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Zwo 174 or 290 for guiding a 8"edge

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#1 miwitte

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Posted 26 June 2019 - 10:25 PM

I've come to the conclusion that my ASI 120 won't cut it for guiding with my 8" edge/reducer and ZwO OAG combo. Given the larger pixels of the 174 in the longer FL scope it would seem this is the choice I should purchase VS the 290. Both seem to have same sensor size and sensitivity just the pixel size is different.

Would like to hear reasons why either one.

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#2 mistateo

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Posted 26 June 2019 - 10:55 PM

For the exact reason you already mentioned, the larger pixels of the 174.



#3 RossW

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Posted 27 June 2019 - 08:25 PM

Hello miwitte,

 

The 174 is the way to go with your Edge, due more to the huge sensor (1/1.2") than the larger pixels, but unfortunately it won't work well with your ZWO OAG because it has such a small prism and thus a small FOV illuminating the sensor. You'll see strong vignetting and only a tiny portion of the 174's sensor will be illuminated, so most of the sensor will be wasted.

 

On the other hand, the sensor in the 290 is basically the same area as the 120, so IMHO it won't provide you with any major advantage over the 120 you already have; you'll still suffer from a lack of suitable guide stars. With an Edge and any long focal length scope IMHO the main problem is a lack guide stars in the FOV of the OAG guide camera. I've faced situations where I had zero guide stars in the FOV when using an ASI120mm on the ZWO OAG.

 

Based on the above, if you want to stay with the ZWO OAG, you may want to consider the ASI178mm or the QHY equivalent (QHY5-III 178M Mono). The 178's sensor is about twice as large as the 120/290, so on average you'll see twice as many stars in the FOV. It does vignette a little on the ZWO OAG but all the sensor's area is usable. I have used the ASI178mm successfully on the ZWO OAG by fitting it directly to the M42 connector, but adjusting focus was difficult.

 

I've now moved to the Celestron OAG and it is a huge improvement over the ZWO OAG in terms of FOV area. It barely squeezes into the imaging train of my EDGE HD 1100 while maintaining the 146mm backfocus spec (external focuser/OAG/EFW/Camera). If I understand correctly the backfocus of the 8" Edge might not allow such a large OAG though, assuming you use an external focuser. If it will fit then that is what I would recommend: The ASI 174 and the Celestron OAG. A pig heaven combination.

 

Cheers,

 

Ross


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#4 kisstek

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Posted 27 June 2019 - 08:44 PM

For an OAG, don't you want the sensor to capture the entire OAG image circle? Anything smaller seems like you could be missing potential guide stars. Anything bigger is just a bunch of unused pixels.

 

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#5 JP50515

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Posted 27 June 2019 - 11:13 PM

Hello miwitte,

The 174 is the way to go with your Edge, due more to the huge sensor (1/1.2") than the larger pixels, but unfortunately it won't work well with your ZWO OAG because it has such a small prism and thus a small FOV illuminating the sensor. You'll see strong vignetting and only a tiny portion of the 174's sensor will be illuminated, so most of the sensor will be wasted.

On the other hand, the sensor in the 290 is basically the same area as the 120, so IMHO it won't provide you with any major advantage over the 120 you already have; you'll still suffer from a lack of suitable guide stars. With an Edge and any long focal length scope IMHO the main problem is a lack guide stars in the FOV of the OAG guide camera. I've faced situations where I had zero guide stars in the FOV when using an ASI120mm on the ZWO OAG.

Based on the above, if you want to stay with the ZWO OAG, you may want to consider the ASI178mm or the QHY equivalent (QHY5-III 178M Mono). The 178's sensor is about twice as large as the 120/290, so on average you'll see twice as many stars in the FOV. It does vignette a little on the ZWO OAG but all the sensor's area is usable. I have used the ASI178mm successfully on the ZWO OAG by fitting it directly to the M42 connector, but adjusting focus was difficult.

