telescope for guiding actually gain? More stars?

Trying to figure why a guide scope needs to be ~1/3 the main telescopes FL.

That said, my 50 mm is admittedly large for guiding considering my main is an 80 mm, but it works fab.

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It's useful for those who want to do Astrophotography  and especially long exposures as usually the mounts themselves do track but depending on the mounts accuracy you might get 2 minute exposures like on mine. With the autoguider you add it so the accuracy of the mount can be kept perfectly centred on your star. So it makes small adjustments for you. 

The 1:3 focal length is no longer necessary as computers have come a long way and that is now out of date. 

So a small scope with a mono webcam is sufficient. Very basically speaking! 

Kind regards

 

Gerry

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17 hours ago, Gerry Casa Christiana said:

It's useful for those who want to do Astrophotography  and especially long exposures as usually the mounts themselves do track but depending on the mounts accuracy you might get 2 minute exposures like on mine. With the autoguider you add it so the accuracy of the mount can be kept perfectly centred on your star. So it makes small adjustments for you. 

The 1:3 focal length is no longer necessary as computers have come a long way and that is now out of date. 

So a small scope with a mono webcam is sufficient. Very basically speaking! 

Kind regards

 

Gerry

Hi Gerry,

Yep, exactly why going in I planned an auto guide into my equipment. To give the mount an eye on the sky, if you will.

At first I tried to do it 'my way' using an off axis guider. That, of course, doesn't work with a small refractor. I finally learned the hard way OAG's are for SCT's. So I relented and got the 50mm and used my already owned guide scope camera.

Eureka! That worked!

Later, I encountered this 1:3 focal length palaver.

But it makes no sense, do to the one star guiding these guide scopes actually do.

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On 04/05/2017 at 03:15, SonnyE said:

Musing here.... Bare with me....

What is a guide scopes function? To find and lock onto a single star, correct? And to draw the guiding from that single star. Right?

So what does using a larger (and more expensive) telescope for guiding actually gain? More stars?

Trying to figure why a guide scope needs to be ~1/3 the main telescopes FL.

That said, my 50 mm is admittedly large for guiding considering my main is an 80 mm, but it works fab.

Bobro's earlier post explaining the relationship between pixel scale at the imager and pixel scale at the guider will answer your question. Focal length is just a shorthand way of discussing the matter.

What I don't think has come up is the matter of flexure, particularly when imaging with reflectors. It is very hard indeed to remove all possibility of primary mirror movement in a reflector because if the mirror is held too tightly it will experience distortion. If the mirror moves slightly, so does the image. A separate guide scope cannot 'know' this so cannot correct for it. However, an Off Axis Guider uses the same light cone as the imager for guiding, so mirror movement (known as mirror flop, though this is an exaggeration!) will be seen and corrected by the guider. Under no circumstances would I guide a catadioptric with a guide scope. I've always used an OAG for this. The small mirror of the 150PDS might let you get away with it but, in principle, reflectors are best guided via OAG. You need to be sure that you have enough backfocus for an OAG, though. It pushes the camera further down the light cone.

Olly

PS You don't need to calculate pixel scale yourself. Here's the easy way to find it! http://www.12dstring.me.uk/fovcalc.php

Edited by ollypenrice
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9 hours ago, ollypenrice said:

Bobro's earlier post explaining the relationship between pixel scale at the imager and pixel scale at the guider will answer your question. Focal length is just a shorthand way of discussing the matter.

What I don't think has come up is the matter of flexure, particularly when imaging with reflectors. It is very hard indeed to remove all possibility of primary mirror movement in a reflector because if the mirror is held too tightly it will experience distortion. If the mirror moves slightly, so does the image. A separate guide scope cannot 'know' this so cannot correct for it. However, an Off Axis Guider uses the same light cone as the imager for guiding, so mirror movement (known as mirror flop, though this is an exaggeration!) will be seen and corrected by the guider. Under no circumstances would I guide a catadioptric with a guide scope. I've always used an OAG for this. The small mirror of the 150PDS might let you get away with it but, in principle, reflectors are best guided via OAG. You need to be sure that you have enough backfocus for an OAG, though. It pushes the camera further down the light cone.

