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Wide-field scope options: AT102ED vs. 6" f/5 Newtonian (w/ optional AT80ED)

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#1 astro.runner

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Posted 25 October 2020 - 02:55 PM

My first post. Quick intro - I was super keen on astronomy as a teenager (even ground my own 4" mirror @ age 13), but life/career choices put the hobby on hold for the next 25+ years. My interest was recently rekindled after travel to a couple of high-altitude Bortle 1 sites and Neowise. Purchased an Evolution 8 GoTo SCT a few months ago and have been enjoying views of Jupiter's GRS, Saturn (Cassini's division still eludes me!), Mars' dark regions and polar caps, as well as the brighter Messier objects. My backyard is Bortle 8 but I have made one trip to a Bortle 4 site and enjoyed dark sky views of M27, M31, and M13. In terms of accessories, so far I've invested in the Baader 8-24 Zoom EP, 0.63X focal reducer, and a few cheap color/UHC filters.

 

While the Evolution 8 is a nice scope, its limitations are becoming clearer to me now. Wide-field is one of them - the Pleiades are rising and I get barely half of the cluster in my FOV. The Double Cluster is another difficult target. So I put in an order for the AT102ED last month (Astronomics supplied an ETA of December). In the meantime, I came across some fast higher-aperture Newtonians (e.g. Apertura/GSO 6" f/5) which are wide-field capable at significantly lower cost ($199 vs. $599). In fact, I could get both the Newtonian and the AT80ED ($399) for the same price as the AT102ED. So, my questions for the wise folks on this board:

 

1. What are the performance tradeoffs between AT102ED and the 6" f/5 Newtonian? I understand the latter may need a coma corrector for wide-field 2" EPs. Will this be a problem if I use a 40mm 1.25" Plossl or the 56mm 2" Plossl?

 

2. Is there a significant portability difference between AT102ED and the Newtonian for grab-n-go? Spec-wise their weight seems similar (9 lbs vs. 10 lbs).

 

3. If I choose the Newtonian route, is it worth getting the AT80ED as an additional option for larger FOV/portability?

 

4. Is it possible to use the Evolution 8 GoTo mount with any of these scopes? How does their clearance/moment arm compare with the C8 OTA? Eventually I plan to get a manual AltAz mount (Versago II, Voyager 2, etc.) for the wide-field scopes.

 

If it helps, I am value conscious but not necessarily budget-constrained. And I do plan to "get a Dob" (12-14") within the next couple of years to complete my stable smile.gif

 

-- astro.runner


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#2 Echolight

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Posted 25 October 2020 - 05:06 PM

I went with an 80mm ED f7.5 refractor first. Neither of the others can compete for wide field or low weight and stability on a mount. Although the AT102ED is surely a more well rounded scope with more light gathering and ability to reach a little deeper. But for the same stability the 4 inch will likely need a bigger more expensive mount.

 

A 6 inch newt should beat the 4 inch in resolution and magnitude. So yeah, the combo of the 6 inch and the 80 would cover more sky.
The one thing about the newt is that the focuser is on the high end. So you'll either need a low tripod or a tall chair. Or you could stand all the time.
Also most of those 6 inch f5 newts are optimized for photography, not visual. So they'll have a larger obstruction, secondary mirror, and will need a focuser extender. And the f5 will limit you to around a 30mm eyepiece due to oversized exit pupil with longer focal lengths

 

Think I'd throw in another $200 and get a base model 8 inch f6 dob and an 80ED on a light alt/az mount. Or $300 extra and get the 80mm refractor and a deluxe dob with 2-speed focuser.

But this may end up being less in the end if you're not happy with the stability of the 102 on the lightweight mount.

The 8 inch f6 dob with 2 inch focuser will be capable of nearly three times the field of view of a C8 with 1.25 inch diagonal....assuming widest field eyepieces for each.

 

One fairly inexpensive accessory you could add to the SCT to get a wider field of view is a .63 reducer.

 

Of course with any of the scopes you'll need the appropriate wide angle eyepieces to get the widest TFoV.


Edited by Echolight, 25 October 2020 - 05:20 PM.


