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Need advice on guidescope and camera

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#1 ajlmarques

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Posted 19 February 2019 - 05:50 PM

Hi,

 

First post on the forum.

 

I have a Celestron 8 edge on a CGX mount. Started doing some imaging recently and, as probably expected (or maybe I can't get it properly aligned), any exposures longer than 3-4 minutes start showing some oblong stars.

Now I'm looking to get a guidescope and camera for it to plug into PHD, but the enormity of choice for both is getting me really confused - help please.

 

What guidescope and camera would you recommend? I don't want to make a massive investment, but also don't want to be frustrated soon by limitations on the equipment.

 

Any advice is very much appreciated.



#2 GraySkies

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Posted 19 February 2019 - 06:05 PM

I'm also looking at guide cameras, right now I have my eye on the ZWO ASI150MM-Mini as the front runner, there are more expensive guidecameras out there but this sounds like it will get the job done.



#3 Stelios

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Posted 19 February 2019 - 06:13 PM

First of all, welcome to Cloudy Nights!

 

You should bite the bullet and get an OAG (off-axis-guider) and camera for your Edge HD.

 

The Celestron OAG and ASI174MM-mini are an ideal combo (the large sensor of the 174 and large prism of the Celestron OAG mate well together).

 

If the ASI174MM is too much money, the ASI290MM-mini (smaller sensor but very sensitive) should be OK. 

 

There are other OAG's of course. But the Edge can accommodate the Celestron without problems.

 

A guidescope will have severe differential flexure issues on an SCT. As a relative beginner who hasn't guided, you can't quite appreciate the problem, but it happens when you guide well but stars are elongated because of mirror shift or drag (by cables and/or gravity) on the scope. With an OAG it just doesn't happen.

 

There's nothing to fear about an OAG. You need to spend some time initially in order to make sure that the guide camera and main camera come to focus together. You can practice this during the day using something like Sharpcap (free). Once you've found the common focus point, you just lock the OAG there. 

 

If you have a reducer, you'll need to accommodate it. The Edge reducer has 105mm backfocus which will require some spacers (spacers come with the Celestron OAG, and additional ones are easily purchased if needed--this will depend also on whether you use a DSLR).


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#4 ajlmarques

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Posted 19 February 2019 - 06:26 PM

I'm also looking at guide cameras, right now I have my eye on the ZWO ASI150MM-Mini as the front runner, there are more expensive guidecameras out there but this sounds like it will get the job done.

Thanks, I'll have a look at it as well.



#5 ajlmarques

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Posted 19 February 2019 - 06:31 PM

First of all, welcome to Cloudy Nights!

 

You should bite the bullet and get an OAG (off-axis-guider) and camera for your Edge HD.

 

The Celestron OAG and ASI174MM-mini are an ideal combo (the large sensor of the 174 and large prism of the Celestron OAG mate well together).

 

If the ASI174MM is too much money, the ASI290MM-mini (smaller sensor but very sensitive) should be OK. 

 

There are other OAG's of course. But the Edge can accommodate the Celestron without problems.

 

A guidescope will have severe differential flexure issues on an SCT. As a relative beginner who hasn't guided, you can't quite appreciate the problem, but it happens when you guide well but stars are elongated because of mirror shift or drag (by cables and/or gravity) on the scope. With an OAG it just doesn't happen.

 

There's nothing to fear about an OAG. You need to spend some time initially in order to make sure that the guide camera and main camera come to focus together. You can practice this during the day using something like Sharpcap (free). Once you've found the common focus point, you just lock the OAG there. 

 

If you have a reducer, you'll need to accommodate it. The Edge reducer has 105mm backfocus which will require some spacers (spacers come with the Celestron OAG, and additional ones are easily purchased if needed--this will depend also on whether you use a DSLR).

Holy smokes, Batman... I came here looking for xxx with yyy are a good combo, and you're taking me down a rabbit hole. lol.gif

 

Very much appreciated. I hadn't thought of the OAG solution and will definitely look into it now as a possible choice. As I don't have a permanent site for the gear and it will get moved around, is this a problem with  OAG? 

