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Tak FC field flattener for visual use?

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#1 Ihtegla Sar

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Posted 29 September 2020 - 12:46 AM

Is there such a thing as a field flattner for visual use that will correct field curvature in a Tak FC doublet?

Tak offers reducers/correctors for photography and they have this flattener for the FC but from the description it doesn't sound like it corrects for field curvature.

https://www.takahash...tener-104x.html

Is there any type of field flattener that will correct for field curvature for visual use in an FC Tak to make it perform more like a petzval?

#2 sg6

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Posted 29 September 2020 - 01:57 AM

If it is a flattener then it has to flatten the prime image immaterial of imaging or visual. The camera sensor is flat.

Just read the Tak page and it reads either odd or wrong. They say it corrects for the Spherical Aberration which I doubt. Mainly because I would expect almost zero SA present - I think they may have the wrong term, or something. The image would be "spread"over a spherical surfcae but that is not spherical aberration, that is curvature. Spherical Aberration is a poor set of surface radaii and spacing of the primary lens. And Tak are not exactly known for poor lens production.

 

A bit of the description just reads odd:

This flattener works now only with all FS and FC telescopes currently in production, but also works with many older models.

 

So it "now only with all FS and FC telescopes currently in production", and then the only gets in effect dumped and it "also works with many older models".

 

As best, or least, I would say it is a "flattener" and it should therefore produce a flat (flatter) image at the image plane. If you put a camera or an eyepiece at the appropriate place the image used should be flat. Depending on your specific scope you should have a flat image of 40mm or 44mm diameter to either look at with an eyepiece or splat on to a sensor.

 

What I suppose is my problem is had it come from Takahashi Japan I would have said it was a translation error, however no reason for that these days. But it says Takahashiamerica.com and someone there should know enough to get it right.

 

I think the description on the Tak page is just wrong. If you want a laugh reference the page and ask Takahashi how much Spherial Aberration they expect in their scopes and lenses.


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#3 Ihtegla Sar

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Posted 29 September 2020 - 02:26 PM

If it is a flattener then it has to flatten the prime image immaterial of imaging or visual. The camera sensor is flat.

Just read the Tak page and it reads either odd or wrong. They say it corrects for the Spherical Aberration which I doubt. Mainly because I would expect almost zero SA present - I think they may have the wrong term, or something. The image would be "spread"over a spherical surfcae but that is not spherical aberration, that is curvature. Spherical Aberration is a poor set of surface radaii and spacing of the primary lens. And Tak are not exactly known for poor lens production.

 

A bit of the description just reads odd:

This flattener works now only with all FS and FC telescopes currently in production, but also works with many older models.

 

So it "now only with all FS and FC telescopes currently in production", and then the only gets in effect dumped and it "also works with many older models".

 

As best, or least, I would say it is a "flattener" and it should therefore produce a flat (flatter) image at the image plane. If you put a camera or an eyepiece at the appropriate place the image used should be flat. Depending on your specific scope you should have a flat image of 40mm or 44mm diameter to either look at with an eyepiece or splat on to a sensor.

 

What I suppose is my problem is had it come from Takahashi Japan I would have said it was a translation error, however no reason for that these days. But it says Takahashiamerica.com and someone there should know enough to get it right.

 

I think the description on the Tak page is just wrong. If you want a laugh reference the page and ask Takahashi how much Spherial Aberration they expect in their scopes and lenses.

Glad I am not the only one who thought that description was mostly a bunch of gibberish.  Even Google translate usually does a better job of not mixing terms like "field curvature" and "spherical aberration" but maybe it was a translation error.

 

I sent Takahashi America a message and I tried not to be too flippant.  We will see what they say . . .



#4 Ihtegla Sar

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Posted 30 September 2020 - 09:41 AM

Takahashi America responded very quickly. Long story short, it was a translation error. The flattener does correct for field curvature, not spherical abberation and it can be used for visual astronomy but there won't be enough backfocus to use a diagnol, so only straight through viewing. That sounds like a literal pain in the neck.

Hmm. I wonder if I can screw the DL lens cell onto my DF tube to increase the back focus to make it work with a diagonal, perhaps with an extension tube? Seems like that would work but probably not worth the effort since I wanted it more for use with the wider field of the DF.

