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Advice regarding eyepieces and barlows\powermates

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#1 Dyptorden

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Posted 03 January 2015 - 07:55 PM

Hello,

I need and advice, especially somebody who used powermates "known to be exponentially" better than barlows.

Due to lack of money I intend to buy those 2x and 4x powermates from TV so that I won't need to buy smaller eyepieces... this way the 12mm would become 6mm and 3mm, the 16 would become 8mm and 4mm and so on.

How much quality\light would I sacrifice by using those barlows? Should I better wait more and buy smaller eyepieces (in mm) and use no barlows?

 

And second question, if I want to go for planet photo, would the 2x or 4x powermate be too much magnification (some say that the best planet photos need no barlows).At the moment I have no t2 adapter so that I can test, so I need your experience

I own an 8" DOB / 1200 FL / Nikon D40.

 

Thank you,


Edited by Dyptorden, 03 January 2015 - 07:57 PM.


#2 astrophile

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Posted 03 January 2015 - 08:32 PM

Welcome to Cloudy Nights!  As to your first question-- You won't sacrifice any appreciable light or quality of view by adding a Powermate.  What you will add, though (especially for the 2-inchers), is a bunch of weight and length to your eyepiece; plus extra time on every ep swap.  I recommend looking for used powermates so if you decide the system is not for you, you can sell them without taking a loss on the experience.

 

Sorry, someone else will have to chime in on your photography question, I have no knowledge there.



#3 Dyptorden

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Posted 03 January 2015 - 08:56 PM

I understood that there might be some drawbacks due to adding Powermate's 4 extra lenses in the in the system ... that is why I asked about that quality loss. Any extra lens is supposed to decrease quality... right?


Edited by Dyptorden, 03 January 2015 - 08:57 PM.


#4 Peter Besenbruch

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Posted 03 January 2015 - 09:33 PM

I understood that there might be some drawbacks due to adding Powermate's 4 extra lenses in the in the system ... that is why I asked about that quality loss. Any extra lens is supposed to decrease quality... right?

 

Sure do. That's why all of us observe with single lens eyepieces. ;)

 

On a more serious vein, manufacturers use more lenses to accomplish something. Often that something involves presenting a wider field of view while maintaining edge sharpness. Very often part of the eyepiece will include something called a Smyth lens. These are basically built in Barlow lenses that help sharpen the edge of the field, while offering more eye relief. A Barlow, (or Powermate, as Televue calls it) provides that for all eyepieces.

 

I just picked up an Explore Scientific 3x, 4 element Barlow. I use it with a Baader zoom eyepiece. The zoom has seven lenses. Combine that with 4 in the Barlow, and we're up to 11. The views look wonderful. Perhaps a few decades back, it would not have been a viable combination, but now it is. I attribute this to improved coatings, and generally a high level of optical quality.

 

When a company adds lenses to a Barlow, it is usually trying to correct for changes in eyepiece behavior caused by the Barlow. This includes a bit of vignetting of the edge of the field, and an increase in the eyepiece's eye relief. Many, perhaps most eyepieces don't need the extra help, but a 4 element Barlow is nice to have when you do have such an eyepiece.



#5 SteveG

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Posted 04 January 2015 - 03:05 PM

I understood that there might be some drawbacks due to adding Powermate's 4 extra lenses in the in the system ... that is why I asked about that quality loss. Any extra lens is supposed to decrease quality... right?

Simply put, no, the Powermate's 4 lenses will not degrade the view.

 

And if you want to save some money, the Explore Scientific Focal Extenders are the same basic design of the Powermate at a much lower price.



#6 GeneT

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Posted 04 January 2015 - 04:41 PM

Hello,

I need and advice, especially somebody who used powermates "known to be exponentially" better than barlows.

Due to lack of money I intend to buy those 2x and 4x powermates from TV so that I won't need to buy smaller eyepieces... this way the 12mm would become 6mm and 3mm, the 16 would become 8mm and 4mm and so on.

How much quality\light would I sacrifice by using those barlows? Should I better wait more and buy smaller eyepieces (in mm) and use no barlows?

