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Which off-axis guider?


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I need an off axis guider for a long FL reflector (2350mm). Can anyone recommend a really good one? I don't mind spending up to £300-350, but I am rather bewildered by the choice and could do with some advice.

Or maybe even an on-axis guider (whatever they are!)...

Thanks

Edited by StuartT
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1.  Does your "long FL reflector" have enough focus range to accommodate an OAG ?

2. Focusing the guidecam can be fiddly, so I recommend one with a helical focuser for the guidecam.

Again. if the OTA will focus with that extra image path length.

3.  The prisms vary in size from 8x8mm to 12x12mm. The bigger prism gives a wider FOV, so more chance of getting a bright-enough guidestar.

If the guidecam sensor is large enough to cover the prism size.

Michael

 

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I'd look for one with a large prism,  an 8mm prism is too small, a 12 is much better and with as many solid fittings as poss.. just make sure you have a sensitive guidecam

What size tube are you using?

Edited by Same old newbie alert
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On 12/04/2022 at 18:34, michael8554 said:

1.  Does your "long FL reflector" have enough focus range to accommodate an OAG ?

2. Focusing the guidecam can be fiddly, so I recommend one with a helical focuser for the guidecam.

Again. if the OTA will focus with that extra image path length.

3.  The prisms vary in size from 8x8mm to 12x12mm. The bigger prism gives a wider FOV, so more chance of getting a bright-enough guidestar.

If the guidecam sensor is large enough to cover the prism size.

Michael

 

 

12 hours ago, Same old newbie alert said:

I'd look for one with a large prism,  an 8mm prism is too small, a 12 is much better and with as many solid fittings as poss.. just make sure you have a sensitive guidecam

What size tube are you using?

Thanks both. It is a Celestron Edge HD 9.25" - just arrived today.

I've been reading about on-axis guiders too as they provide much more choice of guidestar. Would that be better than OAG do you reckon?

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1 hour ago, StuartT said:

 

Thanks both. It is a Celestron Edge HD 9.25" - just arrived today.

I've been reading about on-axis guiders too as they provide much more choice of guidestar. Would that be better than OAG do you reckon?

No idea how they work or know of anyone that uses one.. sorry... Nice scope btw, enjoy

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Beware the OAG. I'm sure they're excellent. I had one and it improved my guiding from 1.5-2" AVG to around 0.7-1.2" at 360mm FL during one night (so nothing compared to what you're looking to do). It however was a swine to find focus, even after having pre prepared it during the day and locking everything down, the helical focuser I had lying around wasnt compatible as it pushed the focal plane out beyond the physical depth of the camera (ie a physical stop), it lost focus during a shoot even though everything was tightened up securely grubscrews and all, refocusing is an issue if you're shooting mono and if you need to refocus the OAG due to a filter change. If you'd like to know it was a zwo one. I'm sure many people use theirs well.

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Once you've got both cameras in focus, it's best to keep the whole OAG / image cam / guidecam setup assembled, then focusing the image cam will always focus the guidecam too.

Mono imaging will be as Elp describes, unless you have the filters before the OAG, but then some filters could attenuate the guidestar too much.

Adding a helical focuser to the guidecam doesn't alter the focal plane, that is set by the prism to imaging cam spacing, so hasn't altered.

It just pushes the guidecam sensor further away from the focal plane.

If the imaging cam is already as close to the OAG as it can be then a helical focuser isn't workable.

Michael

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2 minutes ago, michael8554 said:

Adding a helical focuser to the guidecam doesn't alter the focal plane, that is set by the prism to imaging cam spacing, so hasn't altered.

My wording was bad, the helical focuser I had bottomed out (physical surface at the bottom of the helical focuser) was around 5mm beyond the focal plane of the OAG for the guidecam side so I physically couldn't push the camera any further in to achieve focus, it was an svbony focuser so some other designs may account for this. A helical focuser will definitely help with refocusing as the range is extremely minute between seeing stars/blobs and seeing nothing.