I've now moved to the Celestron OAG and it is a huge improvement over the ZWO OAG in terms of FOV area. It barely squeezes into the imaging train of my EDGE HD 1100 while maintaining the 146mm backfocus spec (external focuser/OAG/EFW/Camera). If I understand correctly the backfocus of the 8" Edge might not allow such a large OAG though, assuming you use an external focuser. If it will fit then that is what I would recommend: The ASI 174 and the Celestron OAG. A pig heaven combination.

Cheers,

Ross


You literally just saved me from making a new post. Thanks mate.

Are you using a moonlight?
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#6 bobzeq25

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Posted 28 June 2019 - 10:17 AM

RossW nailed it.  I also have the Celestron OAG and a 174.  On a ZWO OAG you'd be wasting a lot of money.



#7 miwitte

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Posted 28 June 2019 - 10:26 AM

the challenge is i have the moonlite on there so the celestron OAG wont fit. Plus i already have the ZWO OAG. the 120 doesnt work at all so i am stuck right now and need to get a guide camera. After all the grief on gone through with this setup and collimation i need to move forward and get guiding working so i can finally start imagining. Right now its a giant paperweight and i want to make sure i get the right camera that others have used with the ZWO and EDGE combo.



#8 John Miele

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Posted 28 June 2019 - 12:01 PM

I'm using the ASI174mini to guide my Edge8 with the Orion TOAG and it works just fine. I would rather have the Celestron OAG with it's larger prism but as was already pointed out, it just won't allow you to maintain the 105mm backfocus. I posted this thread the other day detailing how tricky it was to get the 105mm even using a thin bodied guider like the TOAG with a Moonlite focuser. But it can be done.

 

https://www.cloudyni...-edge8-reducer/

 

It's true using the 174mini means I am wasting a lot of the sensor real estate but I'm still getting all the guide stars that are possible to get using a thin bodied OAG like the Orion (or ZWO). Right? Plus the mini is a capable planetary camera to use when the planetary imaging urge strikes. I bought mine used and it was not all that expensive. YMMV...John


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#9 gundark

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Posted 28 June 2019 - 12:47 PM

I use the 174 with an OAG that has a 12 mm prism. Works beautifully.

#10 Stelios

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Posted 28 June 2019 - 02:04 PM

I agree that the 174 vignettes with the ZWO OAG. 

 

HOWEVER:

 

1) I've never had a problem finding guide stars, pre- or post-meridian flip (without having to fiddle with guidecam position). If someone can give me a notoriously difficult object for finding guide stars for, I'll be able to report how it does there.

 

2) *Most importantly*: I am not married to my ZWO OAG. I may buy a Celestron or different OAG in the future, which will utilize the 174 at its best. 

 

ALSO:

 

I have heard little mentioned here about camera sensitivity. My understanding is that the 290 is much more sensitive (defined how?) than the 120, and that the 174 is about as sensitive as the 290 but with a larger FOV. Is this true, urban legend, or what? Same question with respect to "sensitivity" being key to PhD2 being able to both find and not lose stars. Right? Wrong?


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#11 miwitte

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Posted 28 June 2019 - 02:23 PM

So how does image scale play into guiding? I know we want to be at a good image scale for the imaging camera. The 178 works out to .35", the 290 works out to .42" and the 174 is .85". Im imaging at .55" with the 1600. Is the larger scale of the 174 better for guiding in my case. Also as Stelios said i may end up with a OAG with bigger mirror(if i can fit one.) 



#12 Stelios

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Posted 28 June 2019 - 03:03 PM

I don't think the image scale is relevant here. The rough guidelines is to keep the guider scale to no more than 3-5 times the camera scale. For example, the popular Orion "magnificent mini" guidescope gives an image scale of 6.62" -- so you should not use it to guide more than around 1.3". Thus with an ASI1600 you would need to be imaging at x = 206.3*3.8um/1.3 ~= 600mm or less. With a DSLR (larger pixels) you could do more. And that's an approximate guide.

 

*Any* OAG camera will comfortably beat even the most conservative of such ratios. 

