59 replies to this topic

[Mike Lockwood]

How small could you make the scope then?
I’d be a bit perplexed if it was longer then say a C11 which is a lot slower, with a reducer and TV67 the effective speed is F3.

I’m sure TEC made some compound scope with a similar aperture to a C11 with half the physical length or smaller.

The 12.5" f/2 OTA that I built is 16"x16" square and 34" long.  It could be smaller (and round), but I built it for myself and needed to finish it in a reasonable amount of time with non-exotic materials.  The coatings are specifically for NV.

 

I think it's likely the C11 would vignette pretty seriously in that configuration, not sure about correction.

 

If you do not care about money... there are (corrected across the entire field):

 

- Riccardi Honders https://www.officina...oce-telescopes/

- Riccardi Fast https://www.officina...ast-telescopes/

Those are F/8 and F/3.8 respectively, nowhere near F/2, and coatings are not tailored to NV.

Edited by Mike Lockwood, 28 July 2024 - 03:07 PM.

[Deadlake]

The 12.5" f/2 OTA that I built is 16"x16" square and 34" long.  It could be smaller (and round), but I built it for myself and needed to finish it in a reasonable amount of time with non-exotic materials.  The coatings are specifically for NV.

 

I think it's likely the C11 would vignette pretty seriously in that configuration, not sure about correction.

 

Those are F/8 and F/3.8 respectively, nowhere near F/2, and coatings are not tailored to NV.

I've not seen the C11  or a Edge11HD vignette in this configuration, Gavstar has published multiple images he has taken thru such scopes. 

 

How are the coating specific to NV, presume will reflect in infra red band and not absorb in it?

 

Hope this is not a similar circumstance to dielectrics diagonals absorbing in the infra red band making us go out and buy BBHS 2" diagonals!

[Mauro Da Lio]

The Riccardi Honders system is F/3. And they have a full frame corrected flat field.

 

And, let me, for a moment, express a criticism. As for the F/2 ratio, in my opinion it may be useful for narrowband filters, but for galaxies, broadband reflection nebulae, planetary nebulae etc., and without filters, the image scale is too small.

Take the picture of M82, for example. I think it is meant to show what is visible (*). I can see better details afocally with 10" aperture at about 24-16 mm (F6-9). 

As another example, take M51 and compare the picture with drawings made with a 10" and pure glass here: https://www.deepsky-...alo/dsdlang/fr 

 

(*) for astrophotography, one can get better images with a 499$ telescope e.g., https://www.cloudyni...2#entry13578257

Edited by Mauro Da Lio, 28 July 2024 - 04:37 PM.

[Mike Lockwood]

Deadlake, I did not know that about the C11/Edge.

 

The custom corrector lenses and the mirrors for the 12.5"/16" f/2 are coated for best reflectivity at a bit longer wavelengths, ~500-900 nm.  Galaxy did the coatings, and the secondary coating is slightly different due to 45-degree angle of incidence.

 

Correctors and eyepieces are generally coated for ~400-700, I believe, and there are a lot of surfaces in those.  So for this 12.5", I went for maximum throughput, minimum loss, and fastest feasable system because if I was going to do prime focus, I was going to do it as well as I possibly could!

 

Mauro,

 

I see now that the telescope does say f/3, but their table on the same page says f/8, and that's what I looked at.  They should really fix that error.  And it's still f/3 and not f/2, and that's a big difference.

 

Image scale depends on the size of the object.  This was never really intended to be used on galaxies, but when I saw the contrast it was like nothing I had seen before (and far superior to a 10" scope visually), so I shared the photos to show that as best I could.  As I wrote in the article, and you should have read, the camera images do not capture all of the detail visible in the objects.

 

You did read the other articles about the telescope first, before you stared offering criticism, right?

 

While you may wish to criticize what you see based on your observing preferences, in my opinion you should take into account all types of objects, not your personal favorites.  This is a prototype, and the contrast shown on galaxies is far superior to anything I've seen visually.  With an eye toward public observing, which I think this is best suited for, there is nothing like it.

 

A 24" f/2 is possible, and I hope to build one to increase image scale.