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15 hours ago, Elp said:

Beware the OAG. I'm sure they're excellent. I had one and it improved my guiding from 1.5-2" AVG to around 0.7-1.2" at 360mm FL during one night (so nothing compared to what you're looking to do). It however was a swine to find focus, even after having pre prepared it during the day and locking everything down, the helical focuser I had lying around wasnt compatible as it pushed the focal plane out beyond the physical depth of the camera (ie a physical stop), it lost focus during a shoot even though everything was tightened up securely grubscrews and all, refocusing is an issue if you're shooting mono and if you need to refocus the OAG due to a filter change. If you'd like to know it was a zwo one. I'm sure many people use theirs well.

 

3 hours ago, michael8554 said:

Once you've got both cameras in focus, it's best to keep the whole OAG / image cam / guidecam setup assembled, then focusing the image cam will always focus the guidecam too.

Mono imaging will be as Elp describes, unless you have the filters before the OAG, but then some filters could attenuate the guidestar too much.

Adding a helical focuser to the guidecam doesn't alter the focal plane, that is set by the prism to imaging cam spacing, so hasn't altered.

It just pushes the guidecam sensor further away from the focal plane.

If the imaging cam is already as close to the OAG as it can be then a helical focuser isn't workable.

Michael

This is all beginning to sound very complicated!! 😬

I'm clearly going to have to watch some videos, as I don't really follow what you guys are talking about 🤷‍♂️

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You basically set the OAG as close as to the back of the telescope you can so it gets the most light, filters follow behind with rings etc until you have your required backfocus for your main camera. You then set the prism side and push it into the image circle making sure it's clear of the camera sensor square/rectangle/field of view. You then rack the focus for your main camera until it's roughly focussed. You can measure this focus distance to your camera sensor and apply the same distance from the centre of the OAG prism to your guidescope camera sensor, both cameras effectively need to be parfocal. It's easier to setup during the day focusing your telescope on a far off object.

The issue with filters (for me anyway) is that the camera focus needs adjusting slightly when changing filters, by tinkering the focus the OAG focus will also be altered and will need adjustment. If you're imaging OSC this isn't an issue unless you get massive temperature fluctuations and need to adjust focus.

When it works it's brilliant, but I'm sticking to a guidescope as my imaging focal length is fairly short anyway.

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Posted (edited)
7 hours ago, Elp said:

You basically set the OAG as close as to the back of the telescope you can so it gets the most light, filters follow behind with rings etc until you have your required backfocus for your main camera. You then set the prism side and push it into the image circle making sure it's clear of the camera sensor square/rectangle/field of view. You then rack the focus for your main camera until it's roughly focussed. You can measure this focus distance to your camera sensor and apply the same distance from the centre of the OAG prism to your guidescope camera sensor, both cameras effectively need to be parfocal. It's easier to setup during the day focusing your telescope on a far off object.

The issue with filters (for me anyway) is that the camera focus needs adjusting slightly when changing filters, by tinkering the focus the OAG focus will also be altered and will need adjustment. If you're imaging OSC this isn't an issue unless you get massive temperature fluctuations and need to adjust focus.

When it works it's brilliant, but I'm sticking to a guidescope as my imaging focal length is fairly short anyway.

Err.. ok. I think.

I've added a Diamond Steeltrack focuser to the back of mine (in order to be able to use a Pegasus motor focus unit with it - I don't do anything manually), so when you say "you basically set the OAG as close as to the back of the telescope you can" do you mean as close to the telescope or as close to the focuser? I guess what I'm asking is - do I put the OAG between the telescope and the focuser, or after the focuser?

Edited by StuartT
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17 hours ago, Elp said:

You basically set the OAG as close as to the back of the telescope you can so it gets the most light, filters follow behind with rings etc until you have your required backfocus for your main camera. You then set the prism side and push it into the image circle making sure it's clear of the camera sensor square/rectangle/field of view. You then rack the focus for your main camera until it's roughly focussed. You can measure this focus distance to your camera sensor and apply the same distance from the centre of the OAG prism to your guidescope camera sensor, both cameras effectively need to be parfocal. It's easier to setup during the day focusing your telescope on a far off object.