#3 spereira

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Posted 25 October 2020 - 05:27 PM

Moving to Equipment.

 

smp



#4 izar187

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Posted 25 October 2020 - 05:50 PM

The 6" is capable of a 3 degree widest field in view.

I believe the 102 will yield larger.

These 102's are pretty popular now, with lots of happy owners. This says something about their performance.

The 6 will out resolve it, noticeably to my eye, revealing more.

The 102 a bit less, but it will be cleaner, due to little slower focal ratio, no coma, no obstruction.

 

You do not need a coma corrector for the 6. But the field in view will benefit from one.

Could wait until the 12" to 14" you are planning for later. Or not. : )

 

40mm and 55mm ep' are too low power for an f/5.

Or rather there is a very good chance you will see the central secondary obstruction with the 40mm.

Pretty much a guarantee of seeing it with the 55mm.

Use a 32mm in my short 6, and it works perfectly for me out under rural sky.

If you are in town under brighter sky, then you may opt for a 28mm 2". Or less, but with wider field.

That would again, transfer right over into use in your future 12 to 14.

 

The 6 for me is one position seated viewing, alt-az pipe mounted.

The 102 must be very close to that, with a diagonal that can rotate. 

 

BTW, welcome aboard too.


Edited by izar187, 25 October 2020 - 05:54 PM.


#5 vtornado

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Posted 25 October 2020 - 06:23 PM

With a bortle 8 backyard, a widefield telescope is really crippled.   The big wide fields are washed out.  So maybe, you want to limit how much

you spend on one???   I have two wide fields, an 80mm f/5 Celestron first scope with a 2 inch focuser.   Massive 6 degree field of view without

too much invested ($175 and a lot of that money is in the focuser).  I also have an Orion ED80.  Not quite as wide, but if you spot something

while sweeping (like a double star) it can handle having the power pushed up.  Where the 80 f/5 achro does not do high power well.

BTW to get a well corrected big field in an f/5 scope you do have to drop a lot of money into an eyepiece.  (Probably north of $250)

 

I do have a 6 inch f/5 newt.   The view is close to a 100mm ED.  I am not so picky about coma.  If you are it could use a coma corrector.

 

Others are correct about the 100 f7.  They are great scopes, but you do have to go up from the lightweight cheap alt az mount to

a heavier more expensive one.  The 150 f/5 newt is about the same, the low cost mounts hold it for low - mid power,

high power things get tricky.


Edited by vtornado, 25 October 2020 - 06:27 PM.


#6 Tony Flanders

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Posted 25 October 2020 - 07:23 PM

The 6" is capable of a 3 degree widest field in view.

I believe the 102 will yield larger.

Barely so. The 6-inch Newt has a 762-mm focal length, the 102 ED scope 712 mm. So the refractor's field of view is less than 8% wider.

 

I agree that it makes no sense at all to use a 55-mm eyepiece in an f/5 Newtonian. In fact, it doesn't really make sense to use a Plossl at all. I would consider either a wide-field eyepiece in the 40-mm range or a superwide in the 30-mm range.

 

Another solution to the OP's narrow-TFOV dilemma would be to enhance the SCT with a reducer/corrector, a 2-inch visual back, or both.


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#7 astro.runner

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Posted 25 October 2020 - 10:06 PM

Thanks, all, this is super useful information. After reading your comments, I've decided to stick to my original plan of getting the AT102ED for wide-field views and also to have one capable, quality refractor in my stable given my future plans to get a larger reflector. The suggestion to invest in 2" EPs for the 8" SCT is also a good one (I already have the 0.63X focal reducer). I would also use these EPs on the AT102ED. So here are a couple of follow-on questions:

 

1. What are some good 2" EPs to get TFOV ~2 degrees on the C8 w/reducer and > 3 degrees on the AT102ED? Currently I have a 43 degree 40mm Plossl that maxes out at 1.3deg TFOV on the SCT. Some candidates:

Which one to try first? I believe the 50mm+ EPs might produce exit pupils that are too large, despite the larger TFOV. And will these EPs work for the SCT, or will they also show the central obstruction?

