Large / Fast / Diffraction-spike-less telescopes for ultra narrow-band ... do they exist?

John Stone:
Hi, I live in a Bortle 7 (SQM 18.7) city and I have limited imaging time due to light pollution and limited open sky.

 I own a C8-A XLT with a HyperStar IV and a 0.63x SCT Corrector IV.  The Hyperstar gives me 394mm focal length and the 0.63x reducers gives me 1475mm focal length. 

Both are a pain to use due to collimation problems (especially the HyperStar) and edge of field star distortions because my ASI2600MC Pro's APC camera's sensor size is at the maximum published image circle for the two reducers. 

I'd like to move over to monochrome imaging and I've settled on the the Antlia 2.5nm HSO filter set (but the LRGB is up in the air, but probably will be from Antlia as well). 

http://www.antliafilter.com/pd.jsp?fromColId=146&id=140#_pp=146_647 

These filters are spec'd:

Focus Ratio of ≥ f/3.5 for best optical performance

 I have an EQ6-R Pro mount and I want to find the largest telescope I can put on that mount that gives me F3.5 - F4 performance (at full-frame) WITHOUT diffraction spikes.

The only ones I have been able to find are F4/F4.5 Newtonians with coma reducer/correctors which would be great (Like the ONTC 10"/250P) but they have spider veins which produce diffraction spikes.  Check out https://www.astrobin.com/zlc8g6/ to see what I don't like.   Do such large/fast/diffraction-less telescopes exist?

Also, it would be nice for such a scope to have a set for flattener/reducers that would provide a few options of focal length (where the fastest/most-reduced version would have a F-ratio of >F3.5) Please let me know if you can help.

*** Not entirely clear on why you want the biggest telescope that you can get? Bigger isn't always better.

And, as far as I'm aware most of the very fast scopes are some form of Newtonian design. 

If you don't like diffraction spikes then consider that some people will use a fast scope for capturing narrow band data that will also have a rig with a small refractor to capture RGB data.

You can remove the stars from the fast scope and replace them in post processing with the RGB stars.

***

If you don't like diffraction spikes then consider that some people will use a fast scope for capturing narrow band data that will also have a rig with a small refractor to capture RGB data.

You can remove the stars from the fast scope and replace them in post processing with the RGB stars.

I'm pretty sure that StarXterminator can't remove spikes that go like 1/2 - 1/3 across the image and you're left with a giant 'X-s' in your starless images.

Otherwise, great idea.

Even under the best circumstances there are phantom traces left by SXT (or even worse, SN) so that isn't a 100% proof method.

@Chris White- Overcast Observatory Thanks for these suggestions but they're a little on the small side at 100mm - 110mm objectives.

I think a F3.6 RASA type telescope would be perfect, but they're only made in F2 which is too fast for ultra narrowband like my HyperStar.

I've also looked at the SkyWatcher/Orion MN 190 which seems pretty good except it's a little slow at F5.3 doesn't have reducers/extenders and is limited to APS-C cameras.   I also wonder about how well corrected off-axis as I think the marketing blurb I've read says it's 10x better than an uncorrected Newtonian...  how does that compare to a good coma corrector like the Paracorr?  I've never seen a spot diagram for one either.

So, it's looking like the only telescopes without diffraction-spikes are:

refractors:  small aperture (100-140mm), fast (esprit 100 F3.6 reduced/APC, F5.5 flat/FF,  Askar 140 APO F4.5 reduced/APC, F5.6 reduced/FF, F7 flat/FF)
SCT: large aperture (203 - 235mm), slow (F7 reduced/APC 203, FF 235)
Mak-Newt: moderate aperture (152 mm, 190 mm), moderate (F4.8, F5.3), no reducer available
Mak-Cass: moderate aperture (90 - 180mm), very slow (F12 - F14)

any others?

Of these:

the largest is the MN190 from Orion/Skywatcher, F5.3, APS-C
the most flexible is a 140 APO with (0.65x - F4.5 - APS-C), (0.8x - F5.6 - FF) and (1.0x - F7 - FF)
the fastest is the Esprit 100 with (0.65x - F3.6 - APS-C) and (1x, F5.5 - FF)

I'm leaning towards the 140 Apo as the best compliment to the C8 - XLT I currently own.

You can get a Takahashi FSQ-106 with the 645RD reducer.

It's costly but it's also f/3.6 (perfect for your filters) at 380mm focal length (maybe a bit short for you). The declared corrected image circle is 60mm but you may have to check some reviews.

Daniele

I have an Astro-Physics 305mm Riccardi-Honders. I think that would meet your requirements–no spiders and f/3.8. No longer made, hard to find, and expensive. I believe Officina Stellare still sells their RH Veloce series of telescopes, ranging in aperture from 8" to 16" and having focal ratios from f/2.2 to f/5.6. Those would be possibilities if you want something larger than a 6" Mak Newt. Expensive.