The issue with filters (for me anyway) is that the camera focus needs adjusting slightly when changing filters, by tinkering the focus the OAG focus will also be altered and will need adjustment. If you're imaging OSC this isn't an issue unless you get massive temperature fluctuations and need to adjust focus.

When it works it's brilliant, but I'm sticking to a guidescope as my imaging focal length is fairly short anyway.

No, don't agree.. focus and spacing are a different thing.... You don't need to know or even have the scope focused to get the spacing right.. spacing is to fully illuminate the sensor.. you can have the scope in focus and the center stars will be nice and round, but at the edges ( off axis) won't if the spacing is out.... It's of no wonder you had issues .. but please don't give that kind of advice and put the barrier up...can't see why you would want to use a oag on a 360fl scope anyway...

To use a oag you need to set the spacing to the imaging camera first according to the scopes spec( with a SCT this is abit tricky as the spec from Celestron isn't right, so use the spec as a guide.. note to Stuart, if using a reducer this will change if using a reducer)...

So once the spacing is correct, incorporating the camera sensors backfocus, filter wheel or anything that you want in the imaging train, measure the distance between the prism and the imaging sensor, this distance needs to be the same to the guide camera sensor incorporating the backfocus of the guide cam...

A good idea is to put the long side of the guide camera along side the long side of the imaging camera and the long side of the prism... I don't rotate my camera to find guide stars, if the camera is sensitive enough you won't need to.. so setup your imaging software( think you use Nina don't you Stuart?) Focus the scope, have PhD on loop, I use 3 secs with my oag setup , with PhD looping look for crescent shaped stars, and fine tune the guide cam turret up or down.. don't touch the focus of the imaging cam and get the stars as small and as round as you can.. PhD has no issues guiding on funny shaped stars btw... If you can't find stars, or your prism depth is too far into the imaging train this will show if you take flats, adjust until you can't see the prism in the flats but you can see stars...

It's fairly straightforward, not really complicated and once setup it's done..

Edited by newbie alert
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11 hours ago, StuartT said:

do I put the OAG between the telescope and the focuser, or after the focuser?

After the focuser

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20 minutes ago, newbie alert said:

you can have the scope in focus and the center stars will be nice and round, but at the edges ( off axis) won't if the spacing is out.... It's of no wonder you had issues ..

I've never had an issue setting my imaging backfocus and the advice I've given is correct.

"filters follow behind with rings etc until you have your required backfocus for your main camera"

Once this is done for the main camera, then the focus can be made to determine the focus distance needed for the OAG.

I was testing the OAG so when I do use my longer FL scope I would know it was working okay with the camera I intended to use.

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8 minutes ago, Elp said:

I've never had an issue setting my imaging backfocus and the advice I've given is correct.

Once this is done for the main camera, then the focus can be made to determine the focus distance needed for the OAG.

 

So when you wrote this would you say that was correct?

 sure it's clear of the camera sensor square/rectangle/field of view. You then rack the focus for your main camera until it's roughly focussed. You can measure this focus distance to your camera sensor and apply the same distance from the centre of the OAG prism to your guidescope camera sensor, both cameras effectively need to be parfocal. It's easier to setup during the day focusing your telescope on a far off object.

As focusing during the day will leave you out at night as focus should be at infinity and not a few miles away... The first part was the bit I disagreed with.. focus and spacing are different, as explained... Changing your focus won't change the spacing

Edited by newbie alert
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I would be interested to know, does anybody use the Baader flip mirror OAG? I have and use the flip miiror and never having done guiding, I keep thinking about it. I might add that both the Baader flip mirror and OAG would be within Stuart's budget.

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Read it again, I never mixed up backfocus and focusing as you mentioned. I advised setup the imaging train with filters and rings until backfocus is set for the main camera then rack focus until the focus on main camera is achieved. Some assumption was made that the individual will know their required backfocus first before setting up. It's obvious changing the focus won't change backspacing to anyone who's familiar with AP.

