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Guide Scope Choice

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#1 svsalako

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Posted 21 February 2020 - 06:44 PM

Hi All --

 

I ordered an Astro Tech AT72EDII telescope today as the first piece in an AP setup. I will be using an unmodified Canon 77D on either an HEQ5 or, better yet,  an EQ6 R Pro if I can wait for it to come on sale. From my reading it's clear that ultimately auto guiding is the way to go, so I want to get an appropriate auto guiding rig to go along with the above. I have a few questions:

 

First, "Image Scale": I calculated the image scale of my telescope/camera configuation at 1.78. What is the optimum, or best range, for an image scale for the guide camera/scope set up?

 

Second, I notice that a number of folks are using the Svbony 50 or 60 mm guidescopes, that look suspiciously similar to the ones that are sold by Astro-Tech and Agena, among others. Are they the same scope, because the Svbony brand is less expensive. I'm willing to pay more if the "brand" names are better, but if they are the same.....

 

Would the 50 mm be a good choice? I have also looked at the ZWO 30mm mini guide scope. I like this because of the small size, but I don't think you could use it as a finderscope as well. However, do I need a finderscope on the AT72, or can I just use a low power lens on its own? Recommendations?

 

Lastly, I am considering either the ZWO ASI 120MM mini or the ASI 290MM mini. Paired with the 50mm scope, the 190 has an image scale of 4.06, and the 290 has an image scale of 3.14. Which is the better choice, and what is the relationship between the image scale of the guide scope vs. the imaging scope?

 

Sorry to be so picky, but I am trying to master the details of all of this as I go, instead of just guessing.

 

Thanks in advance for all replies.

 

Cheers.



#2 DmitriNet

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Posted 21 February 2020 - 06:52 PM

Go with guidescope/guide camera that has pixel scale as close to main scope/camera as possible (or even smaller).



#3 OldManSky

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Posted 21 February 2020 - 07:17 PM

I have the 60mm f/4 SVBONY guide scope.  It is, indeed, the exact same one as some other "brand" name ones, at a lower price.

I'm convinced (though it's only a good guess) that the lower price gets you essentially no QA.  I think the factory manufactures a whole bunch of them, and picks ones that meet minimum quality levels for the "brand names," and SVBONY gets the rest.

Which means you can get a good one or one with problems (loose focuser, poor optics, even a number of reviews mentioning optics that were put in the tube backwards -- which is annoying but easily fixable!).

So if you go with the SVBONY, get it from Amazon or somewhere else where returns are quick, easy, and free.  If you get a good one on the first try (I did, lucky I guess), keep it.  If you don't, keep returning it until you do. :)

 

I use the 60mm to guide with an ASI290mm-mini for a GT-71 and a ZS103.  Guide pixel scale is great for those scopes.  



#4 bobzeq25

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Posted 21 February 2020 - 08:29 PM

Hi All --

 

I ordered an Astro Tech AT72EDII telescope today as the first piece in an AP setup. I will be using an unmodified Canon 77D on either an HEQ5 or, better yet,  an EQ6 R Pro if I can wait for it to come on sale. From my reading it's clear that ultimately auto guiding is the way to go, so I want to get an appropriate auto guiding rig to go along with the above. I have a few questions:

 

First, "Image Scale": I calculated the image scale of my telescope/camera configuation at 1.78. What is the optimum, or best range, for an image scale for the guide camera/scope set up?

 

Second, I notice that a number of folks are using the Svbony 50 or 60 mm guidescopes, that look suspiciously similar to the ones that are sold by Astro-Tech and Agena, among others. Are they the same scope, because the Svbony brand is less expensive. I'm willing to pay more if the "brand" names are better, but if they are the same.....

 

Would the 50 mm be a good choice? I have also looked at the ZWO 30mm mini guide scope. I like this because of the small size, but I don't think you could use it as a finderscope as well. However, do I need a finderscope on the AT72, or can I just use a low power lens on its own? Recommendations?

 

Lastly, I am considering either the ZWO ASI 120MM mini or the ASI 290MM mini. Paired with the 50mm scope, the 190 has an image scale of 4.06, and the 290 has an image scale of 3.14. Which is the better choice, and what is the relationship between the image scale of the guide scope vs. the imaging scope?

