Small RFT.(Rich Field Telescope)

[tico]

Hello,
I am very interested in this type of small instruments and their use for large stellar fields and large nebulae... A long time ago I used a 7x50 binocular with a 7.5° field, it fascinated me... but, and it is a very big but for me, on the one hand you could not change the magnification and on the other I could not use nebular filters, and this is where these small telescopes or finder engines come into play that with their small focal lengths can offer those large fields that I like so much with those two possibilities... ,

Have you used them with low powers and large fields to observe objects like NA, Veil, Rosetta, Pleiades... Andromeda... well framed? because I like to see objects with that kind of sensation as if they are surrounded by the universe... wide angle vision as I say.
I would be delighted to hear your experiences.
Very thanks

Tico

[Louis D]

I enjoy using my 72ED refractor for low power sweeping of the skies.  In particular, I can pick out large star clusters and associations that are not obvious at higher powers.  Rich star fields in Orion and Perseus come to mind.

[vlaiv]

It depends on what sort of magnification you are after.

I'm going to assume 7mm exit pupil limit, but you should check it, some people have wider pupils than 7mm and some have narrower, so that is individual.

For x7 magnification - there is this little scope:

https://www.teleskop-express.de/shop/product_info.php/info/p13750_TS-Optics-50-mm-f-4-ED-travel-refractor--spotting-scope-and-guiding-scope-with-Crayford-focuser---perfect-optics.html

If you can live with x9 magnification - there is this one:

https://www.teleskop-express.de/shop/product_info.php/info/p14747_TS-Optics-62-mm-f-8-4-4-Element-Flatfield-Refractor-for-Observation-and-Photography.html

or perhaps something like 60mm TS doublet:

https://www.teleskop-express.de/shop/product_info.php/info/p10095_TS-Optics-PhotoLine-60-mm-f-6-FPL53-Apo---2--R-P-Focuser---RED-Line.html

With 70mm you have quite a bit of choice - 70/500 as cheap option, but you'll have to change focuser for 2" version so you can use 2" eyepieces to get wide field of view. Then there are number of 70-72 ED doublet variants - however, you'll be limited to x10 magnification with those.

 

[PeterW]

You can use filters with binoculars… on the eyepiece end, you do need a widget to hold them on and you do get issues with bandshift as you go out from the centre of the field. You could also consider larger large exit pupil binoculars of fast Newtonian too.

 

Peter

[Mr Spock]

With my 102mm I can get x17 with fov of 4.2°. Not actually tried it yet!

[ollypenrice]

My best ever was an elderly TeleVue Genesis (4 inch F5 refractor) with 2 inch widefield eyepiece (I think a TV 35mm Panoptic at that time.) With an OIII or UHC filter it could show the Rosette or the entire Veil, etc. I now have a 70mm Pronto for these tasks but the larger aperture of the Genesis was worth having.

Olly

[Highburymark]

I can get 5.37x with a TV55, adapted to 67mm with a night vision lens, in my Tecnosky 60ED! Haven’t tried it to be honest. But I have used the little scope with an APM 20mm 100 degree for 18x, which is stunning under the right skies. Does show some field curvature of course.

[Stu]

It’s a type of astronomy I love, widefield filtered views of objects like the Veil and NAN.

I agree with Olly that my Genesis is probably the best at this, 100mm f5, and will give a 5 degree field with a 31mm Nagler.

Smaller scopes can do a great job too, I’ve used FS60C, WO66, TS72 and TV76 for this kind of observing when I go away on holiday somewhere dark. It’s all about dark skies, good dark adaptation and often a UHC or OIII filter. Don’t listen to the opinion that an OIII is only any use in scopes over 8”, it’s not true!

There are quite a few reports around which I’ve done over the years with references to these sorts of observations. A few posted here for info.

 

[Ships and Stars]

1 hour ago, PeterW said:

You can use filters with binoculars… on the eyepiece end, you do need a widget to hold them 

 

Peter

Filtered binoculars were a revelation for me. My first views of the North America nebula and Pelican were unforgettable. TV nebustar on one side and UHC on the other. The Helios Apollo bins and a few others have filter threads. 