I had no issue setting up during the day on a far off object and finding stars at night, many people suggest this online also for a ballpark estimate rather than trying from scratch on tiny points of light when it's dark.

Like I said oags are good when they're setup working and for longer FL scopes.

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On 13/04/2022 at 23:02, Elp said:

Beware the OAG. I'm sure they're excellent. I had one and it improved my guiding from 1.5-2" AVG to around 0.7-1.2" at 360mm FL during one night (so nothing compared to what you're looking to do). It however was a swine to find focus, even after having pre prepared it during the day and locking everything down, the helical focuser I had lying around wasnt compatible as it pushed the focal plane out beyond the physical depth of the camera (ie a physical stop), it lost focus during a shoot even though everything was tightened up securely grubscrews and all, refocusing is an issue if you're shooting mono and if you need to refocus the OAG due to a filter change. If you'd like to know it was a zwo one. I'm sure many people use theirs well.

Yeah I did

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19 hours ago, Elp said:

set the OAG as close as to the back of the telescope you can so it gets the most light, filters follow behind with rings etc until you have your required backfocus for your main camera. You then set the prism side and push it into the image circle making sure it's clear of the camera sensor square/rectangle/field of view. You then rack the focus for your main camera until it's roughly focussed. You can measure this focus distance to your camera sensor and apply the same distance from the centre of the OAG prism to your guidescope camera sensor, both cameras effectively need to be parfocal. It's easier to setup during the day focusing your telescope on a far off object.

Why rack the focuser until it's nearly focused and then measure the focus distance  and apply it to the guide cam ... It won't change .  You just confused the OP

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err.. ok. Sorry to have given rise to a disagreement here. I must admit I am now quite lost by this discussion and am feeling a bit apprehensive about setting up OAG. But having now bought the scope, I think I have no choice as it's a long FL (unless I just use it for planets).

I watched the James Lamb video this morning (Setting up an OAG) which seemed understandable. But reading your various posts has made me confused again. Would you both agree that the advice from Lamb is sound? (then at least I can follow him!)

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1 hour ago, StuartT said:

err.. ok. Sorry to have given rise to a disagreement here. I must admit I am now quite lost by this discussion and am feeling a bit apprehensive about setting up OAG. But having now bought the scope, I think I have no choice as it's a long FL (unless I just use it for planets).

I watched the James Lamb video this morning (Setting up an OAG) which seemed understandable. But reading your various posts has made me confused again. Would you both agree that the advice from Lamb is sound? (then at least I can follow him!)

I be honest, James lamb on the oag lost me on his rotation technique.. sat there thinking am I going to need one of those measuring tools... 

I reality it's far simplier than that, I've not needed to rotate my qhy 290 ever.. it's simple, set your spacing to the imaging camera, measure the distance from prism to imaging sensor and set the same for the guidecam incorporating the backfocus distance.. point to a  decent starfield and fine tune the guide cam on the turret.. that's it... Like you say that's probably the best choice you have with a sct

 

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Stuart, maybe you need to try it out! I found it quite confusing the first time out and it took a lot of experimentation and mistakes to get it right (C8 XLT + FF/FR + TS-Optics OAG) but I think the guys above and James Lamb have covered it all.

If you're not using the 0.7x reducer (and I believe you're not) then it's more straightforward but still not simple. The James Lamb video is pretty good - I had to draw my own CAD diagrams (I did it on paper) to know exactly what distances were available, but the critical part is that the distance from prism to imaging sensor is the same as prism to guide sensor. Focuser goes ahead of the entire assembly, whether that's the EdgeHD primary mirror or the Diamond Steeltrack. The design of the OAG may constrain you somewhat - the closer the prism is to the imaging sensor, the further in to the OAG stalk the guidecam needs to be inserted. You may not be able to insert it far enough; if so you'll need an extra spacer between OAG and imaging cam to give some extra room.

If you're using a filter wheel or holder it goes between the OAG and the imaging camera so that you're getting full non-filtered illumination on the guidecam sensor (especially if using narrowband filters). James Lamb's proposed arrangement of imaging camera-filter wheel-OAG-spacers-telescope makes a lot of sense and that's what I've used.