 

This is not an astrophography forum, and the telescope described here is designed for real-time nightvision viewing.  Astrophotograpy requires a mount, a camera, and other equipment to process, so your comparison is invalid, and entirely off-topic.

Edited by Mike Lockwood, 28 July 2024 - 05:44 PM.

[Mauro Da Lio]

With a background of visual observing, I knew (like all DSO and planetary observers) that the contrast sensitivity of the human eye has a peak at a particular spatial frequency. Take this picture from here: https://www.research..._fig4_340311673

 

 

For Scotopic vision, i.e., DSO with glass only, the peak is at very low spatial frequency. Hence, with glass, experts observers use high and very high magnifications ( https://www.cloudyni...gratulations/).

 

Night vision devices elevate the surface brightness, to somewhat between mesopic and photopic regimes. However the peak in contrast sensitivity is still there, only at a higher frequency. This means that there still is an optimal magnification, albeit the optimal magnification is smaller than with glass.

 

When I bough my NVD, I heard about using as low magnification as possible (e.g., TV 67mm) and as fast F ratio as possible. This was in contrast with what I was theoretically expecting (the existence of an optimal image scale, which is not the lower the better).

 

I later realized that recommending as fast as possible ratio was always in relation to the use of narrowband filters where a a lot of light is blocked. 

 

After some time of use, I have indeed realized that when using narrowband filters (in practice only for H-alpha) a fast F ratio is necessary (for me 40 mm is good enough). However, when using the NVD without filters or with broadband filter (IR-cut under dark sky) then the optimal magnification argument still, holds. For galaxies, globulars, planetary nebulae I need to magnify at somewhere to F/5 to F/10. Otherwise I get an image brighter than necessary with less details.

 

So, if one takes into account all types of objects for me the conclusion should: H-alpha -> Fast F/ratio, all other objects (galaxies, globulars, reflection nebulae, planetary nebulae) -> optimal magnification around F/5 to F/10. 

Edited by Mauro Da Lio, 29 July 2024 - 04:54 AM.

[sixela]

It’s easier to transform an f/2 scope into an f/6-f/8 scope with a Barlow than to get to f/2 with an f/8 scope.

So for “dual use” H-alpha/OIII (with your Photonis if you like the UHC-S you should really get a dual-band 4nm Altair Astro filter, it rocks!) and regular Astronomik L1 usage I’d prefer a fast scope. Especially for wide field observation!

I’ll agree that if you’re only interested in f/6 use on smaller objects such aggressively fast scopes are unnecessary, but that’s simply not their design space.

[Mike Lockwood]

Well, thanks for the inputs guys, but this is an f/2 telescope design and it's going to stay that way.  That's what the thread is about, and it's been a fun project for me.

 

For more magnification, I'll build a larger one.  The topic starter here is building a 16" f/2 so eventually we should hear from him.

 

I only posted galaxy photos because I wanted to show the brightness and contrast that was possible, but it definitely makes me want a larger aperture version.  Then I'll have both.

[WheezyGod]

I would love to get one of these at some point and would prefer f/2 as opposed to f/3 or higher. There’s tons of slower options out there for small objects but nothing that’s as fast as f/2. I’m not sure if a 16in one of these could fit or nearly fit the NA nebula with the TV67 setup, but if so that would be incredible.

[Mike Lockwood]

I need photos of the Veil, Gamma Cygni, NA nebulas and all of them north of the NA.  It's on my list, but I didn't get to it in April.  Might have to do all of those at Okie-Tex if I don't have a chance before that.

 

The field of view in the 12.5" is 1.64 degrees, 16" is 1.27 degrees.

[WheezyGod]

I need photos of the Veil, Gamma Cygni, NA nebulas and all of them north of the NA. It's on my list, but I didn't get to it in April. Might have to do all of those at Okie-Tex if I don't have a chance before that.

The field of view in the 12.5" is 1.64 degrees, 16" is 1.27 degrees.

I got a rough idea using SkySafari if the size of the NA nebula is accurately to scale. With the 16in using the TV67 setup, the FOV would nearly or just barely fit the NA and pelican nebulas. Pretty impressive!