- Jared

There is no free lunch…. fast, wide field, low cost… you can choose any two, but not all three

Is there any information on the performance of that Meade SN10?

looks like it’s pretty old

Yes, the AP is around 65 pounds before you add dovetails and guide scope.

John Stone:
Hi, I live in a Bortle 7 (SQM 18.7) city and I have limited imaging time due to light pollution and limited open sky.

 I own a C8-A XLT with a HyperStar IV and a 0.63x SCT Corrector IV.  The Hyperstar gives me 394mm focal length and the 0.63x reducers gives me 1475mm focal length. 

Both are a pain to use due to collimation problems (especially the HyperStar) and edge of field star distortions because my ASI2600MC Pro's APC camera's sensor size is at the maximum published image circle for the two reducers. 

I'd like to move over to monochrome imaging and I've settled on the the Antlia 2.5nm HSO filter set (but the LRGB is up in the air, but probably will be from Antlia as well). 

http://www.antliafilter.com/pd.jsp?fromColId=146&id=140#_pp=146_647 

These filters are spec'd:

Focus Ratio of ≥ f/3.5 for best optical performance

 I have an EQ6-R Pro mount and I want to find the largest telescope I can put on that mount that gives me F3.5 - F4 performance (at full-frame) WITHOUT diffraction spikes.

The only ones I have been able to find are F4/F4.5 Newtonians with coma reducer/correctors which would be great (Like the ONTC 10"/250P) but they have spider veins which produce diffraction spikes.  Check out https://www.astrobin.com/zlc8g6/ to see what I don't like.   Do such large/fast/diffraction-less telescopes exist?

Also, it would be nice for such a scope to have a set for flattener/reducers that would provide a few options of focal length (where the fastest/most-reduced version would have a F-ratio of >F3.5) Please let me know if you can help.

Have you considered a Canon "white" telephoto lens? I used a Canon 600mm f/4 L II lens for years. It had some slight issues with the IS (image stabilization) group, however right near the end of my use of that lens as a telescope (I needed to repurpose it back to bird and wildlife photography for which it was originally purchased), I picked up an Astromechanics EF adapter. This powered the lens, allowing me to use its built in focus, and seemed to resolve the slight IS group float. 

The lens, for its size, is very fast, EXCEPTIONALLY sharp, and as far as refractors go has a whopping 150mm aperture. It is not super long, and it supports a 44mm image circle with excellent performance nearly from corner to corner (due to field obstruction you can end up with split star halos in the periphery stars). These days, the lens is a lot less than I purchased it for (a bit over ten grand at the time, IIRC 2012 or 2013). I think these days you can find them between $6k to $7k. 

I ended up picking up an FSQ106 for my astrophotography. I spent a lot of time debating between the FSQ106 and VSD100. In the end, I kept finding things about the VSD having slightly bluer halos on stars, and it has its own unique focusing system. I ended up getting the FSQ106 so I could also attach a Nightcrawler focuser, which is a match made in heaven. The FSQ106 is a great scope...however it is actually not as good as the Canon 600mm lens. Its spot size is less ideal, spot aberrations increase more to the periphery of the field, and the details are just not as fine and sharp. I will probably be using the Canon 600mm lens again for AP, doing full color with an EOS R5 camera at a dark site for a while here before I put the FSQ106 back into action. The Canon "great white" telephoto lenses are great options for light guzzling refractors, if you have the dough to get one. Thankfully with the shift to the EOS R system, and a whole new set of lenses specially designed for that system, the EF versions have been coming down in price a lot for used or refurbished, so they are becoming more accessible as a result. 

IMO, you would be hard pressed to find a scope as sharp and easy to use (at least, with the Astromechanics EF adapter, which is powered and supports ASCOM) as the Canon 600mm f/4 L II. There are probably some newer refractors that are sharper, but they are also a lot more expensive. The L II series of great white lenses made use of nanocoatings on the internal surfaces of some of the larger optical elements. These nanocoatings are part of the reason the images from these lenses is so exceptional...it helps refract the light with practically no reflection and better refractive characteristics. I've never had stars as small, with any scope (RC, Newt, Refractor), as what the 600mm lens produced, nor details as sharp.

How BIG do you need? Like, you know a 10" f/3 is not faster than a 4" f/3? So unless you want the focal length of a 10" f/3 (750mm), size shouldn't really be that big of a deciding factor. 

I'd be looking for a Tak FSQ106N with the 0.63 reducer giving you 106mm f/3.9. 

thats going to give you speed, sharpness, no spikes, flat field over MASSIVE sensors, way bigger than 35mm sensors…