 

Sorry to be so picky, but I am trying to master the details of all of this as I go, instead of just guessing.

 

Thanks in advance for all replies.

 

Cheers.

None of the choices you're discussing are bad.  But you left off the most important thing, by far.  _This_ is what you need to be picky about.

 

The guidescope must be rigidly mounted, or you could be in serious trouble.  A finder shoe is inadequate.  Scroll down this page to see how it's done.  Two spaced rings trinagulate the load, and are much stiffer.   Yes, it's not necessarily cheap.  But people aren't doing what they do to look good.  <smile>

 

https://www.cloudyni...ardrails/page-4


Edited by bobzeq25, 21 February 2020 - 08:30 PM.

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#5 svsalako

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Posted 23 February 2020 - 10:56 AM

I'm guessing that OldManSky is correct about the difference between the brand names and the Svbony version of the scope. I will try the Svbony route, but if I get a bad one, I'll return it and get one of the brand names.

 

The pixel size on the ASI 120mm mini is essentially the same as the Canon 77d, so I think I'll go with that. It's half the price of the 290, and I need to save money somewhere (hahahahahah!).

 

Understood about flexure. My idea is to

1) remove the dovetail base supplied with the guide scope

2) fab a piece of aluminium bar stock and fasten it to the tops of the two rings on the AT72

3) fasten the two rings of the guide scope to the same piece of bar stock

 

This should be rock solid, and pretty much imitates the pics that were referenced.

 

I've pretty much got my equipment list worked out except for this dilemma. Keeping in mind that I have until mid-July before I can actually use this stuff:

 

--I can wait for the EQR6 Pro to go on sale for $1350 or so

 

-- I can get right now a new HEQ5 for $935.

 

In the long run the EQR6 is a better mount, but that's a pretty big price difference and I think the HEQ5 would work for now.  I think I could probably get most of my money back in a year or two if I sell it to upgrade. Thoughts anyone?

 

Cheers, and thanks.


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#6 Peregrinatum

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Posted 23 February 2020 - 11:05 AM

I'm guessing that OldManSky is correct about the difference between the brand names and the Svbony version of the scope. I will try the Svbony route, but if I get a bad one, I'll return it and get one of the brand names.

 

The pixel size on the ASI 120mm mini is essentially the same as the Canon 77d, so I think I'll go with that. It's half the price of the 290, and I need to save money somewhere (hahahahahah!).

 

Understood about flexure. My idea is to

1) remove the dovetail base supplied with the guide scope

2) fab a piece of aluminium bar stock and fasten it to the tops of the two rings on the AT72

3) fasten the two rings of the guide scope to the same piece of bar stock

 

This should be rock solid, and pretty much imitates the pics that were referenced.

 

I've pretty much got my equipment list worked out except for this dilemma. Keeping in mind that I have until mid-July before I can actually use this stuff:

 

--I can wait for the EQR6 Pro to go on sale for $1350 or so

 

-- I can get right now a new HEQ5 for $935.

 

In the long run the EQR6 is a better mount, but that's a pretty big price difference and I think the HEQ5 would work for now.  I think I could probably get most of my money back in a year or two if I sell it to upgrade. Thoughts anyone?

 

Cheers, and thanks.

 

Limiting factor is payload, you don't want to load your mount more than 1/2 to 2/3 of it's rated capacity when using it for AP.

 

HEQ5 = rated for 30 lbs. (maybe Lol, atleast 15 lbs), so all your imaging gear < 20 lbs (camera, guide scope, OTA, dew heaters, cables, etc.)

 

EQ6-R = rated for 44 lbs., all imaging gear < 30 lbs

 

HEQ5 would probably be fine with up to a 110mm refractor, and I use a C925HD on my EQ6-R and it guides well with that large scope.

 

Both good choices, depends what you want to do with your imaging.  Personally, I would wait for the EQ6-R, but if you gotta scratch the AP itch now then go for the HEQ5 knowing that you may have to upgrade later.

 

edit:  there is a EQ6-R in the classified at your price point

https://www.cloudyni...-price-reduced/
 


Edited by Peregrinatum, 23 February 2020 - 11:26 AM.


