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Size of refractor

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#1 Fred Sensabaugh

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Posted 22 June 2021 - 08:47 PM

New to astrophotography but old to visual. Looking for an apo triple less than $2500 that does not need a field flatterer for imaging. I would appreciate any advice on actual tubes or general f/stop and aperture that controls curvature. Hope I’m asking this correctly.



#2 unimatrix0

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Posted 22 June 2021 - 08:58 PM

New to astrophotography but old to visual. Looking for an apo triple less than $2500 that does not need a field flatterer for imaging. I would appreciate any advice on actual tubes or general f/stop and aperture that controls curvature. Hope I’m asking this correctly.

https://astromart.co...d-only-6-nights



#3 idclimber

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Posted 22 June 2021 - 08:59 PM

SVX80T-3SV by Stellarvue. These are hand figured high end scopes specifically intended for imaging wide field. You may wait though to have one built for you.


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#4 RogeZ

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Posted 22 June 2021 - 09:02 PM

Fred what camera are you going to be using?

#5 Fred Sensabaugh

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Posted 22 June 2021 - 09:08 PM

I knew as soon as I posted I should have mentioned the camera size. I can use either an aps-c or full framed dslr. I guess the larger the sensor the more curvature?



#6 idclimber

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Posted 22 June 2021 - 09:11 PM

Avoid Aperture fever at any and all costs. Imaging is completely unlike visual. The large aperture scopes that are really common in visual are really not helpful or appropriate for a beginner.

 

An 80mm refractor with a 4/3", APS, or even a full frame DSLR will frame perfectly much of the deep sky targets most beginners are interested in. A 8" scope or larger, that is far too often tempting for a beginning imager simply can't frame targets like the North American Nebula, Andromeda, or even the good stuff over in Orion. 

 

I tried to start with a big scope (12"SCT). I eventually succeeded in imagining well with it, but only after backing up and perfecting my skills on a smaller refractor. This was absolutey critical to that success. Even so I image with the refractor most of the time. 

 

The Stellarvue scope I mentioned earlier will fill a full frame sensor. It will also do it pretty well. Most refractors can't. 


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#7 ChrisWhite

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Posted 22 June 2021 - 09:12 PM

Why won't you use a field flattener?
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#8 bobzeq25

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Posted 22 June 2021 - 09:19 PM

New to astrophotography but old to visual. Looking for an apo triple less than $2500 that does not need a field flatterer for imaging. I would appreciate any advice on actual tubes or general f/stop and aperture that controls curvature. Hope I’m asking this correctly.

This would be an excellent choice for a scope to start out in DSO AP with.  You do understand that the mount is more important?   It's unclear since you're asking about the scope first.

 

https://www.teleskop...-Objective.html

 

It's not aperture or f stop that controls curvature.  You need an additional lens.  Some flatfield scopes have more than one.

 

In stock, not a trivial consideration these days.

 

The two big beginner mistakes are:

 

skimping on the mount

 

starting with too big a scope

 

With the 71Q an HEQ5Pro would be a good mount.  Or its clone, the Sirius.  Or an ioptron 30Pro.  All about $1200.

 

Yes, you spend as much (or more) on the mount than you do on the scope.  1/1000 of an inch tracking error, and you've wasted your money on the good scope.


Edited by bobzeq25, 22 June 2021 - 09:23 PM.


#9 Fred Sensabaugh

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Posted 22 June 2021 - 09:22 PM

Why won't you use a field flattener?

I have a couple of fast reflectors that use a coma corrector for wide angle lenses so I thought field flatteners would be basically the same for refractors and if could get by without one would be better. If they are needed for correct framing of deep sky images then it is what it is. I’m just learning.



#10 Fred Sensabaugh

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Posted 22 June 2021 - 09:25 PM

This would be an excellent choice for a scope to start out in DSO AP with.  You do understand that the mount is more important?   It's unclear since you're asking about the scope first.

 

https://www.teleskop...-Objective.html

 

It's not aperture or f stop that controls curvature.  You need an additional lens.  Some flatfield scopes have more than one.

 

In stock, not a trivial consideration these days.