I use the 290mm Mini guidecam and have never had to search/rotate for a suitable star. PHD2 is excellent in guiding on the oddest shaped stars, including bean-shaped and seagull-shaped. It also doesn't need focus to be spot on. If you use parfocal filters the small difference in optical path length through different filters shouldn't impact you.

Your focuser must go between OAG and telescope, because if it's between OAG and imaging camera it will change the critical distance between prism and imaging sensor, but the distance from prism to guide cam sensor will stay the same (the two cameras are no longer parfocal).

If you decide to add the reducer all of this will change and your distances will become more critical, as the back-focus distance from the back edge of the reducer to the sensors (both cameras) must be 146mm. You will not be able to put the Diamond Steeltrack, Crayford or R&P anywhere between the reducer and camera as it will change this back focus. I don't believe it can go ahead of the flattener (i.e. scope-side) either so you may be stuck with focusing by adjusting the primary mirror. (As I'm writing this I wonder does the EdgeHD have a built-in flattener with a fixed back-focus?)

Finally, when you have it set up correctly, try to keep the entire imaging assembly intact even if you take it off the scope. One advantage of the OAG setup is that once the two cameras are set correctly you never need to adjust them again. 

Pegasus Astro do an interesting OAG with a focus motor on the stalk. This gives more automation than most of us need, but sounds like a good idea for remote setups if the OAG could go out of alignment.

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Posted (edited)
23 hours ago, newbie alert said:

I reality it's far simplier than that, I've not needed to rotate my qhy 290 ever.. it's simple, set your spacing to the imaging camera, measure the distance from prism to imaging sensor and set the same for the guidecam incorporating the backfocus distance.. point to a  decent starfield and fine tune the guide cam on the turret.. that's it... Like you say that's probably the best choice you have with a sct

Ok, thanks for confirming. I must say I have been a bit surprised hearing OAG people say you have to rotate to find a guidestar. There is not exactly  a shortage of stars in the galaxy (about 10^11 I believe). Besides, I operate my scope from indoors and I really don't want to have to disconnect everything, take the laptop outside and reconnect it and then fiddle about rotating a guider before disconnecting it, bringing it back inside and reconnecting everything. That's exactly why I use a motor focuser instead of twiddling knobs on the scope! 

22 hours ago, Padraic M said:

Stuart, maybe you need to try it out! I found it quite confusing the first time out and it took a lot of experimentation and mistakes to get it right (C8 XLT + FF/FR + TS-Optics OAG) but I think the guys above and James Lamb have covered it all.

If you're not using the 0.7x reducer (and I believe you're not) then it's more straightforward but still not simple. The James Lamb video is pretty good - I had to draw my own CAD diagrams (I did it on paper) to know exactly what distances were available, but the critical part is that the distance from prism to imaging sensor is the same as prism to guide sensor. Focuser goes ahead of the entire assembly, whether that's the EdgeHD primary mirror or the Diamond Steeltrack. The design of the OAG may constrain you somewhat - the closer the prism is to the imaging sensor, the further in to the OAG stalk the guidecam needs to be inserted. You may not be able to insert it far enough; if so you'll need an extra spacer between OAG and imaging cam to give some extra room.

If you're using a filter wheel or holder it goes between the OAG and the imaging camera so that you're getting full non-filtered illumination on the guidecam sensor (especially if using narrowband filters). James Lamb's proposed arrangement of imaging camera-filter wheel-OAG-spacers-telescope makes a lot of sense and that's what I've used.

I use the 290mm Mini guidecam and have never had to search/rotate for a suitable star. PHD2 is excellent in guiding on the oddest shaped stars, including bean-shaped and seagull-shaped. It also doesn't need focus to be spot on. If you use parfocal filters the small difference in optical path length through different filters shouldn't impact you.

