16 replies to this topic

[CosmicShadow]

So I'm considering this massive achromatic refractor as an upgrade from my Zenithstar 73.

I've got an ZWO ASI2600MM with Antila 4.5nm SHO and LRGB filters in a wheel. I've got a ZWO EAF. I'm in Bortle 8.

 

If I'm shooting each filter individually, since it's mono, does it even matter if the scope is achromatic? Won't it just focus each filter until it works and then I get the advantages of a 6" refractor at a fraction of the price? I'm going to be processing in PixInsight with SPCC for colour correction of stars anyway.

 

I understand it's going to be big, heavy and long and I will likely have issues regarding weight for my mount, as well as the risk of it hitting a leg.

 

I've seen a handful of decent pictures, and the comments that go with them seem to say that people had a blast imaging with it, however there really aren't that many available to peruse on Astrobin.

 

I'm looking for more data points about what I'm missing here, or why this won't work.

 

Thanks,

- Ivan

[unitron_man]

I've been happy focusing at each filter with a doublet ED refractor.  Just don't expect to use a luminance channel.

 

Examples

 

https://www.astrobin...rs/unitron_man/

Edited by unitron_man, 22 September 2023 - 02:01 PM.

[t-ara-fan]

A very very high end 6" refractor  with the flattener is $16K. Plus adapters etc. 

 

ES makes a triplet that is $6K.  So of course the one extra piece of glass in the triplet object is not $5K.  There will be a lot of details in the scope you linked that were made economical not "the best they can be".    One thing, and this is hearsay, is that even going narrowband you do not get as good an image with the econo-refractor that you get with higher end scopes.  The stars just don't come together in a 5um sized spot.  You get a much bigger spot.

 

Maybe that scope is for visual?

Edited by t-ara-fan, 22 September 2023 - 02:03 PM.

[Echolight]

For visual use, the big achromat will bury any 73mm scope.

[Cbaxter]

1. No, you won't get the advantage of a 6" apochromatic refractor at a fraction of the price. The more expensive scopes generally have higher quality components throughout. Such as a focuser that is ideal for astrophotography.

2. A mono camera is not immune to chromatic aberrations. CA is still captured by a mono camera, bloating stars and having a monochromatic halo where a color camera would show it as violet or blue.

3. Even focusing each filter individually the poorly corrected wavelengths will still have bigger/bloated spots.

4. Others have mentioned imaging with an ED doublet, the AR152 has no ED elements in it. It is an a chromatic doublet that will have significantly more CA than an ED doublet.

5. Visually, it has fsr more light gathering and resolving power than any 73mm scope. However, vies will be marred by significant CA that will be worse as magnification increases. Whether that is acceptable or not for visual use is completely subjective and up to you.

6. There are other aberrations besides CA and higher end ED and SD doublets and triplets are usually better corrected overall. You usually do get what you pay for.

My subjective recommendation, if your primary use will be imaging, a 4" triplet or ED/SD doublet will give you very noticeably higher quality images than the 152mm achromat and a lot more resolution than your current 73mm. A 4" ED scope would also ride and track better on smaller mounts.

Edited by Cbaxter, 23 September 2023 - 11:13 AM.

[Wildetelescope]

Folks do image narrowband with Achromats.   Depends on what your expectations are.   I have seen some very nice images coming from D and G scopes.  Your challenge with a fast achromatic will not be so much blurring due to lateral color, but other types of aberrations, like field curvature, spherical aberration etc..    If you HAD a larger fast achromat already, I would say give it a try.  But I would not buy one for this purpose.  It is not what it was designed for.    If you want 6 inches of aperture goodness, I would consider one of the F8 ed doublets that are out there.   

 

JMd

[GTom]

Do you guys READ the OP? He's asking about ACHROMATIC refractors. You can find a 6" Istar achro for less than $1k. 

A very very high end 6" refractor  with the flattener is $16K. Plus adapters etc. 

 

ES makes a triplet that is $6K. ...

 

I've been happy focusing at each filter with a doublet ED refractor.  ...

Edited by GTom, 23 September 2023 - 07:03 PM.

[GTom]

Doesn't matter if your sensor is mono or color. What matters is the bandwidth you are covering with your filters. 

R, G and a restricted B that starts from 450nm could deliver useable results on an f/10 6" achro. The AR152 is however f/6.5, hopeless for proper wide-band imaging.

 

I'd save up another grand and get a 6" apo doublet, this side of the pond 2k$ may fetch you an Altair or TS 150/1200 F-PL53+Lanthanum doublet that blows any practical amateur achro below 10" out of the water.

Edited by GTom, 23 September 2023 - 07:20 PM.

[CosmicShadow]

Here are some samples of nice images I've found from someone using the AR152:

• https://www.astrobin.com/jz8ffi
• https://www.astrobin.com/3ms9ic
• https://www.astrobin.com/rqhsij
• https://www.astrobin.com/qz34te/D

Some of those have interesting comments about the usage of the scope and the guy sounds pretty stoked about how well it works, except he doesn't use it anymore.