 

And second question, if I want to go for planet photo, would the 2x or 4x powermate be too much magnification (some say that the best planet photos need no barlows).At the moment I have no t2 adapter so that I can test, so I need your experience

I own an 8" DOB / 1200 FL / Nikon D40.

 

Thank you,

 

Although I own a 2X Powermate, I don't agree that Powermates are 'exponentially' better than barlows. They are both excellent extenders to one's eyepiece collection. TeleVue's web site gives a good explanation of both. I prefer to use single focal length eyepieces, not because that the views suffer but because I like to move through a set of eyepieces quickly, changing them in and out of the focusser. The Powermate slows me down. There are weight issues and a Barlow or Powermate might require some adjustments to balance your telescope. You might consider a good zoom eyepiece. Several have posted on Cloudy Nights that the Baader Mark III zoom (8-24mm) is an excellent value for the money. Too much magnification? You can go beyond 50X per inch on rare occasions, and for limited objects. I like to cruise at between 20 and 30X per inch. Lastly, I would not recommend buying both a 2X and 4X Barlow or Powermate. Rethink your goals and what magnifications you like to work with, then match your eyepieces to those. I recommend you then decide between a 2X or 4X Barlow or Powermate. 



#7 MitchAlsup

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Posted 04 January 2015 - 05:33 PM

Hello,

I need and advice, especially somebody who used powermates "known to be exponentially" better than barlows.

Due to lack of money I intend to buy those 2x and 4x powermates from TV so that I won't need to buy smaller eyepieces... this way the 12mm would become 6mm and 3mm, the 16 would become 8mm and 4mm and so on.

How much quality\light would I sacrifice by using those barlows? Should I better wait more and buy smaller eyepieces (in mm) and use no barlows?

 

And second question, if I want to go for planet photo, would the 2x or 4x powermate be too much magnification (some say that the best planet photos need no barlows).At the moment I have no t2 adapter so that I can test, so I need your experience

I own an 8" DOB / 1200 FL / Nikon D40.

 

Thank you,

 

PowerMates are NOT Exponentially better than Barlows. PMs have properties that make them better focal length magnifiers (that property is known as Telecentric), another property is that as expressed in "Telescopes Eyepieces Astrographs" that PMs have higher Strehl than either long or short Barlows. (That is they do less damage to the light going through them than do Barlows at a theoretical level.

 

Realistically, you only need a factor of 6 from widest EP to highest power EP, corresponding to 6mm exit pupil (low power) and 1mm exit pupil (high power). A well chosen set of 3 EPs and a single 2X barlow (or PowerMate) can satisfy your need for focal selection. So, given a choice between 2 PMs and 1 EP, or 3 EPs and one PM, I suggest the 1 PM and 3 EPs.

 

Note that the 2X and 4X PMs are 2" only, while the 1.25 PMs are 2.5X and 5X and are harder to fit an EP set around (compared to the 2X and 2", but the 2" is a lifetime focal length multiplier.)



#8 David Knisely

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Posted 06 January 2015 - 02:14 AM

Peter Besenbruch wrote:

 

 

 

On a more serious vein, manufacturers use more lenses to accomplish something. Often that something involves presenting a wider field of view while maintaining edge sharpness. Very often part of the eyepiece will include something called a Smyth lens. These are basically built in Barlow lenses that help sharpen the edge of the field, while offering more eye relief. A Barlow, (or Powermate, as Televue calls it) provides that for all eyepieces.

 

Well, this isn't quite accurate.  The Smyth lens is not a Barlow lens at all: it is a field flattener.  In wide-field eyepiece design, it is possible to minimize off-axis astigmatism in a given eyepiece set, but at the expense of introducing notable field curvature.  The Smyth lens acts ahead of the rest of the eyepiece to compensate for that field curvature and flatten that field, yielding a sharper focus all across the field while still preserving the lack of astigmatism produced by the rest of the eyepiece design.  Thus, the Smyth lens is an integral part of the eyepiece and not just some kind  of "built-in Barlow".  If you take the Smyth lens out of a Nagler for example, the eyepiece will not perform nearly as well, since it is missing a vital part of its architecture.  There are some less-expensive eyepieces that do just stick in a simple Barlow ahead of the rest of simple eyepiece designs to help them perform better, but this is not what a Smyth lens really does.