Olly

PS You don't need to calculate pixel scale yourself. Here's the easy way to find it! http://www.12dstring.me.uk/fovcalc.php

Thanks Olly.

The inherent curiosities with reflectors as a whole was what turned me away from them originally.

And in looking towards a telescope as an actual lens for imaging is how I landed in a Refractor as my choice.

Put simply, I suppose I'm Galilean. Less to go wrong = more to enjoy.

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Hello all

Well I have got my guide scope. I went with the qhy mini guide scope. In the end I got it from Modern Astronomy http://www.modernastronomy.com/shop/accessories/qhy-accessories/qhy-miniguidescope-kit/

Really good people I got mine delivered from the Uk to Italy in one day! I would have bought it from FLO if they had it. 

Well does it work do you ask? I've had it a week now at least but the moon! and now it's been raining! So I'm still waiting. I blame global warming and every person with a 4x4 :) just kidding. Yes just going to have to be patient. Not my greatest talent. 

I'll post my results when I get it going. 

Gerry

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On 05/05/2017 at 21:07, SonnyE said:

Thanks Olly.

The inherent curiosities with reflectors as a whole was what turned me away from them originally.

And in looking towards a telescope as an actual lens for imaging is how I landed in a Refractor as my choice.

Put simply, I suppose I'm Galilean. Less to go wrong = more to enjoy.

God willing I too would like a nice apochromatic refractor but as I'm new to it I want to get the most out of what I have.

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On 02/05/2017 at 09:58, Gerry Casa Christiana said:

Well I've just had a nightmare with Altair Astro just trying to buy one of their guide scopes. 2 weeks nothing! Very disappointed I cancelled the order today so now I need to look for somewhere else to get a decent guide scope. I'm also going to be involved in a observatory build. They lost out there. 

 

So guys and girls. What can I do now? 

Just so you know what I've got. Skywatcher 150pds which is 750 focal length on a Eq6.  I already bought the ASI 120mm so I just need a guide scope!

Shame FLO don't have other options. 

Any ideas will be gratefully received. Maybe they did me a favour if I can get something better. 

Gutted. 

I believe Altair is having issues generally at the moment with their Chinese suppliers.  I was waiting 6 weeks just for a simple 15mm spacer for my Esprit, so ended up having one made.  Problem is their mini guide scopes are actually very good for that purpose, so maybe worth hanging on for a bit, or keeping a keen eye on ABS.

Edited by RayD
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I use the QHY miniGuideScope and a QHY5L-II-M for my 500mm OTA/lens. I already had the QHY5L-II-M camera before getting the little scope but I really like the combo, small and rigid.

What is nice about it is that I can use the combo with SharpCap for polar alignment. And since I have got the QHY5L-II-M camera I can also use a cheap 25mm CCTV lens and use PoleMaster software as well if I need to.

The miniGuideScope only has a 130mm focal length but with the small 3.75um pixels of the QHY5L-II-M (same as your ASI) I think it would be suitable for way longer focal length than what I use it for.

I don't think you can go too wrong with your choice.

One slight flaw that I noticed with it is that if you focus too far out (i.e. unscrew the barrel) and then try to lock focus, you can damage the threads on the barrel with the grub screw so be careful. It isn't normal to screw focus out much so it shouldn't really be a problem.

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I know nothing about statistics, I just do what works.  The skywatcher finderscope on your 150PDS will work OK as a guidescope, I had one and that's what i used.  No need to buy a guidescope and worry about how to mount it.  Just need a guide camera and the relevant  finderguider adapter.  

If FLO can't help I know a retailer who does them but as they are Sponsors of this site I should not post up another retailer.  

Image I did with it.  Bright stars are a bit squiffy as the focusser protruded into the  tube too much as it was the 150P version.  N.B.  These are 900sec subs, so you can see the guiding was OK. 

724f5bab0ee62f3233bd09c776b43cd7.1824x0_

Carole 

Edited by carastro
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