 

It looks like he's using a Starizona Apex ED .65x reducer / flattener. He says he's pretty impressed with how pinpoint the stars are (albeit it looks like it's all narrowband).

 

Here's one comment from the last image posted:
 

 

I did change the stock focuser out for a Moonlite CFL. The stock focuser is a pretty typical low-end Crayford. I didn't trust it to maintain focus under the weight of an APS-C camera and filter wheel.
 
  I will say that this combination of scope and reducer has produced the smallest, pin-point star sizes I've seen for anything in the 500-800mm focal length (when shooting narrowband anyway). Even my FLT132 reduced to ~655mm produces slightly larger stars. That was an interesting surprise.
 
  The one draw-back I encountered is that this combination of optics doesn't appear to have the same field flatness for Ha/Sii vs Oiii imaging. As a result, the stars do not perfectly line up when combining these filters. This generally manifests as something that looks a bit like atmospheric dispersion (e.g. stars that are more red on one side, more blue on the other). You can see that in this image if you zoom in.
 
  Shooting with this scope convinced me that for at least some focal lengths, refractors are superior even if you give up some aperture in the process. The stars convinced me that I needed to invest in an APO triplet, hence the FLT132, giving up another inch of aperture but gaining a ton of star quality.
 
  I'm not sure what to do with the achromat at this point. It's kind of redundant for me now.

 

Just seemed like someone was getting some really nice images and if I can pick up an AR152 for $500 + $650 for the reducer, it would work out to less than the price of an ES 102ED APO triplet alone and I'd be 6" at f/4.28. But yeah, it seems like maybe the stars would cause issue, unless SPCC in PixInsight just clobbers the star colour anyway.

[Cbaxter]

Here are some samples of nice images I've found from someone using the AR152:

• https://www.astrobin.com/jz8ffi
• https://www.astrobin.com/3ms9ic
• https://www.astrobin.com/rqhsij
• https://www.astrobin.com/qz34te/D

Some of those have interesting comments about the usage of the scope and the guy sounds pretty stoked about how well it works, except he doesn't use it anymore.

It looks like he's using a Starizona Apex ED .65x reducer / flattener. He says he's pretty impressed with how pinpoint the stars are (albeit it looks like it's all narrowband).

Here's one comment from the last image posted:


Just seemed like someone was getting some really nice images and if I can pick up an AR152 for $500 + $650 for the reducer, it would work out to less than the price of an ES 102ED APO triplet alone and I'd be 6" at f/4.28. But yeah, it seems like maybe the stars would cause issue, unless SPCC in PixInsight just clobbers the star colour anyway.

Notice that comment says they switched out to a Moonlite CFL focuser. Crayford focusers are not very good for astrophotography. A low-end crayford would be even worse. That Moonlite CFL 2.5" focuser is something like $495.00 so add that to the cost of the scope plus the Apex reducer. Also, maybe consider that that person doesn't use that setup anymore. Going the cheap achromat route is a false economy unless you just want some simple live stacked images on a small sensore and or don't care about aberrations inherent/common in achromats.

Edited by Cbaxter, 23 September 2023 - 08:07 PM.

[GTom]

Notice that comment says they switched out to a Moonlite CFL focuser. Crayford focusers are not very good for astrophotography. A low-end crayford would be even worse. That Moonlite CFL 2.5" focuser is something like $495.00 so add that to the cost of the scope plus the Apex reducer. Also, maybe consider that that person doesn't use that setup anymore. Going the cheap achromat route is a false economy unless you just want some simple live stacked images on a small sensore and or don't care about aberrations inherent/common in achromats.

Absolutely valid point, most eco achromat OTAs come with useless focusers, proper replacement doubles the price immediately. Adding filters to mitigate CA is also the wrong direction, you may waste 40% of the valuable photons easily - why not buy a size smaller proper APO then?

[Wildetelescope]

Here are some samples of nice images I've found from someone using the AR152:

• https://www.astrobin.com/jz8ffi
• https://www.astrobin.com/3ms9ic
• https://www.astrobin.com/rqhsij
• https://www.astrobin.com/qz34te/D

Some of those have interesting comments about the usage of the scope and the guy sounds pretty stoked about how well it works, except he doesn't use it anymore.

 

It looks like he's using a Starizona Apex ED .65x reducer / flattener. He says he's pretty impressed with how pinpoint the stars are (albeit it looks like it's all narrowband).

 

Here's one comment from the last image posted:
 

 

Just seemed like someone was getting some really nice images and if I can pick up an AR152 for $500 + $650 for the reducer, it would work out to less than the price of an ES 102ED APO triplet alone and I'd be 6" at f/4.28. But yeah, it seems like maybe the stars would cause issue, unless SPCC in PixInsight just clobbers the star colour anyway.

Those images show what can be done with an Achromat in Narrow band.  Note that he is using 3 nm filters.  So add 1000 dollars in filters to your cost for the scope.  Now if you were going to shoot narrow band to begin with, then that cost is a wash.  But with an ED scope, you gain get away with OSC cameras or less expensive filters.   Those images also speak to a highly skilled individual with respect to processing.  All depends on what you want to get out of the experience.    