 

The two big beginner mistakes are:

 

skimping on the mount

 

starting with too big a scope

 

With the 71Q an HEQ5Pro would be a good mount.  Or its clone, the Sirius.  Or an ioptron 30Pro.  All about $1200.

 

Yes, you spend as much (or more) on the mount than you do on the scope.  1/1000 of an inch tracking error, and you've wasted your money on the good scope.

My mount is an Atlas eq-g. 



#11 bobzeq25

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Posted 22 June 2021 - 09:39 PM

My mount is an Atlas eq-g. 

Fine mount for this. 

 

I've used both flatfield refractors, and refractors with flatteners.  Flatfield refractors are a very nice luxury.  No need to space a flattener, put the camera on and focus.  Makes learning DSO AP easier, always a goal of my advice. 

 

There's _no_ reason to avoid one other than cost.  Flatteners are not somehow necessary instead.


Edited by bobzeq25, 23 June 2021 - 12:33 AM.


#12 idclimber

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Posted 22 June 2021 - 10:19 PM

So one of the reasons I suggested the SV scope was because you said "does not need a flattener" I apparently incorrectly assumed you meant it was already included, which this scope does. 

 

The scope that bobzeq25 suggested is a lot less expensive as are many other equivalent scopes. You will be hard pressed to see a difference between a 1500 scope mid level and a higher end scope at 2,500. This is especially true for a beginning imager. Go ahead and look over at Astobin and see what people can do with very modest scopes. 

 

If however you are like me and just enjoy premium glass then by all means get one. The 2500 budget is above what most will spend. The other high end scope I considered was the the FSQ 85. I even considered their bigger version. 

 

If you intend to use your full full frame camera be careful. Not all scopes have an image circle large or flat enough. Full Frame is simply harder. APS should fit nearly any of the scopes. But consider this, if your APS DSLR is a lot older and does not have live preview it is probably not a good choice either. I have an older APS Nikon (D200) that really is obsolete and there is little point in trying to image with it. 

 

If you don't already have a DSLR for this purpose. Consider one of the cooled astro cameras instead. 


Edited by idclimber, 22 June 2021 - 10:21 PM.

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#13 Pluggednickels

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Posted 22 June 2021 - 10:26 PM

I have enjoyed the crap out of my William optics redcat    I am so glad I listened to the experts and went with a smaller focal length 

scope to start out with.  I started out with a WO z61 and that was a nice scope as well 

I have used the redcat with all types of cameras from a Full Frame sony to a ASI553 and it performs flawlessly with all of them 

I use it with a ioptron Smart EQ Pro+ mount  with ZWO guide scope and focuser all runs off an ASIair Pro

 

I love having everything in one piece of software to do everything other than post processing with 

it has changed my experience from spending literally hours on non goto tracking mounts to find stuff to literally be up and running 

in most cases less than 20minutes to get polar aligned focus and guiding running and camera cooled down to temperature 

 

I also have a 102 ES  but it doesnt' see nearly the use that my redcat does  

the redcat paired with the small ioptron SmartEq has turned my past nights of frustrations into enjoyable nights of getting good images 

 

with this setup I have no problem getting 5 minute plus good subs all night long 

i have even tried some longer subs of 10 to 15 minutes and got good results as well even though I rarely go more than 3-5 minute subs 

 

and for your budget of 2500 you could get into a redcat with asiair pro and even a cooled camera for that 

The ASiair IMHO almost makes it feel like I'm cheating as I haven't had the problems with astrophotography that I heard about when I started 

I still have those nights just like everyone does but they are fewer and fewer as I learn more and more of this hobby 

 

now if I could just learn how to post process I would be set LOL 

 

go with what the experts say start small and go bigger as you learn   I am so glad i didn't start with some big monster as from what I know now 

I probably would have ditched the hobby if I had went that route 


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#14 TelescopeGreg

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Posted 23 June 2021 - 12:05 AM

Before recommending a new scope, what scopes do you currently have? 



#15 Stelios

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Posted 23 June 2021 - 01:03 AM

 You will be hard pressed to see a difference between a 1500 scope mid level and a higher end scope at 2,500. 