 

2. I understand 2" diagonals for refractors and SCTs are different in how they fit in the visual back/focuser. If I plan to get just one 2" diagonal initially, which one should I get? Is there an adapter to cross-use on the other scope?

 

3. What's the best AltAz mount for the AT102ED? The Voyager 2 claims 18lb payload and Vixen Porta II claims 20lb. Will one of these be sufficient?

 

Thanks in advance for your advice.

 

-- astro.runner


Edited by astro.runner, 25 October 2020 - 10:07 PM.


#8 Echolight

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Posted 25 October 2020 - 11:38 PM

I don't think the reducer is designed to work with 2 inch, is it? From what I understand, if you try to get more than 1.3 degrees TFoV in a C8 there will be significant vignetting.

 

You really don't want to go over 7mm exit pupil. I'd probably try and stay closer to 6mm.

 

Max TFoV for the refractor would be a 38-40mm 70 degree.

 

You could probably use the Porta II head with a heavier tripod.


Edited by Echolight, 26 October 2020 - 12:02 AM.

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#9 Thomas Marshall

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Posted 26 October 2020 - 02:16 AM

You can get a 2" adapter that screws on where the visual back on the Sct. is, but does not have a collar. Then a 2" refractor diagonal will work in both scopes.    2" eyepieces like my Orion Q70/26mm and my Arcturus 32mm 70*  are great fun in the C8 on DSO's with my Stellarvue 15mm 1 1/4" 82* and Baader 8-24mm zoom for higher power looks. I had to tap an extra hole in the 2" adapter, for an additional thumbscrew, as 1 screw on the compression ring was not enough to hold things tight when using a binoviewer. 1 thumbscrew is enough for even the 2" eyepieces and diagonal alone though. 


Edited by Thomas Marshall, 26 October 2020 - 02:39 AM.


#10 howardcano

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Posted 26 October 2020 - 02:43 PM

What's the best AltAz mount for the AT102ED? The Voyager 2 claims 18lb payload and Vixen Porta II claims 20lb. Will one of these be sufficient?

Echolight is correct; the Porta worked well with my AT102ED and SV102 Access (the Access being a tad heavier), after I upgraded the legs.  I made a simple set from pre-cut oak 1x2's, and it now has sufficient stability for planetary magnifications.


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#11 astro.runner

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Posted 26 October 2020 - 05:44 PM

 

Echolight said:

 

I don't think the reducer is designed to work with 2 inch, is it? From what I understand, if you try to get more than 1.3 degrees TFoV in a C8 there will be significant vignetting.

This is interesting. Since I'm already getting 1.3 degrees TFOV on my C8 with the focal reducer and 40mm Plossl (43deg AFOV), does this mean there's not much additional value in getting 2" EPs for the SCT? If this is the case, I can focus my 2" EPs to optimize performance on the refractor instead. Where can I find more information on the optical theory behind the vignetting effect for SCTs? What impact does it have for visual?

 

 

Echolight said:

 

You could probably use the Porta II head with a heavier tripod.

I'll probably do this - the Porta II head is available separately and I can just use the Evolution tripod, which seems pretty sturdy.

 

 

Thomas Marshall said:

 

You can get a 2" adapter that screws on where the visual back on the Sct. is, but does not have a collar.

I'd appreciate a link to this adapter. I plan to get the Astronomics 2" refractor diagonal.

 

-- astro.runner

 



#12 jonee523

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Posted 26 October 2020 - 07:46 PM

Were you planning to run the scopes at the same time? I ask because the AT80ED should be able to mount onto your current mount without and issue. I would think you would be able to use either scope the AT80ed when you are wanting those wonderful wide views. Then for the detail planetary or small DSO you have your big scope. If you want a separate mount for both scopes. I have a porta mount with wood tripod legs I made, that got my vibrations down to under 2 secs closer to 1. I use a SV102 access about 10 lbs... before the leg upgrade the vibrations was closer to 3-4 secs. I found the plans for the legs here in the CN archives. The total cost was about 40 in materials....

