97 replies to this topic

[Sarkikos]

AFAIK, the spacing required is between TSFLAT2 lens assembly and the field stop of the eyepiece.  The field stop is where the final image is formed.  Otherwise, how could the field stop look sharp when you view through the eyepiece?

 

So to measure the correct spacing for the TSFLAT2, you need to think about the distance between the TSFLAT2 and the field stop of the eyepiece.  Any other considerations can confuse the issue and result in an incorrect spacing. 

 

Tele Vue provides the distance between field stop and shoulder of the eyepiece, which is helpful.  (Eyepiece dimension "F" in the table.)

 

https://www.televue....page.asp?id=214

 

But in practice, the easiest thing to do is just perform field experiments.  Try each eyepiece with TSFLAT2 screwed onto the diagonal barrel, and then with TSFLAT2 screwed directly onto the diagonal without the diagonal barrel. 

 

So far I haven't had to add barrel extensions to the diagonal barrel to increase the spacing in order to achieve a flat field. 

 

On the other hand, if the distance is still too long without the diagonal barrel, you can't get any closer any way. 

 

Mike

Edited by Sarkikos, 09 November 2020 - 07:53 AM.

[Sarkikos]

Correct. Only FC is corrected. But for visual if you at least approximate the distancing per guidelines from TS within ~ 5%, it will prove effective for what it is supposed to do but not for what is outside its design mandate.

 

Phil

I have astigmatism, so I always test the TSFLAT2 with my glasses on and then with them off, to see if astigmatism is from my eyes or from somewhere in the optical system ...  or possibly from both.

 

Mike

[Sarkikos]

Last Friday and Saturday nights I did field tests of my AT92ED with the TSFLAT2 and Baader 2" BBHS diagonal.  Without the TSFLAT2, field curvature was visible to my eyes for Ethos 13, 10, 4.7 and 3.7, and also for Leica Zoom, NAV HW 17, and XW 40, 30, 20 and 14.  With TSFLAT2 on the barrel of the diagonal, the field was flat for the Ethos, Leica Zoom and NAV HW 17.  With TSFLAT2 directly connected to the diagonal, without the diagonal barrel, the field curvature was also corrected, but I don't think quite as well corrected as when keeping the barrel on the diagonal.

 

For the XW's, field curvature was obvious both with and without the TSFLAT2.  The TSFLAT2 didn't seem to make much difference for these XW's.

 

For the NAV HW 17, the TSFLAT2 flattened the field, but aberration was still visible toward edge of field.  The remaining aberration looked like coma.

 

For the Leica Zoom, the TSFLAT2 also flattened the field, but with remaining aberration near edge of field.  This looked like either astigmatism or coma, or a combination of these.

 

For the Ethos, the TSFLAT2 flattened the field with no aberrations remaining.  There was no coma, no astigmatism at edge of field.  (I did see some astigmatism at EOF, if I viewed without my eyeglasses.  I have astigmatism.)

 

Mike

[213Cobra]

I have astigmatism, so I always test the TSFLAT2 with my glasses on and then with them off, to see if astigmatism is from my eyes or from somewhere in the optical system ...  or possibly from both.

 

Mike

For astigmatism I rely on DioptRx.

[Jon Isaacs]

I been using the TSFLAT2 with a William Optics diagonal with the flattener used as the nosepiece.  The scope is my AT-80LE, an 80 mm F/6, 480 mm focal length.

 

I am measuring the insertion length of the William Optics diagonal. We all should be doing that. I measured my TV Everbrite as exactly 4.00 inches, 102 mm but the nosepiece added about 36 mm. The body is one piece so the nose piece does not unscrew. The proper spacing for a 480mm focal length is about 120 mm.

 

I measured the WO at 5 mm longer than the Everbrite which makes it 107 mm.  The 31 mm Nagler and 21 mm Ethos both add 9.6 mm to that so with the WO diagonal and TSFLAT2 as the nosepiece, the spacing is 116mm -117 mm, very close.

 

The 31 mm Nagler seems essentially perfect.  The 13 mm and 8 mm do as well. The 21mm Ethos seems better but exhibits an aberration that's coma like on bright stars. 

 

I did play with the spacing on the 21 mm Ethos with no improvement but I plan to do a more careful evaluation.  

 

As far as astigmatism, on axis astigmatism can be the scope or your eyes, off-axis is the eyepiece or flattener.

 

I probably have slight astigmatism but my prescription has none and I dont see it at large exit pupils.

 

Jon

[Sarkikos]

For astigmatism I rely on DioptRx.

I have tried DioptRx but don't like them.  I not only have astigmatism, but also myopia and presbyopia.  With a DioptRx on the eyepiece, the astigmatism is corrected, but I still can't see the sky clearly naked eye and I can't read charts.  So DioptRx does not work for me.  What works for me is progressive eyeglasses.  They correct for myopia and presbyopia, as well as astigmatism.