Edited by Ihtegla Sar, 30 September 2020 - 09:41 AM.


#5 edif300

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Posted 30 September 2020 - 10:10 AM

Hmm. I wonder if I can screw the DL lens cell onto my DF tube to increase the back focus to make it work with a diagonal, perhaps with an extension tube? Seems like that would work but probably not worth the effort since I wanted it more for use with the wider field of the DF.


That's not a good idea, will get a lens diameter vignetting. Same happen by removing lens cell coupler.

Edited by edif300, 30 September 2020 - 10:11 AM.


#6 Ihtegla Sar

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Posted 30 September 2020 - 10:31 AM

That's not a good idea, will get a lens diameter vignetting. Same happen by removing lens cell coupler.


What is "lens diameter" vignetting and why would I get it by putting a DL lens cell on a DF tube? The only difference between a DF and a DL is focul length (900 v 740).

#7 edif300

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Posted 30 September 2020 - 11:55 AM

Objective lens diameter=100mm

OTA OD = 95mm

If you don’t take care about the spacing between lens cell and OTA you will get a good FC-90DL wink.gif

 

FC-100DL_Takahashi.jpg


Edited by edif300, 30 September 2020 - 11:56 AM.


#8 edif300

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Posted 30 September 2020 - 12:10 PM

The best petzval like is a petzval. To say FSQ.

 

I didn’t understand what you are trying to do since the FD-100DL has the higher F/D (9) in the FC series. Has that high field curvature to be the field corrector a must have for visual? What eyepieces are you using? Tak UW? 



#9 Spikey131

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Posted 30 September 2020 - 12:12 PM

I’m a bit surprised that field curvature is that noticeable in these f/7 and f/9 refractors.



#10 Ihtegla Sar

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Posted 30 September 2020 - 01:52 PM

The best petzval like is a petzval. To say FSQ.

 

I didn’t understand what you are trying to do since the FD-100DL has the higher F/D (9) in the FC series. Has that high field curvature to be the field corrector a must have for visual? What eyepieces are you using? Tak UW? 

Yeah, I had thought of getting a TV Petzval but that's a whole lot more money, even used, and then I would have another OTA to care for and feed and I am running out of space, plus I am a bit leery of TV petzvals since I hear that a hard knock can put them out of collimation and then its a trip back to TV and I'd rather not have to deal with that.  Was thinking that a two hundred dollar field flattener might improve the views in the Taks I already own, but straight through viewing would be too hard on my neck and back.  

 

You are right that DL has less field curvature and a narrower field than the DF so it may not even be worth the effort. I really wanted it to use with my DF and my 42mm Vixen LVW or my 40mm Pentax XW eyepieces.  Doesn't sound like that will be possible without straight through viewing.

 

But the Tak lens cells screw on and off easily so it would only take a few minutes to pop the DL lens cell onto the DF tube, just to see how it works.  I may try that to see how it works with my binoviewer.  I cannot get low power views with the bino viewer because it takes a barlow to bring it to focus due to a lack of back focus. Maybe with the DL lens cell on the DF OTA I could get the bino viewer working without the barlow.


Edited by Ihtegla Sar, 30 September 2020 - 01:52 PM.


#11 gezak22

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Posted 30 September 2020 - 02:42 PM

Is there such a thing as a field flattner for visual use that will correct field curvature in a Tak FC doublet?

Tak offers reducers/correctors for photography and they have this flattener for the FC but from the description it doesn't sound like it corrects for field curvature.

https://www.takahash...tener-104x.html

Is there any type of field flattener that will correct for field curvature for visual use in an FC Tak to make it perform more like a petzval?

Are you absolutely, positively certain that you are seeing field curvature and not astigmatism? I am asking because I have made the same mistake in my TV petzval scope. People were quick to point out that my views could be compromised by astigamatism, and once I installed a dioptrx on my widest field eyepiece, I proved them right.



#12 Ihtegla Sar

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Posted 30 September 2020 - 04:04 PM

Are you absolutely, positively certain that you are seeing field curvature and not astigmatism? I am asking because I have made the same mistake in my TV petzval scope. People were quick to point out that my views could be compromised by astigamatism, and once I installed a dioptrx on my widest field eyepiece, I proved them right.