[mikeDnight]

If you're looking to get a rich field telescope, you might consider a Skywatcher StarTravel refractor. They range from 80mm to 150mm in aperture and give spectacular wide field views. The 150mm is truly awesome as an RFT.

Another, but way more expensive, are Televue refractors. It's probably better to buy these second hand.

[tico]

Thanks, All opinión is good!! 

[PeterW]

Getting to 6-7mm exit pupil needs a reasonably short focal ratio or the eyepiece focal lengths get hard to find.

I have made some small foam disks that can hold 1.25” filters into the binocular rubber eyecups with friction, no threads needed. Using anti-fog spray helps avoid frustration on dewy evenings.

 

Peter

[Merlin]

Can anti-fog spray be used on first surface mirrors?  I doubt it, but maybe.

[PeterW]

I wouldn’t use anything on a first surface coated mirror, but use on a hard multi coated filters I’ve no concerns. Just keep obvious dirt blown off and use lint free tissues to minimise and risk of scratches and you should be fine.

 

peter

[johninderby]

Some anti fog sprays can attack the coatings on optics so not a good idea to use them. 

[PeterW]

Any specific brands or chemicals?

 

peter

[johninderby]

Just something I read on a Camera forum. The conclusion seemed to be no one was sure which anti-fog solution was safe for a camera lens. 

Edited March 16, 2022 by johninderby

[PeterW]

As they result in you needing to wipe your optics more than normal there will be additional risk of scratches, so care is needed. Only needed in those chilly and damp nights. I’ve asked Baader for a comment.

 

peter

[PeterW]

In response to Zeiss anti fog use on Baader filters, Baader provided the following…

”No problem. But if it comes to cleaning - less is more.

If you scrubb off each dust-grain seperately no coating will withstand in the long run. so it is much preferable to cleane once a year and not to spare on the cleaning fluid then.”

Peter

[Merlin]

Anti-fogging sprays might be okay for dielectric coatings.

[scarp15]

I used to use a TV-76, which did a good job but for some field curvature. I now use a TV-85 and combine with a Pan 41mm; 4.4 degree field, 5.8mm exit pupil at 14.6x. With both of these instruments, depending upon the target, have used a H-beta filter as well as O-III and UHC filters. 

[tico]

I like to observe big field of the Sky, like the wide Field astro photos,... 

Any have seeing NAN well frame..? 

[Stu]

55 minutes ago, tico said:

I like to observe big field of the Sky, like the wide Field astro photos,... 

Any have seeing NAN well frame..? 

Yep. First image is a 31mm Nagler in a Genesis (500mm f/l), second is a 24mm Panoptic in an FS60C (355mm f/l). The Genesis gets my vote due to the larger aperture and larger exit pupil (6.2mm vs 4.1mm) but under a dark sky, with OIII filter, anything along these lines would be good.

[tico]

Stu, 

Thanks for the images, fantástic!? 

[Andrew_B]

The Takahashi FSQ models are apparently superb wide field scopes due to their short focal lengths and flat image fields. Adding a matching focal extender can sharpen them up even more and make them into good planetary refractors. Only downsides are their weight and high cost.

Scopes like those and the TeleVue Nagler-Petzval models (Genesis, NP-101, NP-127, etc) are a bit like regular refractors with the addition of a permanently built-in reducer and field-flattener. For imaging you can achieve something similar by adding a reducer to a standard doublet or triplet design but this combination often aren't practical for visual use because there isn't enough back focus to accommodate a star diagonal.

Borg do some very lightweight fast focal ratio doublets (72mm, 90mm, and 107mm all f5.6) that would make great RFTs but they're not cheap. Field curvature might be an issue depending on your choice of eyepiece and how sensitive you are to it, but you can add a non-reducing flattener that would solve the problem and could be used visually.