Field flattener for visual use?

[Stu]

Question... Is there such a thing? I find with my TV76, combined with very wide eps like the 21 Ethos, the stars in the outer parts of the fov cannot be brought to focus at the same point as the central stars. This does have an impact on viewing open clusters etc so I was wondering if a flattener would help? I don't need a focal reducer, just a flattener.

I have seen this....

http://www.teleskop-express.de/shop/product_info.php/info/p1010_TS-Universal-2--Field-Flattener-for-Refractors-f-5-to-f-8---Flat-Field-for-Astroph.html

Any chance it would work? I have asked the question but would be interested in hearing if anyone has tried one. Is spacing as critical as for imaging?

Cheers,

Stu

Edited November 23, 2013 by BigMakStutov

[michael.h.f.wilkinson]

I have used a TS flattener in my APM 80 mm for visual, but it us difficult to get the spacing right. It did help a bit, but currently I do not bother with it. I find that I can find a point of focus at which my eye can accommodate for the slight difference.

[Stu]

Thanks Michael. Is your APM a triplet? Possibly has a flatter field than the TV doublet?

It is only really a problem with the 17 and 21 Ethos, have not tried the the 31t5 in it yet. I can't find a position which is an acceptable compromise so might be worth trying?

Stu

[Stu]

Just as an update to this thread, I acquired a TS flattener from the forum with the aim of trying it in the TV76 with 21 Ethos. This combination has not been working well, with a significant different in focus position between centre and edge of the field, negating any benefit of the widefield. The 24mm Panoptic does not show the same issues so it is obviously the 100 degree field combined with the short focal length doublet which is causing the issue.

Well, after a relatively brief trial I can say that it does sort the problem. I screwed it into the front of the diagonal, and whilst I may still have to play around to get the spacing closer to ideal, I can now find a position where the stars are crisply focused across the field.

Interestingly, TS said that the flattener was not for visual use, and amended their website to remove this reference after I asked about it. I chose to ignore them anyway and give it a go and this now makes the 21e useable in the 76 which is good news.

Stu

[Space Dragon]

Stu

I have been pondering a flattener or flattener/reducer for visual with  F/5.9 and F/7 refractors, both doublets and the ES 30mm 82º. Interesting to hear that you had some success with it, as I weirdly couldn't find any evidence of them being used for just visual.

It's not awful as it is but could be better, could you post a link to the one you have ?

Cheers

Duncan

[Stu]

Hi Duncan,

This is the one...

http://www.teleskop-express.de/shop/product_info.php/language/en/info/p1010_TS-Universal-2--Field-Flattener-for-Refractors-f-5-to-f-8---Flat-Field-for-Astroph.html

As I mentioned, the TS website did mention that it was useable for visual, which is why I posted in the first place. When I asked them though, they said that was a mistake and removed the reference. I decided it was worth the risk secondhand and I do think it has been worthwhile. The field curvature was quite pronounced and now, with minimal effort, I think it is quite ok. I'm sure with a bit more experimenting with spacing it could be even better.

It is strange that not so many people have commented on this effect, probably not many 21 Ethos' being used in small doublets! At 70 degrees, or with shorter focal lengths eps it is not really a problem but it is quite clear on 21mm with 100 degree afov.

Stu

[Space Dragon]

Thank you Stu.....will look out for one.

[Paz]

I have been researching exactly this issue and wondered if anyone can comment on whether this field flattener would help for visual use.

https://www.365astronomy.com/ts-optics-photoline-1.0x-field-flattener-for-photoline-apo-refractors-with-72mm-aperture.html

I've acquired a 72mm TS Photo line scope for visual and on its first outing noticed more field curvature than I was expecting.

I have a few other queries on this topic...

Where in the optical train would it go? I see from the limited information out there that going in front of the diagonal seems to be the best place to start.

Are field flatteners very specific to individual scopes? I've also recently acquired a 102mm f7 apo and although I've not had an opportunity to test that properly would one field flattener do the job on a 102mm f7 and also a 72mm f6?

Would I have to have a 2" diagonal or is there a way to use a 1.25" diagonal.

p.s. it may be worth mentioning I don't have any intentions to do photography.

[Louis D]

It's alive!  This thread is alive!

Seriously though, what you have to watch out for is the working distance.  Most field flatteners have a working distance of 55mm like the one you referenced.  It would require an eyepiece holder that attaches to T-mount threads.  That, and you would need quite a bit of in-focus to make it all work.

Alternatively, you could try what I did with my AstroTech 72ED.  Buy a TSFLAT2, a 15mm SCT nosepiece, and a 2" GSO/Revelation dielectric diagonal.  Remove the original nosepiece from the diagonal, screw the SCT nosepiece into its place (it may not want to thread very far I've found, but it's still solid), and screw the TSFLAT2 into the SCT nosepiece's M48 threads.  You'll have a nearly perfectly flat field with your 72ED scope.

[Paz]

On 24/11/2019 at 23:29, Louis D said:

It's alive!  This thread is alive!

Seriously though, what you have to watch out for is the working distance.  Most field flatteners have a working distance of 55mm like the one you referenced.  It would require an eyepiece holder that attaches to T-mount threads.  That, and you would need quite a bit of in-focus to make it all work.

Alternatively, you could try what I did with my AstroTech 72ED.  Buy a TSFLAT2, a 15mm SCT nosepiece, and a 2" GSO/Revelation dielectric diagonal.  Remove the original nosepiece from the diagonal, screw the SCT nosepiece into its place (it may not want to thread very far I've found, but it's still solid), and screw the TSFLAT2 into the SCT nosepiece's M48 threads.  You'll have a nearly perfectly flat field with your 72ED scope.

Thanks for the response. It looks like a flattener will add a bit of weight, length, and complication so  I'm going to try a few different eyepieces to see if that makes much difference to what I notice and then come back to flatteners if necessary.