 

BTW, I don't quite see why the Celestron OAG won't work with the Moonlite. Agreed that it won't work at the recommended positioning of halfway between the range, but I think that recommendation is for convenience's sake--to leave you the maximum flexibility in focusing (if anyone knows ANOTHER reason, speak up!). The Moonlite focuser can adjust from 36.2mm to 49.2mm. The recommended positioning is at about 43.7mm, but I think that even at 39mm (from reducer) would have plenty of room for temp change refocusing. The Celestron OAG is 29mm, the ZWO EFW8 + camera is 26.5, so if we can keep the required spacers for connections to 105-39-29-26.5 = 10.5mm, we're in business. 

 

The only reason I haven't tried that out, is that for my refractors all reducers and flatteners require 55mm--which is more than 29 + 26.5. If I could find a reducer or flattener with *more* than the 55mm standard (a pox on you, DSLRs!!! :)) then I would do it in a flash.



#13 John Miele

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Posted 28 June 2019 - 04:33 PM

Hi Stelios,

 

"if we can keep the required spacers for connections to 105-39-29-26.5 = 10.5mm, we're in business."

 

That is exactly the problem. If I used a shorter spacer between the OAG and the ASI071 camera, I could not lower the guide camera close enough to reach focus. I just don't think you can maintain the 105mm backfocus by using shorter spacers and get the main camera and the guide camera to  reach focus at the same time. I may be wrong and I would sure love to see how to solve the problem!

 

John

 

Edit: On third thought...maybe there is a way to fit the Celestron OAG in the train depending on the distance from the prism to the camera chip. I'm thinking maybe it could work after all. I may just have to buy one and try!


Edited by John Miele, 28 June 2019 - 04:42 PM.

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#14 RossW

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Posted 29 June 2019 - 01:24 AM

Are you using a moonlight?

 

No I'm using the Baader SCT focuser on my Edge 11:

 

https://www.baader-p...steeltrack.html

 

Take off the 2" eyepiece clamp and you're down to only about 85mm of backspace taken. It fits directly onto the 3.25" thread of the Edge. I then go M48 to the Celestron OAG and then M42 to the filter wheel.

 

By coincidence I just purchased a Moonlight CHL 2.5" SCT focuser, to fit my new Meade 16" ACF, but I also purchased a spare flange to fit the Edge too, just in case. Backfocus-wise the Moonlight seems to be about the same length as the Baader when you include the adapters (haven't measured it accurately), however, it can be easily rotated to framing a DSO; the Baader cannot be easily rotated.

 

Cheers,

 

Ross



#15 RossW

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Posted 29 June 2019 - 01:51 AM

Edit: On third thought...maybe there is a way to fit the Celestron OAG in the train depending on the distance from the prism to the camera chip. I'm thinking maybe it could work after all. I may just have to buy one and try!

If I understand it correctly, adjusting the prism height will not change the guide camera's focal position, it simply changes the guide camera's FOV. I think that's what I have read here on CN, and it is also my own experience (from memory). 



#16 miwitte

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Posted 29 June 2019 - 08:57 AM

So sounds like the 174 mini is the recommendation? Looks like several with the zwo OAG are using it successfully, then if I find a better OAG that fits down the road I can use more of the sensor.

 

and I think part of the celestron issue is trying to get the image sensor and guide sensor at same focus which will probably require a spacer between OAG and imagining camera. The 8" reducer is only 105mm backspacing the larger 9-14 edges is 146mm so no issues with them.



#17 Stelios

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Posted 30 June 2019 - 12:38 AM

Hi Stelios,

 

"if we can keep the required spacers for connections to 105-39-29-26.5 = 10.5mm, we're in business."

 

That is exactly the problem. If I used a shorter spacer between the OAG and the ASI071 camera, I could not lower the guide camera close enough to reach focus. I just don't think you can maintain the 105mm backfocus by using shorter spacers and get the main camera and the guide camera to  reach focus at the same time. I may be wrong and I would sure love to see how to solve the problem!