 

I have a Canon 5d3 using prime focus.



#6 Stelios

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Posted 19 February 2019 - 06:36 PM

Holy smokes, Batman... I came here looking for xxx with yyy are a good combo, and you're taking me down a rabbit hole. lol.gif

 

Very much appreciated. I hadn't thought of the OAG solution and will definitely look into it now as a possible choice. As I don't have a permanent site for the gear and it will get moved around, is this a problem with  OAG? 

 

I have a Canon 5d3 using prime focus.

No, the OAG can easily stay attached to the imaging train. If you need to remove and reinsert the camera, a mark on the tube where focus is will help reestablish it.

 

If you didn't have an SCT, I would've recommended different items. But for an SCT (and long-focal-length imaging) OAGs (and ONAGs for larger scopes) rule.



#7 Stelios

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Posted 19 February 2019 - 06:38 PM

Here's a link to the image of Celestron OAG on an Edge 8" with a DSLR.

 

Note the guide camera shown is pancake-style. The recommended -mini versions are slimmer and easier to insert further into the OAG if needed.



#8 Bigdan

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Posted 19 February 2019 - 06:41 PM

You may want to try Hyperstar imaging with your Edge telescope. It is very fast, and delivers beautiful images. You must use a guide scope with Hyperstar..... OAG will not work. Hyperstar imaging would be shorter focal length than rear cell imaging with your scope. You could do Hyperstar for a while, and then when you move into rear cell imaging, try it with a focal reducer first, and with an 8 in. SCT and focal reducer, you may be able to get away with using guide scope instead of OAG.

I did rear cell imaging fine with a Meade 8 in. SCT and guide scope, with a focal reducer on the main scope. I am doing OAG with C14 Edge and .7X focal reducer now, but OAG only because the C14 Edge, even with focal reducer, is pretty long F/L at 2737mm.

Orion ST-80, at 80mm aperture, and 400mm F/L, and only 2 lbs., was a good one. Unfortunately, it is discontinued, and there are threads to the effect that the replacement is not as good in quality. You may be able to find an ST-80 at a retailer somewhere. The longer the focal length you can get in a guide scope, the more accurate your guiding will be. You can get the Losmandy 3 ring guide scope setup.... 3 rings instead of 2.... to help reduce flexure.

ZWO or QHY guide cameras would be good. Calculate the image scale with whatever scope/camera combination you plan on using (main scope for OAG or guide scope) to see what combination would be best.

Edited by Bigdan, 19 February 2019 - 06:51 PM.


#9 bobzeq25

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Posted 19 February 2019 - 06:49 PM

Stelios is absolutely correct, but it goes farther than that.  The long, big, heavy scope will make everything harder, right down to finding your target.   It's not the best tool for learning AP with.

 

The rabbit hole is not Stelios' excellent advice.  It's the scope.  Really.  Here's how it goes.  Not my quotes, but I have collected a number of similar ones.

 

"I started out with a CPC 800  on a heavy duty wedge and a Canon 450d.  In hindsight, I'd have started with an 80mm refractor.  I would have saved a lot of money and gotten up the learning curve a lot quicker."

 

I have the Celestron OAG and the 174, it's a good combination.

 

OAG.  $220.  174.  $500.  Reducer.  $120.  $840.

 

Or this, which you'll learn better and faster with.  $469.

 

https://www.astronom...fpl-53-f-6.html

 

There have been a number of people here who spent money trying to learn on an SCT, and found success by switching scopes.

 

"If you're new to astrophotography I highly recommend getting a small refractor.   It's much lighter on the mount, incredibly easier to guide, easier to shoot with, no coma, better images. I used an 8" SCT for my first 7 months of astronomy/astrophotography and it was headaches. I just got an 80mm stellarvue triplet refractor and my first image is bangin' compared to my previous ones. Blows my SCT's out of the water, which I honestly don't think I'll be using for a long time."