 

JMD

[GTom]

Those images show what can be done with an Achromat in Narrow band.  Note that he is using 3 nm filters.  So add 1000 dollars in filters to your cost for the scope.  Now if you were going to shoot narrow band to begin with, then that cost is a wash.  But with an ED scope, you gain get away with OSC cameras or less expensive filters.   Those images also speak to a highly skilled individual with respect to processing.  All depends on what you want to get out of the experience.    

 

JMD

 

6-7nm filters could do the job, but to what end? You can forget about galaxies, reflection nebulae, globular clusters, planets...

[PatrickVt]

This is asked quite often by those wanting to get involved in astrophotography at a far lower price point and there is nothing wrong with getting started on a limited budget and making compromises.  That being said, I think you are being very conservative in estimating costs.

 

As others have pointed out, the cost of a set of narrowband filters will be around $1200.  You are already talking about replacing the focuser and that will be at least a couple hundred more in cost.  And, a field flattener would be a good idea.  All these upgrades are adding to your total startup cost.

 

Back to the narrowband filter set...  some beginners are not bothered by the chromatic aberration.  Actually, most non-photographers wouldn't even think it doesn't belong.  That is a personal preference though.

 

The cost of a used AR152 typically, in today's market, has been around $750+, not the $500 you are estimating (unless you already have a seller lined up, of course...  then that would be a great buy if it is in good condition!).  Incidentally, I thought the AR152 had a rack and pinion focuser but I could be wrong and, if buying used, it could be any focuser on the scope and not necessarily the original stock focuser.

 

Using your CEM26 for imaging with this telescope, guide scope, camera, and whatever else you add into the mix, will be an exercise in frustration.  As with any part of this budget exercise, it can be done but many compromises will need to be made and you should not expect consistent results.  The weight of this configuration is one problem because you will be hovering around the maximum capacity of this mount but the long moment arm of this big scope will actually be a bigger problem.  For observing only, you could probably get by if you are more tolerant of waiting on the scope to settle.  For imaging, it will be an ongoing exercise in frustration.  I would think that a CEM26 might be the biggest problem in this plan.

 

My opinion is that there is nothing wrong with starting down the road to astrophotography with an achromatic refractor as long as you understand your limitations, the compromises, and you are realistic about your expectations in the resulting images.  Again, unfortunately, you might need a heftier mount though.

 

Patrick

[CosmicShadow]

Yes, the mount is definitely a big concern and question mark, especially with the worry about hitting the legs. I've got a line on a $500 CAD AR152 in good condition, and even adding in the $650 reducer/flattener and another $500 on a better focuser, that's still less than a new stock 102mm + flattener.

 

I've already got a great camera, good mono filters and everything setup in an EFW, I've been doing Astro for about a year now on my Zenithstar 73iii, so just looking at the realistic situation of hacking gear for cheap, especially when seeing some impressive shots with it. I think enough folks have given me feedback to say that it's not worth it, but it's definitely worth looking into and learning more about. Obviously some folks have done it with some impressive results, but I don't have spare giant scopes, powerful mounts and excess cash lying around.

 

I'm more likely going to look at going down the imaging newt path next to go bigger and faster at a more affordable price, so I can reuse my camera and filters and not have to worry about swapping out filters on a hyperstar or Rasa setup, which is quite expensive. I can get an 8" newt down to F/3 which is the max of my filters anyway for about $1500-$2000. It's just a question of whether I can push my mount that hard or not, which I'm having doubts on.

[Wildetelescope]

Yes, the mount is definitely a big concern and question mark, especially with the worry about hitting the legs. I've got a line on a $500 CAD AR152 in good condition, and even adding in the $650 reducer/flattener and another $500 on a better focuser, that's still less than a new stock 102mm + flattener.

 

I've already got a great camera, good mono filters and everything setup in an EFW, I've been doing Astro for about a year now on my Zenithstar 73iii, so just looking at the realistic situation of hacking gear for cheap, especially when seeing some impressive shots with it. I think enough folks have given me feedback to say that it's not worth it, but it's definitely worth looking into and learning more about. Obviously some folks have done it with some impressive results, but I don't have spare giant scopes, powerful mounts and excess cash lying around.

 

I'm more likely going to look at going down the imaging newt path next to go bigger and faster at a more affordable price, so I can reuse my camera and filters and not have to worry about swapping out filters on a hyperstar or Rasa setup, which is quite expensive. I can get an 8" newt down to F/3 which is the max of my filters anyway for about $1500-$2000. It's just a question of whether I can push my mount that hard or not, which I'm having doubts on.

An imaging newt makes more sense to me over purchasing an achromat.

 

JMD

[GTom]

An imaging newt makes more sense to me over purchasing an achromat.

 

JMD

Or a Mak-Newtonian if you don't like diffraction spikes. The 190mn costs £1300 new this side of the pond and readily useable with modern lighter weight CMOS cameras up to APS-C.