 

Mike

Edited by Sarkikos, 12 November 2020 - 08:45 AM.

[Sarkikos]

AFAIK, the spacing should be measured from the lens assembly in the TSFLAT2 to the eyepiece end of the diagonal, being careful to measure accurately around the bend.  Add or subtract the distance from the eyepiece shoulder to the eyepiece field stop, depending on whether the field stop is above or below the shoulder.   (Tele Vue provides this information for their eyepieces on their website.)  Also, if the eyepiece doesn't fully insert into the end of the diagonal, you'll have to add the extra distance that the eyepiece shoulder is above the diagonal end.

 

Mike

Edited by Sarkikos, 12 November 2020 - 09:01 AM.

[Jon Isaacs]

As far as measuring the diagonal, I think the most accurate way is to determine the optical path length by focusing on a distant (or not so distant) object and the removing the diagonal and refocusing without the diagonal. This will likely need spacers but an accurate measure,ent can be made without having to guess the location of the mirror. The works with both prisms and mirrors.

 

The length of the nose piece needs to be added if the nose piece will be used.

 

Jon

[tony_spina]

Well I decided to try the TSFLAT2 again.  The one i purchased several month ago had astigmatism.  With the lower price on the TS site i decided to give it another try. I'm hoping this one is not defective.   

 

Will be using it with the ST120 and maybe ST80

[Sarkikos]

Well I decided to try the TSFLAT2 again.  The one i purchased several month ago had astigmatism.  With the lower price on the TS site i decided to give it another try. I'm hoping this one is not defective.   

 

Will be using it with the ST120 and maybe ST80

Are you certain it was the TSFLAT2 that was the source of the astigmatism?   When the TSFLAT2 makes the field flat, astigmatism in eyepieces or other accessories - or even the telescope - could be easier to see.

 

Mike

Edited by Sarkikos, 17 November 2020 - 07:11 AM.

[tony_spina]

Are you certain it was the TSFLAT2 that was the source of the astigmatism?   When the TSFLAT2 makes the field flat, astigmatism in eyepieces or other accessories - or even the telescope - could be easier to see.

 

Mike

100%  certain that it was the TSFLAT2.  Tried multiple combinations to eliminate any other possibilities 

[turtle86]

Just received a TSFLAT2.  Last night I tried it out for the first time with my Tak Sky 90 II by screwing it to the end of an AP Maxbright diagonal.  I really wasn't expecting all that much, but was pleasantly surprised that the boost in performance was very apparent, as I got very nice flat views with a 27 and 35 Pan.  I don't know if it's at the optimal distance in this configuration (and I have little desire to fiddle with spacers and other diagonals) but it's plenty good enough for me.  I'm starting to think that this is almost as much a must for fast refractors as a Paracorr is for fast reflectors.

[Sarkikos]

100%  certain that it was the TSFLAT2.  Tried multiple combinations to eliminate any other possibilities 

Another source of astigmatism could be your eyes.  Just saying.  My eyes have astigmatism.  To eliminate that source of the aberration, I have to either keep my eyeglasses on at the telescope or put a Dioptrx on the eyepiece.

 

Mike

[Doug Culbertson]

Just received a TSFLAT2.  Last night I tried it out for the first time with my Tak Sky 90 II by screwing it to the end of an AP Maxbright diagonal.  I really wasn't expecting all that much, but was pleasantly surprised that the boost in performance was very apparent, as I got very nice flat views with a 27 and 35 Pan.  I don't know if it's at the optimal distance in this configuration (and I have little desire to fiddle with spacers and other diagonals) but it's plenty good enough for me.  I'm starting to think that this is almost as much a must for fast refractors as a Paracorr is for fast reflectors.

 

Do you still have the Orion 120ST? I would be interested to hear how the TSFLAT2 works with it as I just picked up the SkyWatcher version of the 120st. 

[Sarkikos]

Hi Doug,

 

I have an Orion ST120.  The TSFLAT2 works great with it to flatten the field.  

 

Mike

[Doug Culbertson]

Thanks Mike! 

[Sarkikos]

Here is how I setup the ST120 with TSFLAT2, from post #41 above and another thread (Field Flatteners for Achromats, Visual Only):

 

 

Last night I took out my ST120, 10 Ethos, 8 Ethos, Baader 2" ClickLock Mirror Diagonal and TS2FF.  I removed the barrel of the diagonal, exposing the SCT threads.  I attached an SCT-48mm ring adapter to the diagonal, and then screwed on the TS2FF.

 

This setup produced a flat field in the ST120, through both the 8 Ethos and 10 Ethos.  The field stop of the 8 Ethos is 17.8mm below the shoulder.  The field stop of the 10 Ethos is 6.9mm below the shoulder.  (These are Tele Vue specs from their website.)  The 10.9mm difference between the field stop locations of the two eyepieces didn't appear to make any difference in the flatness of the field.