 

Tele Vue does make very good Barlows (and they actually *call* them "Barlows").  I highly recommend them.  However, as for the Tele Vue Powermates, they are NOT Barlows.  A Barlow is a two (or sometimes three) element negative lens set designed as an "image amplifier" to produce higher magnification with a given eyepiece.  Barlows not only amplify the telescope's on-axis light cone making it appear much shallower with a longer effective focal length, but also amplify the angles that some of the near-marginal light rays have when exiting the Barlow lens.  This can introduce problems in the form of vignetting on some wider-field eyepieces, as some of the outer field light is diverted out beyond the edges of the eyepiece's field stop.  For the Powermates, Tele Vue designed a larger positive achromat lens set behind the initial negative doublet to catch that outer light and re-direct it, forming a light cone that more closely simulates that of a telescope that actually does have a longer focal length.  This redirection helps catch that diverted light and send it all to the eyepiece.  The 2x, 2.5x, and 4x Powermates also are nearly "telecentric" so that changes in eyepiece-Powermate lens distances do not produce large changes in the amplification factor that can occur when a simple Barlow is used with varying eyepiece-Barlow distances.  The Powermates also do not extend the eye relief of an eyepiece in the way some Barlows do, preventing some of the "blackout" problems a few people might encounter when using Barlows on some longer focal length longer eye relief eyepieces.  Thus, the Powermates are nearly pure image amplifiers that avoid some of the problems Barlow have.  I also can easily recommend the Powermates as an excellent way to boost the power without losing anything (other than the money spent buying them :) ).  Clear skies to you.         


Edited by David Knisely, 06 January 2015 - 06:27 AM.


#9 Jon Isaacs

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Posted 06 January 2015 - 03:51 AM

It is my understanding that the advantages of a Powermate are important at longer focal lengths, at moderate and shorter focal lengths a Barlow works just fine. If one is interested in economizing, I think Barlws are a better choice. 

 

As far as needing a range of exit pupils from 6 mm to 1mm, double stars can require much higher magnifications/smaller exit pupils.

 

I am like Gene, fixed focal length eyepieces for all but the highest magnifications, when I need more than a 3.5 mm provides, I use a 2x TV Barlow with the 5mm or 3.5 mm.

 

Jon



#10 Dyptorden

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Posted 06 January 2015 - 05:01 AM

T

 

Peter Besenbruch wrote:

 

 

 

On a more serious vein, manufacturers use more lenses to accomplish something. Often that something involves presenting a wider field of view while maintaining edge sharpness. Very often part of the eyepiece will include something called a Smyth lens. These are basically built in Barlow lenses that help sharpen the edge of the field, while offering more eye relief. A Barlow, (or Powermate, as Televue calls it) provides that for all eyepieces.

 

Well, this isn't quite accurate.  The Smyth lens is not a Barlow lens at all: it is a field flattener.  In wide-field eyepiece design, it is possible to minimize off-axis astigmatism in a given eyepiece set, but at the expense of introducing notable field curvature.  The Smyth lens acts ahead of the rest of the eyepiece to compensate for that field curvature and flatten that field, yielding a sharper focus all across the field while still preserving the lack of astigmatism produced by the rest of the eyepiece design.  Thus, the Smyth lens is an integral part of the eyepiece and not just some kind  of "built-in Barlow".  If you take the Smyth lens out of a Nagler for example, the eyepiece will not perform nearly as well, since it is missing a vital part of its architecture.  There are some less-expensive eyepieces that do just stick in a simple Barlow ahead of the rest ot simple eyepiece designs to help them perform better, but this is not what a Smyth lens really does.