I am not absolutely certain of anything. 😊According to my eyeglasses prescription my eyes don't have much astigmatism (less than .25 diopters) and I don't think it's astigmatism in my eye, but next time I have the scope out with these eyepieces I will rotate my head 90 degrees and see if I can see any change in the view or if the stars look elongated rather than just out if focus.

From what I recall it seemed like field curvature because I could focus on the stars near the middle 50 % of the field or the outer portion but not both at once, so that seemed like field curvature.

#13 gezak22

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Posted 30 September 2020 - 04:23 PM

I am not absolutely certain of anything. According to my eyeglasses prescription my eyes don't have much astigmatism (less than .25 diopters) and I don't think it's astigmatism in my eye, but next time I have the scope out with these eyepieces I will rotate my head 90 degrees and see if I can see any change in the view or if the stars look elongated rather than just out if focus.

From what I recall it seemed like field curvature because I could focus on the stars near the middle 50 % of the field or the outer portion but not both at once, so that seemed like field curvature.

That sounds reasonable. Do you have any other eyepieces of the same focal length you could try?


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#14 n2068dd

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Posted 01 October 2020 - 04:51 AM

Is there such a thing as a field flattner for visual use that will correct field curvature in a Tak FC doublet?

Tak offers reducers/correctors for photography and they have this flattener for the FC but from the description it doesn't sound like it corrects for field curvature.

https://www.takahash...tener-104x.html

Is there any type of field flattener that will correct for field curvature for visual use in an FC Tak to make it perform more like a petzval?

All of Takahashi field flattner is designed to less Petzval field curvature. and All of the camera lens, too.

Petzval field curvature expressed by the equation, ' Petzval sum' which have the curve radius and refractive index.

A doublet can't correct Petzval sum to zero. we must add for the more negative function to cancel the positive number. Most of the case one set of convex lens and concave lens is used.



#15 213Cobra

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Posted 01 October 2020 - 05:13 AM

I’m a bit surprised that field curvature is that noticeable in these f/7 and f/9 refractors.

By f/9 (in slowness), the FC problem should be minimal to the point of insignificance. OK, maybe you don't see it that way. You can always try an inexpensive TSFlat2 to solve your Tak f/7 - f/9 visual FC problem. In my f/6 LOMO triplet refractor, TSFlat2 is visually indispensable. In my f/7.5 triplet LOMO, its benefits are much more slight. At f/9 I doubt FC is your primary problem.

 

All this is cleaned up by a Takahashi FSQ 106 or 85, or a Tele-Vue 101 Petzval. Or one of the Tak Q scopes, FS-60Q or FOA-60Q, or even the FC-76 DCU + Q module. But it you love your Tak doublet, the TSFlat2 can fix the visual problem, cheaply and if you are using the Tak UW series eyepieces or any other oculars optimized for flat fields, you're golden.

 

Phil


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#16 n2068dd

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Posted 01 October 2020 - 09:22 AM

Hi, Ihtegla Sar

 

--------I may try that to see how it works with my binoviewer.  I cannot get low power views with the bino viewer because it takes a barlow to bring it to focus due to a lack of back focus. Maybe with the DL lens cell on the DF OTA I could get the bino viewer working without the barlow.

 

Maybe any of your try would be in vain......

 

1. Inherently, binoviewers will add the much more aberrations. The reason is the long glass pass. it will add the CA and AS aberrations.

    That's the reason many binoviewer fringe image is fuzzy or distorted. not by Petzval field curvature. because field flattner have no powers.

2. The barlow is used to recover that aberrations by lower the light pass angle. if you pull out it much more aberration will see.

    Despite, the barlow will add the color aberrations more on prime focus. most of the barlow is designed as achromatic, blue error will be worse.

3. If binoviewers is made of mirrors, 1st aberration CA and AS would be minimum. Though as you know less reflectance will harm the brightness.

    If 2 mirrors, 0.9*0.9=0.81 And mirror is far more sensitive on surface accuracy than lens in twice for one surface. This will degrade the lens accuracy more.

4. A field flattner is not designed for that kind of aberrations. Some of them will lower the spherical aberrations though only in the center.

5. Best way is to use eyepieces as designed for that usage. I don't know where to get, some TV?  If you would modify the binocular, a normal telescope eyepieces is useless. They are not designed for that.

 




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