 

Hyperstar is a possibility, but, honestly, it can be fiddly.  The reason the small refractor works for learning this complicated art is that it's simple to use.  It gets out of the way, and lets you get on with learning AP.

 

It's very much a case of choosing the right tool for the job.  Which is learning AP.  It takes time, there's little from my first year on my astrobin.

 

Whatever you decide, this book will be extremely useful.

 

https://www.amazon.c...d/dp/0999470906


Edited by bobzeq25, 19 February 2019 - 06:53 PM.

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#10 Stelios

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Posted 19 February 2019 - 06:51 PM

You may want to try Hyperstar imaging with your Edge telescope. It is very fast, and delivers beautiful images. You must use a guide scope with Hyperstar..... OAG will not work. Hyperstar imaging would be shorter focal length than rear cell imaging with your scope. You could do Hyperstar for a while, and then when you move into rear cell imaging, try it with a focal reducer first, and with an 8 in. SCT and focal reducer, you may be able to get away with using guide scope instead of OAG.

I did rear cell imaging fine with a Meade 8 in. SCT and guide scope, with a focal reducer on the main scope. I am doing OAG with C14 Edge and .7X focal reducer now, but OAG only because the C14 Edge, even with focal reducer, is pretty long F/L at 2737mm.

Orion ST-80, at 80mm aperture, and 400mm F/L, and only 2 lbs., was a good one. Unfortunately, it is discontinued, and there are threads to the effect that the replacement is not as good in quality. You may be able to find an ST-80 at a retailer somewhere. The longer the focal length you can get in a guide scope, the more accurate you guiding will be.

ZWO or QHY guide cameras would be good. Calculate the image scale with whatever scope/camera combination you plan on using (main scope for OAG or guide scope) to see what combination would be best.

Note that Hyperstar is a $$$$ purchase and significantly reduces the focal length (it's F/2 so about 400mm) for imaging. It's a great way to image widefield--at a cost greater than buying a separate 80mm refractor would entail. 

 

If you will use hyperstar, the Orion 50mm guidescope ($79) and ASI120MM-mini ($149) are a good combo. 

 

I don't see why try to "get away" with using a guidescope rather than an OAG if imaging at 2000 or (with reducer) 1400mm. I have been using my refractors, and even at the shorter focal length they have and with an excellent mount, I had subs lost to flexure before an OAG--and none thereafter.



#11 GraySkies

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Posted 19 February 2019 - 07:00 PM

Note that Hyperstar is a $$$$ purchase and significantly reduces the focal length (it's F/2 so about 400mm) for imaging. It's a great way to image widefield--at a cost greater than buying a separate 80mm refractor would entail. 

 

If you will use hyperstar, the Orion 50mm guidescope ($79) and ASI120MM-mini ($149) are a good combo. 

 

I don't see why try to "get away" with using a guidescope rather than an OAG if imaging at 2000 or (with reducer) 1400mm. I have been using my refractors, and even at the shorter focal length they have and with an excellent mount, I had subs lost to flexure before an OAG--and none thereafter.

So my query as someone also looking at guide-scope options (and I own a 80mm and 8") why not just stick to OAG? Sounds like its the better choice assuming you can pick a good target off the small mirror. Then your image train is set and you just have to apply the correct spacers after each scope and you are off to the races, correct?



#12 Ken Sturrock

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Posted 19 February 2019 - 07:32 PM

You certainly can use an OAG on your small refractor. Many of us do that. It makes a very light weight and compact package.

 

A small refractor can go well with either an OAG or a guide scope, but the guide scope is conceptually easier for some people to wrap their brains around so it is frequently the default recommendation. In the case of the aforementioned Hyperstar, OAG would mean more stuff sticking out in the light path and is probably not a good idea (nor necessary).

 

Best of luck.