Your focuser must go between OAG and telescope, because if it's between OAG and imaging camera it will change the critical distance between prism and imaging sensor, but the distance from prism to guide cam sensor will stay the same (the two cameras are no longer parfocal).

If you decide to add the reducer all of this will change and your distances will become more critical, as the back-focus distance from the back edge of the reducer to the sensors (both cameras) must be 146mm. You will not be able to put the Diamond Steeltrack, Crayford or R&P anywhere between the reducer and camera as it will change this back focus. I don't believe it can go ahead of the flattener (i.e. scope-side) either so you may be stuck with focusing by adjusting the primary mirror. (As I'm writing this I wonder does the EdgeHD have a built-in flattener with a fixed back-focus?)

Finally, when you have it set up correctly, try to keep the entire imaging assembly intact even if you take it off the scope. One advantage of the OAG setup is that once the two cameras are set correctly you never need to adjust them again. 

Pegasus Astro do an interesting OAG with a focus motor on the stalk. This gives more automation than most of us need, but sounds like a good idea for remote setups if the OAG could go out of alignment.

Thank you. this is very helpful. It's also very encouraging that you can use such a small sensor for this (the 290 is weeny!). Various people have told me you need a huge sensor for OAG (even an APS-C which seems a very pricey option!). 

So I am thinking of getting the Celestron OAG because it has a 12mm prism. The Pegasus sounds interesting (having a motor focuser) but it has a smaller prism (10mm) and for what I can gather, you need the largest prism possible to maximise guidestar choice

Edited by StuartT
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10 hours ago, StuartT said:

There is not exactly  a shortage of stars in the galaxy

You’d think that, but some targets eg M51 are very short on stars. I struggled with a 120MM on 100% gain to get a star. I switched to a 290MM and quite often have the gain at 50%.  And my prism is 8mm 
 

I’m switched to an oag recently on a c.500mm dual refractor setup.  It’s not at all complicated to setup or focus.  I could have used a guidescope but this is just a very neat solution that allows me to consistently guide at (less than or equal) to 0.5” RMS (1/3 of my imaging resolution).

I didn’t read all the posts above, so apologies if this is a repeat.
 

This is how I remember doing it - just steps to be done in this general order.

1) position OAG before filter wheel and camera ie closest to focuser.  So either screw onto focal reducer or insert into drawtube.

2) calculate and set back spacing to desired length for reducer - oag might  be 16.5mm if ZWO, so subtract this off back spacing requirement.

3) align prism with long edge of camera sensor - I mark the body of camera with sharpie on a printed label to make it easy to locate.  Undo grub screws and move prism stalk in or out to get it close to the camera sensor without blocking it.  I remove whole assembly from focuser and eyeball to do this step.

4) refit everything and attach guide camera.  During day time adjust position of main telescope camera so it is in good focus on a distant tree or lamppost. Next adjust position of guide camera along stalk (don’t move stalk itself) to get it in focus.

5) Repeat step 4) at night.  I use the moon and recheck on a star after.

 

 

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13 hours ago, StuartT said:

 

So I am thinking of getting the Celestron OAG because it has a 12mm prism. The Pegasus sounds interesting (having a motor focuser) but it has a smaller prism (10mm) and for what I can gather, you need the largest prism possible to maximise guidestar choice

I'm using a Celestron version,while the 12mm prism is nice I find the body bulky and takes up so much room.. I found I needed to use the thinnest adapter with m48 opening and then search the internet and buy a m48 to SCT adapter to attach it to the scope, not inc in the kit and at the time wasn't that easy to find.. this had to be fairly small as it incorporates into the backfocus figure, with a hd version I believe you should have more space... The turret isn't the best and has never  worked ( I bought it secondhand , so I couldn't sent it back)

Once it's set and locked down it works all ok, I don't rotate my imaging camera, and as said previously I don't search for guidestars so that's locked down too, the Qhy 290 is more than sensitive enough

I'm not too sure what else is out there as I've not looked but I think the Celestron was made for DSLR use , wasn't really that we'll thought about which I find is usual for the Celestron brand( other than a tube I don't think I'd by another Celestron product) 

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