I used to think so, too... and then I got my SVX70T. 



#16 bobzeq25

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Posted 23 June 2021 - 08:41 AM

I used to think so, too... and then I got my SVX70T. 

Personal thing, based on personal situation.  Just a few small (<grin> ) relevant points.

 

How many hours did you have in this when you got that scope?

 

How good was your processing at the time?  Were you using PixInsight.?

 

How much of that fabulous other equipment (mounts, cameras, ...) did you have at the time?

 

Beginners often think that this is all about equipment.  But they're well advised to not try and buy a forever scope up front.  Nothing will inform future purchase decisions like experience.  And it must always be kept in mind that this is a complicated system that also involves far more than equipment.


Edited by bobzeq25, 23 June 2021 - 08:42 AM.


#17 WadeH237

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Posted 23 June 2021 - 09:40 AM

Looking for an apo triple ... that does not need a field flatterer for imaging.

I don't think that this exists.

 

Triplets, by design, have field curvature.  You need at least a 4th element to correct for it.  For most of us, that's a flattener.  But there are 4 element designs, such as Petzval's, that are corrected for curvature.

 

With a triplet and a small enough sensor, you might not notice the curvature - but it will be there.


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#18 Jared

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Posted 23 June 2021 - 04:57 PM

I knew as soon as I posted I should have mentioned the camera size. I can use either an aps-c or full framed dslr. I guess the larger the sensor the more curvature?

For refractors, the amount of curvature is generally determined by the focal length of the telescope. If there is no flattener or any "tricks" in the design (ordinary doublet or triplet, regardless of glass materials), the radius of curvature will be approximately ⅓ of the focal length. So, the faster the scope and the smaller the scope the more it will need a flattener. Also, the larger the chip the more you will need a flattener (just because you are incorporating more of that curved focal plane into the image).

 

If you are talking about an APS-C sized sensor or larger, and you aren't discussing larger, long focus refractors (that generally aren't even made any more) you are going to need a flattener of some sort. That could come in the form of an after market flattener that is designed for your approximate focal length, or a flattener made for your exact telescope, or an optical design that already includes a flattener. Any of the above options can work well. Sorting out the correct back focus can be frustrating if you go with a generic flattener--the specified back focus will only be correct for one specific focal length of telescope which may not match what you own. I'd recommend either the second or third approach if you have the budget, that is, a scope with a dedicated flattener, either built-in or sold as an option. 

 

  • The 80mm SVX telescopes by Stellarvue is within your budget even including the (optional but designed-for-the-scope) flattener. 
  • You might find a deal on a used Televue NP101is. That is a Petzval design and so needs no flattner
  • Some of the Orion EON scopes are within your budget and there are dedicated flatteners available
  • The SkyWatcher Esprit refractors come with the recommended reducer/flattener; the 80mm is well within your budget, and the 100mm would stretch you just a bit
  • Astronomics with their Astro-Tech lineup has a bunch of appropriate refractors, though the field flatteners are generally not matched to the scope model; still, they are popular enough that you could easily find which flatteners work with which model scopes and what the correct spacing is

I'm sure there are other options as well. I think I would start seeing who has stock on a scope, since availability has been an issue on an awful lot of models over the past year or so.



#19 joeytroy

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Posted 23 June 2021 - 11:57 PM

SVX80T-3SV by Stellarvue. These are hand figured high end scopes specifically intended for imaging wide field. You may wait though to have one built for you.

+1 on this! I ended up going with the SVX080T-25FT and all the bells as whistles as the 3VS just has the Feather 2.5” focuser instead of the 3”. Call the sales team, they are really nice and know a lot on the hardware side. I just ordered mine on Monday and really excited even though its late July to September time line. I started with a RedCat which is a quad glass scope and is a good starting out scope at 250mm. Might be useful to also know what you want to shoot to know how deep you want to go Nebula, or Galaxies?


Edited by joeytroy, 23 June 2021 - 11:58 PM.


#20 Prudentis

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Posted 24 June 2021 - 08:00 AM

Before recommending a new scope, what scopes do you currently have? 

Second that.

Why don't you start with what you already have?




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