 

Jon



#13 Echolight

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Posted 26 October 2020 - 09:26 PM

This is interesting. Since I'm already getting 1.3 degrees TFOV on my C8 with the focal reducer and 40mm Plossl (43deg AFOV), does this mean there's not much additional value in getting 2" EPs for the SCT? If this is the case, I can focus my 2" EPs to optimize performance on the refractor instead. Where can I find more information on the optical theory behind the vignetting effect for SCTs? What impact does it have for visual?

 

I'll probably do this - the Porta II head is available separately and I can just use the Evolution tripod, which seems pretty sturdy.

 

I'd appreciate a link to this adapter. I plan to get the Astronomics 2" refractor diagonal.

 

-- astro.runner

 

 

If you make a specific thread on the Cats & Casses forum about using the reducer and a 2 inch diagonal and eyepieces with a C8 you will probably get some better answers from some more experienced users than what I could give you.

 

I skipped the reducer and went the 2 inch route because all my scopes and any future scopes I might have will have 2 inch focusers, and I didn't want to buy extra 1.25 eyepieces just for the C8.



#14 Thomas Marshall

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Posted 27 October 2020 - 01:51 AM

This is interesting. Since I'm already getting 1.3 degrees TFOV on my C8 with the focal reducer and 40mm Plossl (43deg AFOV), does this mean there's not much additional value in getting 2" EPs for the SCT? If this is the case, I can focus my 2" EPs to optimize performance on the refractor instead. Where can I find more information on the optical theory behind the vignetting effect for SCTs? What impact does it have for visual?

 

I'll probably do this - the Porta II head is available separately and I can just use the Evolution tripod, which seems pretty sturdy.

 

I'd appreciate a link to this adapter. I plan to get the Astronomics 2" refractor diagonal.

 

-- astro.runner

If you Google 2" visual back for sct, there are 3 or 4 that show up at $20-$25. "High Point" seems the best, as some others are "on the way", but they are "in stock". Plus, theirs has double thumb screws already for extra strength. $25.00. I also use mine with the "Astronomics 2" diagonal" - works great.  



#15 wrvond

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Posted 27 October 2020 - 08:33 AM

Here's what I have on my C8:

 

The Celestron f/3.6 reducer screwed directly to the back of the OTA (2") going to a William Optics 2" SCT visual back with Roto-lock, Baader 2" BBHS mirror diagonal with Clicklock, and Teleview 31mm Nagler T-5.

 

Included is a representation of the view presented through this optical chain looking at M31.

 

 
For comparison, here is a representation of M31 through my Sky-Watcher 120 ED Pro with Baader 2" prism diagonal and the N31T5:
 

 

For good measure, here's an Astronomy Tools depiction of M31 through my XT10g using a Paracorr and the N31T5:

 

 


Edited by wrvond, 27 October 2020 - 08:48 AM.

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#16 astro.runner

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Posted 27 October 2020 - 09:27 PM

Thanks Thomas and wrvond (esp. for the reference to astronomy.tools - very cool!)

 

Any problems achieving focus with the additional 2" SCT adapter and 2" diagonal with the 0.63x reducer? As I understand, the distance between the reducer and eyepiece has to be within a narrow range.

 

-- astro.runner


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#17 wrvond

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Posted 28 October 2020 - 02:14 AM

Initially I had the prism diagonal on the C8, but couldn’t achieve focus. I changed to the mirror diagonal and haven’t had a problem.



#18 Escape Pod

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Posted 29 October 2020 - 02:18 AM

I had an extended convo about minimum magnification and exit pupil size recently. The gist is that the secondary mirror obstruction on your SCT (and my Mak) pose problems with extra long focal lengths that your refractor won’t have, And once the exit pupil gets too big, you won’t be able to receive the extra light. 



#19 Brent Campbell

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Posted 30 October 2020 - 07:58 PM

This is interesting. Since I'm already getting 1.3 degrees TFOV on my C8 with the focal reducer and 40mm Plossl (43deg AFOV), does this mean there's not much additional value in getting 2" EPs for the SCT? If this is the case, I can focus my 2" EPs to optimize performance on the refractor instead. Where can I find more information on the optical theory behind the vignetting effect for SCTs? What impact does it have for visual?