#7 HowardSD

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Posted 23 February 2020 - 03:44 PM

I have the same scope and use the Orion 50mm mini guide scope with an ASI 120MC-s, works perfect for me. The Orion mini comes with a mount that is rock solid on my scope no issues at all, you'll need a finder bracket, i picked up one of Amazon for $15 has the 2 screws rather than one think it was Astronomics brand (more solid). The scope setup is somewhat back heavy especially with a DSLR attached but so far i've had no problems. I use it on an AVX, not the greatest for this but with PHD2 my setup can do 3 min subs... probably longer but not tried that yet.



#8 svsalako

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Posted 24 February 2020 - 11:43 AM

Still ruminating on which guide scope to get. Lots of folks on the imaging forum are gung ho about the Stellarvue 50mm, but it's$149 without rings, which adds another $25 or so, plus shipping so it will end up being around $200. I've talked to both Agena and Astonomics about their guidescopes and both were.  vague about comparing their private label scopes to the ones on Amazon or Ebay. I was hoping for some kind of assurance about quality control but......

 

Separate question: Obviously the 60mm guide scope brings in a bit more light compared to the 50mm, but if I am doing the math correctly, the longer focal length (240mm vs 190mm) produces a lower image scale. If I go with the ZWO ASI12mm mini, the image scale with the 60 mm aperture is 2.86 vs 4.06 with the 50mm. The image scale for my camera with the At72EDII is 1.78. Is this a reason to go with the 60 vs. the 50mm? or does it not really matter. I'm trying to do two things here:

--buy the most appropriate guidescope

--understand how image scale affect what is going on.

 

Thanks for listening to my blather......



#9 DrGomer

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Posted 24 February 2020 - 11:58 AM

From what I recall, you want the guide scope to be anywhere from 1-5x the imaging image scale.  Guiding is done at sub pixel resolution.  Lower guide ratios lead to less stars in the field to be able to guide off off. Higher ratios tend to have more guide error.  



#10 bobzeq25

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Posted 24 February 2020 - 12:17 PM

Still ruminating on which guide scope to get. Lots of folks on the imaging forum are gung ho about the Stellarvue 50mm, but it's$149 without rings, which adds another $25 or so, plus shipping so it will end up being around $200. I've talked to both Agena and Astonomics about their guidescopes and both were.  vague about comparing their private label scopes to the ones on Amazon or Ebay. I was hoping for some kind of assurance about quality control but......

 

Separate question: Obviously the 60mm guide scope brings in a bit more light compared to the 50mm, but if I am doing the math correctly, the longer focal length (240mm vs 190mm) produces a lower image scale. If I go with the ZWO ASI12mm mini, the image scale with the 60 mm aperture is 2.86 vs 4.06 with the 50mm. The image scale for my camera with the At72EDII is 1.78. Is this a reason to go with the 60 vs. the 50mm? or does it not really matter. I'm trying to do two things here:

--buy the most appropriate guidescope

--understand how image scale affect what is going on.

 

Thanks for listening to my blather......

Both could work, and the difference is small.  My kind of decision - neither choice would be wrong.  <smile>

 

If you want my opinion, I'd go with the 60mm.  It could be used on a longer scope later.

 

I used a 60mm with a 910mm refractor for a while, then switched to an off axis guider.  It's all good.


Edited by bobzeq25, 24 February 2020 - 12:19 PM.


#11 svsalako

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Posted 26 February 2020 - 11:27 AM

Hi All --

 

I finally made a decision! After a nice email exchange with Scott at Agenaastro, I ordered their Agena Deluxe 50mm guidescope with the helical focuser -- $120. He assured me that they inspected each of these before they shipped them out to customers to make sure they were in good working order. None of the vendors I talked to were willing to say that theirs were the same scopes that you can get on Ebay or Amazon for much less, which I get, but the fact that Agena inspects each one for QA was worth the extra $$ to me.

 

Now I have to save my $$ for the mount. I'm hoping to have the budget for that in the next 2-3 months, and that moment will coincide with a Skywatcher sale. I saw the EQ6R Pro in the classifieds, but from a domestic expenditure point of view I need to wait a couple of months. Still, I should have a reasonable beginner imaging rig in place by July.

 

Thanks for all the advice -- I'm sure I'll be back with more questions!

 

Cheers and clear skies --

Grey




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