 

John

 

Edit: On third thought...maybe there is a way to fit the Celestron OAG in the train depending on the distance from the prism to the camera chip. I'm thinking maybe it could work after all. I may just have to buy one and try!

 Not sure what the problem would be with the ASI071. Apparently it has 17.5mm sensor depth (as opposed to the 1600's 6.5mm) but needs no FW. So the recommendation is: 16.5mm FW + 21mm spacer + 17.5mm ASI071 = 55mm. 

 

So with the Moonlite and Celestron OAG you'd need 105-39(moonlite)-29(Celestron OAG)-17.5 (ASI071) = 19.5mm worth of spacers. So ditch the 21mm, and use two spacers--this is the standard config:

 

FR => Moonlite => Spacer1 => OAG => Spacer2 => [optional EFW] => camera.

 

Spacer1 + Spacer2 must be 19.5mm. The *smaller* Spacer1 is, the higher up you'll be able to position the OAG camera to make it parfocal. 

 

Spacers are a dime a dozen. There's also the Stellarvue variable 8-12mm spacer. Plus Agena Astro carries a boatload of them.



#18 RossW

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Posted 30 June 2019 - 12:59 AM

Spacer1 + Spacer2 must be 19.5mm. The *smaller* Spacer2 is, the higher up you'll be able to position the OAG camera to make it parfocal.

 

Stelios, mate, you've just blown my mind with that comment. Please help this noob understand why making S2 shorter means moving the guide camera higher, i.e., further away from the OTA?

 

Here's my (faulty) thinking. Let's assume both guide and imaging cameras are in focus. I now reduce S2 by 1mm and thus have to add 1mm to S1 to maintain the 19mm spacing. Imaging camera is still in focus because its distance from the OTA is the same. But the guide camera's distance from the OTA has now increased by 1mm; it's too far away from the OTA and thus out of focus. So, in my mind, to achieve focus again, I would need to move the guide camera closer to the OTA, not further away.

 

I assume that your "higher" = away from the OAG prism?

 

Let me have it wink.gif   

 

Cheers,

 

Ross



#19 bobzeq25

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Posted 30 June 2019 - 11:19 AM

So sounds like the 174 mini is the recommendation? Looks like several with the zwo OAG are using it successfully, then if I find a better OAG that fits down the road I can use more of the sensor.

 

and I think part of the celestron issue is trying to get the image sensor and guide sensor at same focus which will probably require a spacer between OAG and imagining camera. The 8" reducer is only 105mm backspacing the larger 9-14 edges is 146mm so no issues with them.

I didn't have any problem getting the guide camera focused with a Celestron OAG, using the common 1.25 inch form factor for the camera, which is quite flexible. 

 

I did have a problem getting the imaging train to work with a 105mm backfocus field flattener (that required a thread adaptor), the OAG, a filter wheel, and a camera.  I wound up getting a PreciseParts thread adaptor, which was the shortest solution I could find.



#20 Stelios

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Posted 30 June 2019 - 01:08 PM

Spacer1 + Spacer2 must be 19.5mm. The *smaller* Spacer2 is, the higher up you'll be able to position the OAG camera to make it parfocal.

 

Stelios, mate, you've just blown my mind with that comment. Please help this noob understand why making S2 shorter means moving the guide camera higher, i.e., further away from the OTA?

 

Here's my (faulty) thinking. Let's assume both guide and imaging cameras are in focus. I now reduce S2 by 1mm and thus have to add 1mm to S1 to maintain the 19mm spacing. Imaging camera is still in focus because its distance from the OTA is the same. But the guide camera's distance from the OTA has now increased by 1mm; it's too far away from the OTA and thus out of focus. So, in my mind, to achieve focus again, I would need to move the guide camera closer to the OTA, not further away.

 

I assume that your "higher" = away from the OAG prism?

