Should I consider a faster scope or more FL?

Light gathering power depends on the aperture of the lens or mirror and the bigger the lens the more photons you collect. Two Scopes with the same aperture but different f-ratios still collect the same amount of photons per time unit.

A scope with a smaller f-ratio only achieves more “speed” through worse undersampling of the pixels. If you resample the image from the higher fl scope to the “speedy” one you will have the same SNR.

What you want to look at first is your desired FOV and then the biggest aperture scope your mount can handle at that focal length.

If you want to learn more about that topic, John Hayes held a nice presentation at the Astro Imaging Channel on YouTube.

https://www.youtube.com/live/HiJoqQp1qFI

Uhm, okay? That doesn't change what I said earlier.

If you're looking for product recommendations besides the raw metrics, then others might help out here because currently I am not in the possession of a refractor.

Totally agree with what Tobiasz said, it’s an important concept to wrap your head around.

As to your basic question I would lean towards more aperture and more focal length. At 900mm you’re just knocking on the door with smaller objects. Many PN will still be too small but the larger, well known ones will become really interesting. Larger Galaxies like M33 will become a landscape of objects within objects and the run of the mill NGC Galaxies will become fascinating, especially in groupings of two or three. At that focal length tiny background galaxies also start to really become an important part of the image. Essentially, the longer the focal length, the more objects of interest there are to image.

I don’t have time to do your homework for you but I would lay out the specs for all the scopes your looking at and favor the ones that give the longest focal length with the largest apature.

I would probably trade up / sell your HEQ5 and get an EQ6R Pro. It can easily handle every scope you’ve listed and then some. If you really want the harmonic, I won’t say it’s a bad idea. They’re certainly more portable, but the EQ6R is the better choice. The used market is saturated with them right now and it’ll be pretty easy to find one around 1k.

Out of your entire list of scopes, I’d probably go straight for the Askar 130. Disclaimer, I’ve never owned one, but I’ve heard pretty good things about the PHQ line. Also, don’t be afraid of a slower scope. I’m an F/7 user, and honestly if I could, I’d love to find a way to make it F/9 to sample better. Worth checking out the Esprit 120. It’s the baby brother to my scope and I love my 150. VERY sharp. With a full frame camera, I’d certainly go for the longer focal lengths also.

Brian Puhl · Jul 10, 2026, 04:37 PM

I would probably trade up / sell your HEQ5 and get an EQ6R Pro. It can easily handle every scope you’ve listed and then some. If you really want the harmonic, I won’t say it’s a bad idea. They’re certainly more portable, but the EQ6R is the better choice. The used market is saturated with them right now and it’ll be pretty easy to find one around 1k.

Out of your entire list of scopes, I’d probably go straight for the Askar 130. Disclaimer, I’ve never owned one, but I’ve heard pretty good things about the PHQ line. Also, don’t be afraid of a slower scope. I’m an F/7 user, and honestly if I could, I’d love to find a way to make it F/9 to sample better. Worth checking out the Esprit 120. It’s the baby brother to my scope and I love my 150. VERY sharp. With a full frame camera, I’d certainly go for the longer focal lengths also.

thanks Brian! Okay. Dumb question but for Long FL AP can i leave the reducer/flattener off to maintain Image scale or do you always use the reducer for the espirit?

A little aside:

I bought a C9.25 and shot with a Starizona reducer at around 1450mm for a little while. Pixel scale right around .5”/pixel. It was an amazing scope, but I found I wasn’t delivering much more resolution than the 900mm newt I was using prior to the SCT, which was about .86”/pixel. The combination of seeing and exposure (my setup is not protected from wind), meant that I was probably limited to about 800-900mm focal length before my ability to resolve detail dropped off a cliff 😂

In my opinion at least, once you know the max focal length your skies and imaging conditions will support, look for the largest aperture (fastest f ratio) you can get near that focal length. Plus, for most folks imaging from their back yard or traveling to dark sky locations, actual imaging time can be somewhat limited, so it’s nice to maximize the time you have with a faster OTA that can gulp down signal. This is one reason 6-8” f/4 newtonians and RASA/hyperstar setups are so popular with hobby astrophotographers going after DSOs.

If the Takahashi 160-ED fts your budget, that’d be my pick. That said, it wouldn’t be a huge upgrade over your current setup in terms of resolution (1.46”/pixel), mostly speed (f/3.3). So those smaller objects like m51 will still feel somewhat “out of reach.” That said, you could consider picking up a 294mm camera and making use of it’s smaller pixels (2.3 micron in Bin1 mode) to squeeze out more resolution for smaller targets (.9”/pixel).

If you have access to nice dark skies (you mentioned going up into the mountains), f/7 optics can still perform well, so I’d think any of your choices between 100-115mm aperture could be nice (Your pixel scale would be about 1.05”/pixel). But it will take a little bit more exposure time to build SNR compared to a faster focal ratio scope. Also, mostly as an FYI, as I understand it, there’s a noticeably larger bump in length/bulkiness for 130mm+ refractors, so wind will be more of an issue a those higher refractor apertures.