#13 rgsalinger

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Posted 19 February 2019 - 07:33 PM

Find someone who is imaging in your local club and take a look at what they are doing before taking the jump. On this forum we do get people who succeed in imaging with a C8 and a guide scope from time to time. It's not impossible but it requires quite a bit of thought for it to be likely to work. The mirror in these telescopes moves minutely as the mount moves up and down but the tube does not. The guide scope, properly mounted,doesn't move at all. So, you need to lock down the mirror for starters. The mirror locks on these scopes are not really locks and so they may or may not be adequate to control the movement.

 

Now all of this assumes that you are interested in long exposure guided imaging. With the current crop of CMOS cameras, you may find that if you use the companion reducer you can get away with short exposures and so there won't be enough mirror flop (movement) to actually matter. You still need to lock the mirrors and you will also need some kind of auxilliary focuser for best results.

 

I started out with an OAG. I owned it for 6 years. I attached to a filter wheel and a camera. I cut out the foam in a Pelican case to hold the entire assembly including the guide camera. I put a par focal ring around the guide camera just in case. All I had to do was to bolt the thing onto the focuser of whatever OTA I wanted to use. It takes one night (and a bit of forethought) to make an OAG work. However, it's a permanent solution. Over the past 10 years I've owned 6 telescopes (still have 4 of them). All I ever do is just move the two camera systems around using the correct adapters and I'm up and running in minutes.

 

Rgrds-Ross


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#14 GraySkies

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Posted 19 February 2019 - 07:44 PM

You certainly can use an OAG on your small refractor. Many of us do that. It makes a very light weight and compact package.

 

A small refractor can go well with either an OAG or a guide scope, but the guide scope is conceptually easier for some people to wrap their brains around so it is frequently the default recommendation. In the case of the aforementioned Hyperstar, OAG would mean more stuff sticking out in the light path and is probably not a good idea (nor necessary).

 

Best of luck.

Good point on the hyperstar, ideally you want to minimize blocking the front element.

 

Find someone who is imaging in your local club and take a look at what they are doing before taking the jump. On this forum we do get people who succeed in imaging with a C8 and a guide scope from time to time. It's not impossible but it requires quite a bit of thought for it to be likely to work. The mirror in these telescopes moves minutely as the mount moves up and down but the tube does not. The guide scope, properly mounted,doesn't move at all. So, you need to lock down the mirror for starters. The mirror locks on these scopes are not really locks and so they may or may not be adequate to control the movement.

 

Now all of this assumes that you are interested in long exposure guided imaging. With the current crop of CMOS cameras, you may find that if you use the companion reducer you can get away with short exposures and so there won't be enough mirror flop (movement) to actually matter. You still need to lock the mirrors and you will also need some kind of auxilliary focuser for best results.

 

I started out with an OAG. I owned it for 6 years. I attached to a filter wheel and a camera. I cut out the foam in a Pelican case to hold the entire assembly including the guide camera. I put a par focal ring around the guide camera just in case. All I had to do was to bolt the thing onto the focuser of whatever OTA I wanted to use. It takes one night (and a bit of forethought) to make an OAG work. However, it's a permanent solution. Over the past 10 years I've owned 6 telescopes (still have 4 of them). All I ever do is just move the two camera systems around using the correct adapters and I'm up and running in minutes.

 

Rgrds-Ross

I tend to get advice from club members when I can (bit out of the loop the last 2 years as I moved) but from what I'm hearing OAG is becoming more thing serious astronomers are doing as the guide-camera sensitivity are much higher then they used to be and picking up faint stars to guide on is therefore easier.

This is becoming my thinking as well, have it all set-up in a box and then just pop it onto which ever scope I'm using for a particular target and go... less fuss than swapping over cameras and guide-scopes from one setup to another. that also leaves the piggyback mount for a visual scope.

Is there a particular go-to OAG everyone recommends that compatible with most/all scopes given back focus requirements?


Edited by GraySkies, 19 February 2019 - 07:50 PM.


#15 Bigdan

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Posted 19 February 2019 - 07:58 PM

But he has an Edge.... Hyperstar compatible. Hyperstar is faster than a refractor, and he's got large aperture with the SCT.

#16 bobzeq25

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Posted 19 February 2019 - 08:49 PM

Is there a particular go-to OAG everyone recommends that compatible with most/all scopes given back focus requirements?