 

Tele Vue does make very good Barlows (and they actually *call* them "Barlows").  I highly recommend them.  However, as for the Tele Vue Powermates, they are NOT Barlows.  A Barlow is a two (or sometimes three) element negative lens set designed as an "image amplifier" to produce higher magnification with a given eyepiece.  Barlows not only amplify the telescope's on-axis light cone making it appear much shallower with a longer effective focal length, but also amplify the angles that some of the near-marginal light rays have when exiting the Barlow lens.  This can introduce problems in the form of vignetting on some wider-field eyepieces, as some of the outer field light is diverted out beyond the edges of the eyepiece's field stop.  For the Powermates, Tele Vue designed a larger positive achromat lens set behind the initial negative doublet to catch that outer light and re-direct it, forming a light cone that more closely simulates that of a telescope that actually does have a longer focal length.  This redirection helps catch that diverted light and send it all to the eyepiece.  The 2x, 2.5x, and 4x Powermates also are nearly "telecentric" so that changes in eyepiece-Powermate lens distances do not produce large changes in the amplification factor that can occur when a simple Barlow is used with varying eyepiece-Barlow distances.  The Powermates also do not extend the eye relief of an eyepiece in the way some Barlows do, preventing some of the "blackout" problems a few people might encounter when using Barlows on some longer focal length longer eye relief eyepieces.  Thus, the Powermates are nearly pure image amplifiers that avoid some of the problems Barlow have.  I also can easily recommend the Powermates as an excellent way to boost the power without losing anything (other than the money spent buying them :) ).  Clear skies to you.         

 

Thank you very much for the details David, it's all much clearer to me now. Hope the info helps others too.

 

All the best and clear skyes !



#11 David Knisely

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Posted 06 January 2015 - 06:41 AM

It is my understanding that the advantages of a Powermate are important at longer focal lengths, at moderate and shorter focal lengths a Barlow works just fine. If one is interested in economizing, I think Barlws are a better choice. 

 

As far as needing a range of exit pupils from 6 mm to 1mm, double stars can require much higher magnifications/smaller exit pupils.

 

I am like Gene, fixed focal length eyepieces for all but the highest magnifications, when I need more than a 3.5 mm provides, I use a 2x TV Barlow with the 5mm or 3.5 mm.

 

Jon

 

The primary advantages of a Powermate is the lack of outer field vignetting in wider-field eyepieces and the prevention of "eyepiece blackout" due to extended eye relief on longer focal length eyepieces when Barlows are used.  These problems are particularly prominent in some "Shorty" Barlows that can cause some vignetting in wider field occulars.  One other problem which Powermates can help with are with the solar H-alpha filtering systems that require long f/ratios (f/30 in the case of the etalon stacks that sit near just ahead of the eyepiece).  Barlows with their diverging outer rays may not allow the entire field of view to be in the filter's passband if a regular Barlow is used to achieve the f/30 requirement (the dreaded "ring" effect).  However, the 2x, 2.5x, and 4x Powermates are somewhat "telecentric" and produce a more faithful light cone that is closer to what a true intrinsic longer focal length objective will provide.  This will provide full-field H-alpha viewing with the Powermates providing the longer effective focal length, rather than just having H-alpha passband only in smaller sections of the field of view, as happens when using regular Barlows to get f/30.  Indeed, the new DayStar "Quark" H-alpha system uses a built-in telecentric image amplifier to get the long effective focal length that the filter stack requires for proper functioning.  Clear skies to you.  



#12 Hesiod

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Posted 06 January 2015 - 07:03 AM

About planetary shots, the "power" of the focal amplifier depends on the sampling you need: ideally you wish that all the resolving power of your telescope is fully exploited, i.e. each of the camera's pixel frame a portion of the sky smaller than your theoretical resolving power. If we assume a 0.6 arcsec as resolving power for a 8", then each pixel must frame no more than 0.3" arcsec (if the feature is as large as the pixel, it will become a squarish dot, no matter how much pp you may do)

 

A simple formula is

 

Feq= (dp*206265)/S where "S" is the ideal sampling, "dp" your camera pixel size and "Feq" the overall focal lenght (telescope f.l. + amplifiers or reducers).

 

Smaller the camera pixel size, less the Feq required



#13 Dyptorden

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Posted 06 January 2015 - 07:29 AM

Hesiod, could you tell me how do I find the "S" - the ideal sampling? so that I can adjust my overall focal length?