 

Let me have it wink.gif   

 

Cheers,

 

Ross

You are 100% right. I edited my post--I meant Spacer1. I shouldn't post when tired... But at least you got the idea, and how to adjust the two spacers to make sure you can come to focus.


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#21 freestar8n

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Posted 30 June 2019 - 02:20 PM

Hi-

 

I actually recommend making spacer 2 as small as possible - even eliminating it completely.

 

You want the guide sensor to be as close to the prism as possible for a good view of the pupil.  That means brighter guidestars, a wider field, and less aberration.

 

To do that, you want to keep the distance from the oag to the main imaging sensor as small as possible.  You have to have the filterwheel on that side - but other than that you should only use spacers after the oag if either 1)  You need spacers to provide clearance for the filterwheel or something, or 2)  You end up unable to focus the guide sensor because you can't get close enough to the prism.

 

It isn't automatically a problem if you had a small spacer after the OAG - but any spacer there will push the guide sensor away from the prism.  And that will likely lose light from the guidestar.

 

Frank


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#22 Stelios

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Posted 30 June 2019 - 03:03 PM

Hi-

 

I actually recommend making spacer 2 as small as possible - even eliminating it completely.

 

You want the guide sensor to be as close to the prism as possible for a good view of the pupil.  That means brighter guidestars, a wider field, and less aberration.

 

To do that, you want to keep the distance from the oag to the main imaging sensor as small as possible.  You have to have the filterwheel on that side - but other than that you should only use spacers after the oag if either 1)  You need spacers to provide clearance for the filterwheel or something, or 2)  You end up unable to focus the guide sensor because you can't get close enough to the prism.

 

It isn't automatically a problem if you had a small spacer after the OAG - but any spacer there will push the guide sensor away from the prism.  And that will likely lose light from the guidestar.

 

Frank

Excellent point, however I couldn't quite make it work without spacers--and the smallest I had was a 5mm. Even a 2mm would've done it, but I've never had a problem finding stars and so I'm not in a rush to spend $ to solve this "issue."



#23 John Miele

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Posted 30 June 2019 - 08:12 PM

If I understand it correctly, adjusting the prism height will not change the guide camera's focal position, it simply changes the guide camera's FOV. I think that's what I have read here on CN, and it is also my own experience (from memory). 

Ross,

 

Now I am confused (fairly normal for me)...I thought the distance from the camera sensor to the center of the OAG had to to be the same as the distance from the prism to the guide camera sensor. I think you are saying that is not the case?

 

I always thought it had to be like the figure in post #13 of this thread...

 

https://www.cloudyni...er-help-needed/


Edited by John Miele, 30 June 2019 - 08:27 PM.


#24 RossW

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Posted 30 June 2019 - 09:45 PM

Hey John,

 

Well I may well be wrong grin.gif  Probably am. Previously I needed to move a guide cam further "in" but it was as far into the holder as it could go. So I thought that perhaps I could gain a little extra focal quality by moving just the prism stalk, but to my surprise I found that focus really didn't change. Perhaps the focus did change but I just couldn't perceive it (?) Then later when reading CN I read a comment that suggested moving just the prism stalk doesn't change focus. 

 

I hope someone can set me straight if I have this wrong.



#25 kisstek

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Posted 30 June 2019 - 10:57 PM

Ross,

 

Now I am confused (fairly normal for me)...I thought the distance from the camera sensor to the center of the OAG had to to be the same as the distance from the prism to the guide camera sensor. I think you are saying that is not the case?

 

I always thought it had to be like the figure in post #13 of this thread...

 

https://www.cloudyni...er-help-needed/

The common point in both light paths is the front flange of the OAG. The distance from that flange to the mirror to the guide camera sensor should be the same distance as from the flange through the rest of your optical train (EFW, spacers, ...) to your imaging sensor.

 

Since you move the periscope parallel to the front flange, none the distances in that light path change. Only when you move the guide camera relative to the mirror/prism does a distance change and hence, the focus changes.


Edited by kisstek, 30 June 2019 - 10:58 PM.

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