 

I'll probably do this - the Porta II head is available separately and I can just use the Evolution tripod, which seems pretty sturdy.

 

I'd appreciate a link to this adapter. I plan to get the Astronomics 2" refractor diagonal.

 

-- astro.runner

https://www.cloudyni...fov-for-the-c8/

this thread out which will explain the max fov for a c8:  


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#20 Lee D

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Posted 02 November 2020 - 01:18 AM

I have an Evolution too, and have some thoughts on the original post.

 

First, I think sticking to your choice of the AT102ED is excellent. It is half the focal length of your C8 with the "0.63X" reducer installed, and, so configured, both telescopes have about the same f/ratio. So you can get wider fields, lower magnification, and eyepieces that give appropriate exit pupils in one will do so in the other one as well. Very good.

 

Second, I think, but am not sure, that you can mount the AT201ED on the Evolution, straight out of the box. That's a good way to start, and a nice alternative to keep, even if you decide at some point you want to have a second mount.

 

Last, I think there is room to increase the true field of view in both the new refractor and your existing telescope without going to 2" eyepieces. I read here on cloudynights that the 8-24 zoom you mention is quite good, but it has an apparent field of view of only around 45 degrees at 24mm focal length. That is not using all the field available to 1.25" eyepieces. Two eyepieces that do are the Televue Panoptic 24mm and the Explore Scientific 68 24mm. The wider apparent field of 68 degrees in these eyepieces means the observed true field of view would be about 1.5X what you now can see. That increase of 1.5X is quite a nice increase. Any 1.25" eyepiece with a field stop of 27mm will provide a similar max TFOV, like most 30-35mm Plossls. Since you already have a 1.25" diagonal you can insert in either telescope, I would start there. Since you can probably find a suitable eyepiece in stock, you could use it immediately

 

One area to investigate before going down the 2" diagonal route, besides the vignetting, is the change it will have on the focal length of your 8" SCT, both with and without the reducer/corrector mounted. Going from a 1.25" prism diagonal to a 2" mirror diagonal increases the optical path length. Some searching on cloudynights will yield a lot of previous discussion on this topic.


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#21 astro.runner

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Posted 10 November 2020 - 06:41 PM

I have an Evolution too, and have some thoughts on the original post.

 

First, I think sticking to your choice of the AT102ED is excellent. It is half the focal length of your C8 with the "0.63X" reducer installed, and, so configured, both telescopes have about the same f/ratio. So you can get wider fields, lower magnification, and eyepieces that give appropriate exit pupils in one will do so in the other one as well. Very good.

 

Second, I think, but am not sure, that you can mount the AT201ED on the Evolution, straight out of the box. That's a good way to start, and a nice alternative to keep, even if you decide at some point you want to have a second mount.

 

Last, I think there is room to increase the true field of view in both the new refractor and your existing telescope without going to 2" eyepieces. I read here on cloudynights that the 8-24 zoom you mention is quite good, but it has an apparent field of view of only around 45 degrees at 24mm focal length. That is not using all the field available to 1.25" eyepieces. Two eyepieces that do are the Televue Panoptic 24mm and the Explore Scientific 68 24mm. The wider apparent field of 68 degrees in these eyepieces means the observed true field of view would be about 1.5X what you now can see. That increase of 1.5X is quite a nice increase. Any 1.25" eyepiece with a field stop of 27mm will provide a similar max TFOV, like most 30-35mm Plossls. Since you already have a 1.25" diagonal you can insert in either telescope, I would start there. Since you can probably find a suitable eyepiece in stock, you could use it immediately

 

One area to investigate before going down the 2" diagonal route, besides the vignetting, is the change it will have on the focal length of your 8" SCT, both with and without the reducer/corrector mounted. Going from a 1.25" prism diagonal to a 2" mirror diagonal increases the optical path length. Some searching on cloudynights will yield a lot of previous discussion on this topic.

Thanks! That is an interesting point about the ES 68 24mm. I thought I had it narrowed to either ES 68 34mm or APM 70 UFF 30mm. I will add this to my shortlist and evaluate. I guess the shorter FLs have the advantage of darkening the sky background since my usual observation spots have high LP.




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