Also could you tell me whether using a Powermate\barlow or eypiece modifies the formula or not?

Thank you (sorry if the question is too basic.. i am new).



#14 Jon Isaacs

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Posted 06 January 2015 - 09:34 AM

David:

 

It is my understanding when you discuss issues with wider field of view eyepieces,this refers to true field of view rather than apparent field of view.  A Barlow, even a shorty, works just fine with a 5 mm Nagler but with a 32 mm Plossls, there are potential problems.

 

:question:

 

Jon



#15 MitchAlsup

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Posted 06 January 2015 - 09:38 AM

Might as well add this image (again) so everyone knows the difference between Barlow and PowerMate::

 

 



#16 MitchAlsup

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Posted 06 January 2015 - 09:38 AM

Might as well add this image (again) so everyone knows the difference between Barlow and PowerMate::

 

 

Attached Thumbnails

  • barlow vs powermate.jpg


#17 Hesiod

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Posted 06 January 2015 - 11:05 AM

Hesiod, could you tell me how do I find the "S" - the ideal sampling? so that I can adjust my overall focal length?

Also could you tell me whether using a Powermate\barlow or eypiece modifies the formula or not?

Thank you (sorry if the question is too basic.. i am new).

Sorry, my fault.

"S" basically depends on your telescope's aperture: a rough estimate, originally meanted for double stars, is 120/diameter in mm. So 120/200=0.6: it means that your 8" can split a double star as tight as 6arcsec, or solve a Moon craterlets slightly smaller than 1500 m, etc...to make it visible on your pics you have to "splash" the craterlets on at least 2 of the camera's pixel, and better if it is splashed among 3-4 pixel. So you may divide your resolving power by the number of pixel (ex 2) where the craterlets have to appear: 0.6/2=0.3

 

Then you will have Feq= (dp*206265)/0.3. Assuming dp=0,00375 mm*, Feq= 2578mm. Because your telescope has a f.l. of 1200, 2587/1200=2.1, hence you will need a 2x focal amplifier. To spash thre craterlets on 4 pixel, Feq will be around 5100mm, and 5100/1200=4.25; and so on.

The formula basically tell you how strong is the required focal amplifier in order to exploit a given resolving power.

 

Barlows are quite handy because you may vary their power by moving away the sensor from the reference position; but because of the same feature it may be harder to know the actual amplification provided by the system. And with large sensors you may experience vignetting, which may be nasty when shooting at the Moon.

 

*typically DSLR et similia have larger value, around 0.005mm or more



#18 Dyptorden

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Posted 06 January 2015 - 12:51 PM

Again, thank you Hesiod for all these technical answers, they are just so good.

But please give some more details regarding this :

1.How did you get to the value of 1500m for the size of craters ?

2.My Nikon D40 has a sensor of 23.7 x 15.6 mm and 6MPixel

According to Sensor Size's formula : 1000 * sqrt (sensor length * sensor width / number of pixels) I get a value of 7850 while you said about a value of 0.005mm .What is that 7850 measured in then?

3.How can I find the maximum magnification while still getting a clear image ? That is regarding what you've said at 0.6/2=0.3 for finding out the magnification. I guess I might get a foggy image if I make a train of two 4xPowermates before the DSLR :)

 

Thank you again for your time and your patience



#19 Hesiod

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Posted 06 January 2015 - 06:05 PM

Well, the craterlet's size is a rough estimates, based on the fact that 1arcsec= around 1.9 km on the Moon (the Moon is roughly 31arcmin, so 1860 arcsec, and its main diameter is around 3500km).

Yours is the pixel size in microns, if do not multiply by 1000 the formula yelds the size in mm: sqrt[(23.7*15.6)/6000000]=0.0078mm OR 7.8 microns.

(here a link with some specs of DSLR http://www.astropix....P/COMPARE.HTM).

 

You have to choose the sampling according to your telescope specs and the seeing. Ideally you have to employ all time the "true" hi-res sampling, i.e. the sampling which lead to the exploitation of all the telescope resolution. The camera will do a film and only the few (or, if seeing is very good, an hefty heap of) good frames are salvaged and stacked for the pic.