Not really.  It's a matter of tradeoffs.  Thicker OAGs tend to be easier to live with, more sturdy, more stable.  But they use up a lot of backfocus.  The Celestron (which I own) is a good example.  Comes with many adapters, very easy to work with.  Maybe a clear choice, _if_ you have the backfocus for it.  I wound up having to have PreciseParts make a custom adapter, to get one thin enough.

 

Thin ones use less backfocus, work with more setups, but can be fiddly.



#17 GraySkies

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Posted 19 February 2019 - 09:29 PM

Not really.  It's a matter of tradeoffs.  Thicker OAGs tend to be easier to live with, more sturdy, more stable.  But they use up a lot of backfocus.  The Celestron (which I own) is a good example.  Comes with many adapters, very easy to work with.  Maybe a clear choice, _if_ you have the backfocus for it.  I wound up having to have PreciseParts make a custom adapter, to get one thin enough.

 

Thin ones use less backfocus, work with more setups, but can be fiddly.

If I'm only imaging with mirrorless or astro-cameras (also mirrorless) can I get away with a solid OAG or should I still be looking thin? I'm assuming not having the ~54mm back-focus of a DSLR will help in that regard.


Edited by GraySkies, 19 February 2019 - 09:30 PM.


#18 ajlmarques

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Posted 20 February 2019 - 06:13 PM

No, the OAG can easily stay attached to the imaging train. If you need to remove and reinsert the camera, a mark on the tube where focus is will help reestablish it.

 

If you didn't have an SCT, I would've recommended different items. But for an SCT (and long-focal-length imaging) OAGs (and ONAGs for larger scopes) rule.

 

So, after some research, I'm really tempted to go for the OAG solution. Few questions before I make the jump:

 

  • Can I easily attach a motorised filter wheel in the future to this setup?
  • How hard is it, from your experience, to find guide stars with the OAG in a light polluted area?
  • Is the ZWO ASI120mm-mini a good option for camera regarding sensitivity, again in a light polluted area?

 

Many thanks


Edited by ajlmarques, 20 February 2019 - 06:17 PM.


#19 Ken Sturrock

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Posted 20 February 2019 - 09:08 PM

1.) Many people do this, but the precise answer will depend on the depth of the filter wheel, the filter thickness, the OAG depth, the backspace of the imaging camera and the backspace needed for focusing by any field flattener or focal reducer.

 

2.) With my refractors (up to about 800 mm f/l), I've never not found a guide star using a modern guide camera. A longer focal length instrument will be more difficult, so you would benefit from an OAG with a larger prism and a guide camera to take advantage of it.

 

3.) I can't comment intelligently. Of course, that's never stopped me: I would expect it to be fine. In the ZWO guider lineup, the ASI-174 will give you the biggest FOV and the larger pixels are more flexible for long focal lengths (like on an SCT). The ASI-290 has the highest resolution for small refractors or separate guide scopes and the ASI-120 is the compromise, it also costs less.

 

If you haven't seen it before, you may enjoy this episode of the Astro Imaging Channel on OAGs and setting them up.



#20 TelescopeGreg

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Posted 21 February 2019 - 03:02 PM

Filter wheels, OAG, and focus...

 

I've not used any of these (well, except I do focus things), but putting together a few separate threads brings up a question.  I presume that a filter wheel would be placed between the OAG splitter and the imaging camera, back-focus permitting.  You then adjust the guide camera so that it is parfocal with the imaging camera, i.e. so that both come into focus at the same time when the focus knob is adjusted.

 

I also read that when one changes filters it is best to re-focus.  But that would change the focus at the guide camera, too.  I.e. you'd be bringing the imaging camera into focus, and as a result, putting the guide camera out of focus.

 

Is that a problem, or is the focus change at the guide camera not bad enough to cause a loss of the guide star, since it's not quite so sensitive to focus accuracy?


Edited by TelescopeGreg, 21 February 2019 - 03:04 PM.



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