 

The total optical length w/o the diagonal barrel for the 8 Ethos is 112mm - 18mm = 94mm.  The total optical length w/o the diagonal barrel for the 10 Ethos is 112mm - 7mm = 105mm.

 

The total optical length with the diagonal barrel for the 8 Ethos is 138mm - 18mm = 120mm.  The total optical length with diagonal barrel in place for the 10 Ethos is 138mm - 7mm = 131mm..

 

The TS2FF w/o the diagonal barrel flattened the field nicely.  The TS2FF with diagonal barrel did not.

 

According to TS, the optimal spacing for a refractor with a 600mm focal length (like the ST120 or C80ED) is 113mm.  This is 19mm greater than the spacing w/o diagonal barrel for the 8 Ethos, 8mm greater than for the 10 Ethos.  Would inserting a 19mm or 8mm spacing ring in the setup make any visible difference in field flatness?  I doubt it.

 

Keep in mind that the C80ED - which also has a 600mm focal length - showed a flat field with the TS2FF with the diagonal barrel left on.  Would the C80ED show a visibly flatter field with the diagonal barrel removed?  I doubt it. 

 

Use the specs that these companies give you as general guidelines.  But decide what you want to do based on what you actually see in the field.

 

Mike

Edited by Sarkikos, 06 April 2021 - 11:27 AM.

[turtle86]

Do you still have the Orion 120ST? I would be interested to hear how the TSFLAT2 works with it as I just picked up the SkyWatcher version of the 120st. 

 

Yes, I still have that 120ST you sold me a couple of years ago.  I have a lot of fun with it as a grab and go, and before the pandemic it was my go to scope for public outreach.  

 

I bought the TSFLAT2 with the Sky 90 II in mind, but I'll definitely try it with the 120ST.  From Mike's post it sounds like I'll probably need to remove the diagonal barrel.  

[Doug Culbertson]

Yes, I still have that 120ST you sold me a couple of years ago. I have a lot of fun with it as a grab and go, and before the pandemic it was my go to scope for public outreach.

I bought the TSFLAT2 with the Sky 90 II in mind, but I'll definitely try it with the 120ST. From Mike's post it sounds like I'll probably need to remove the diagonal barrel.

Thanks! Yes, I was just reading Mike’s post about removing the diagonal barrel. Since my diagonal is a TeleVue, I might have to buy another diagonal if I want to use the TSFLAT2.

[turtle86]

Thanks! Yes, I was just reading Mike’s post about removing the diagonal barrel. Since my diagonal is a TeleVue, I might have to buy another diagonal if I want to use the TSFLAT2.

 

I have an Astro-Tech diagonal and I just confirmed that the barrel for it is removable.  I'll try the TSFLAT2 with it in my 120ST and report back.

[turtle86]

Update:  I tried the TSFLAT2 in my 120ST earlier this evening using an Astro-Tech diagonal and a 27mm Pan eyepiece (the eyepiece I mostly use with this scope).   My main targets were the Pleiades and the Beehive.  I didn't really see a difference in correction using it with the diagonal barrel removed, but thought I maybe saw a slight improvement when leaving the barrel in place.  I'm sure it would work better with Mike's setup, and I think I've also read that the Williams Optic diagonal would work well too.  I might well try other setups in the future, but for now I'm thrilled that it works so well with my Tak Sky 90 II.

[Doug Culbertson]

Update:  I tried the TSFLAT2 in my 120ST earlier this evening using an Astro-Tech diagonal and a 27mm Pan eyepiece (the eyepiece I mostly use with this scope).   My main targets were the Pleiades and the Beehive.  I didn't really see a difference in correction using it with the diagonal barrel removed, but thought I maybe saw a slight improvement when leaving the barrel in place.  I'm sure it would work better with Mike's setup, and I think I've also read that the Williams Optic diagonal would work well too.  I might well try other setups in the future, but for now I'm thrilled that it works so well with my Tak Sky 90 II.

Thanks for that update. I’m still on the fence regarding the TSFLAT2 since I bought the 120st mainly for white light solar and night vision use, and my only other refractor is a Lunt Ha scope. 
 

Last night while going through a miscellaneous equipment box I found a GSO 2” diagonal that I somehow forgot about and the barrel is removable. In the meantime, I recently had a vitrectomy in my observing eye so I have a temporary bubble that blocks my vision if I look down into an eyepiece, so I won’t be observing for a little while anyway. 

[turtle86]

Thanks for that update. I’m still on the fence regarding the TSFLAT2 since I bought the 120st mainly for white light solar and night vision use, and my only other refractor is a Lunt Ha scope. 
 

Last night while going through a miscellaneous equipment box I found a GSO 2” diagonal that I somehow forgot about and the barrel is removable. In the meantime, I recently had a vitrectomy in my observing eye so I have a temporary bubble that blocks my vision if I look down into an eyepiece, so I won’t be observing for a little while anyway. 

 

Doug, wishing you a speedy recovery!