If the seeing is visually not so good (7/10 or even 6/10) you may manage to snatch instants of steady sky through the camera, and get good frames to stack.

If seeing is bad, most of times nothing can be done because bad seeing "erase" the planet's features (and therefore the planet will be anyway blurred).

You may subsample in some special case, ex to do a mosaic of the full Moon with fewer pics.

However these are not the the "unwritten and unfailing statutes of the gods", are more like guidelines or advices. In a shorter way, a 4x focal amplifier is probably "right" for a 200/1200 optical tube and a camera with fairly large pixels.

The same focal amplifier is however quite awkward to employ visually (huge steps between eyepieces).



#20 Dyptorden

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Posted 07 January 2015 - 03:42 AM

Thank you for the link.

If I reach my target, of having 12mm, 16mm, 20mm and 2x + 4x Powermates, I get good granularity 3/4/5/6/8/10/12/16/20 :)

What I didn't understand.. is regarding my 3rd question... how can I find the maximum amplification that can still offer me a clear image? (the two 4xPowermates example).



#21 areyoukiddingme

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Posted 07 January 2015 - 03:59 AM

Your max magnification will be determined by aperture of your scope, the seeing conditions, and and how well constructed your scope is.

 

In an 8" scope, my guess is that your scope is running out of ideas at about 250x. Nights of excellent seeing will allow you to go higher, but those nights for most of us a very rare.



#22 Dyptorden

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Posted 07 January 2015 - 04:41 AM

I am not speaking about observation magnification, but about photo magnification.. how can I calculate the barlow/powermate maximum magnification before getting a blurred image, in relation to my 8" DOB and my D40 Nikon.



#23 Gazpacho

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Posted 07 January 2015 - 05:25 AM

Your real world maximum magnification will be more limited by the atmospheric conditions above you than by any theoretical limit of the scope itself.  I have experienced conditions that limited me to x25 magnification per inch of aperture.  Infrequently, I can reach x50.  I don't think I have ever reached x100.  Your local conditions might beat that.

 

If your 8" dob is an F/6, then your focal length will be 1200mm.  Magnification is Focal Length of the telescope divided by the Focal Length of the eyepiece.  X25 per inch at 8" is x200 magnification.  You could reach that with a 6mm eyepiece, or a 12mm eyepiece and a x2 barlow, or a 24mm eyepiece and a x4 barlow.  The x50/inch limit you can reach with 3mm, 6mm + x2, or 12mm + x4, each giving you x400 magnification.

 

I trust someone will check my math for me, and correct me if I am wrong.

 

I have very little experience with astrophotography.



#24 Dyptorden

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Posted 07 January 2015 - 05:45 AM

Guys, let me repeat, I speak about photo. For observation I have stated above, ".. reach my target, of having 12mm, 16mm, 20mm and 2x + 4x Powermates, I get good granularity 3/4/5/6/8/10/12/16/20 :) "

 

I need to know strictly for photo, without eyepiece, what is the maximum magnification I can get with barlows\powermates before getting a bad image.



#25 Hesiod

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Posted 07 January 2015 - 07:55 AM

Thank you for the link.

If I reach my target, of having 12mm, 16mm, 20mm and 2x + 4x Powermates, I get good granularity 3/4/5/6/8/10/12/16/20 :)

What I didn't understand.. is regarding my 3rd question... how can I find the maximum amplification that can still offer me a clear image? (the two 4xPowermates example).

 

For HI-Res astrophoto the best advice is to employ an overall focal lenght allowing for ideal sampling: so, assuming 0.6 arcsec, a good idea is to set "S" around 0.2-0.15 arcsec (or, if you prefer, feq= [(0.8*206265)/0.6] multiplied by 3 or 4).

If the seeing will be good, you will get good pics; if seeing is bad, well, there is nothing you can do anyway.

The image will be clear granted that 1)the seeing is good enough 2)the telescope is well collimated, thermally steady, and you get a good focus

 

You do not have to over/undersample  too much to avoid troubles in the later post-processing, when you try to improve contrast and sharpen the pic.


Edited by Hesiod, 07 January 2015 - 07:56 AM.



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