Focal Length & Aperture: Go Small Or Go Big? 

I have a Svbony guide scope and a Svbony stereo microscope. Svbony is a good bang for your buck.

It's a remarkably capable and under rated scope. Since its Dawe's Limit is lower than the limit of seeing in almost all circumstances, you can potentially capture the maximum resolution of many sky objects with it. I have a video coming out on that in a couple hours. But it, too, is a complex topic in which many factors beyond just optics play a role.

What challenges do you experience? Is it light pollution related? I live in an area of about Bortle 1.5, so I don't really get experience with some of the challenges facing suburban and urban astrophotographers?

@@SKYST0RY Well, at times I'd like to use it at native focal length (fl), but our nights for imaging are so few and far between in VT that imaging a particular target at f/10 and getting enough time on target becomes an exercise in frustration. (I'm in a high Bortle 4 area, and Lake Champlain is just to the west, so conditions are often not great.) Also, for some targets, when imaging at f/10 there just aren't enough guide stars around the target and it requires very careful pre-setup of the angle between the main camera and the guide camera to get some stars for guiding (I use a Celestron OAG with an ASI174mm -- which are both great together) and that can take some time to get just right. Then, of course, depending on sky conditions, achieving a low guiding error at native fl isn't always possible although I must say that I haven't had much of a problem not having round stars and if I do, software like BXT is wonderful at taking care of that. The only other thing that gives me pause is that the Celestron Dew Heater ring I use (in conjunction with a Celestron Dew Controller and thermistor) produces spikey stars at times, especially noticeable at native fl. I've mostly remedied this by using a low aggressiveness (2-3) with the Celestron Controller, but there are times the spikey stars still appear. So, there are just a few things that require a more careful setup of the EdgeHD 8 as opposed to a refractors, which are fairly easy to set up and requires less time and attention, especially at shorter fl. But, as I said, I really do like the EdgeHD when everything is working! Just a little more work to get it as good as I'd like it, that's all.

@@gregerianne3880 I've heard about the dew ring creating the spikiness in the outer stars. I haven't bothered with Celestron dew ring controller, since I think it's much overpriced. I had only a fixed output power supply that ran it at 50%, but I recently bought a new rheostat controller for it (since my other was recruited by the Ares-M camera) for about $20 on Amazon. With the new rheostat, I can run the dew ring at a gentler 5% - 10%. Even then, with the aluminum dew shield, perhaps I can forgo even that.

@@SKYST0RY I agree completely: the Celestron Dew Controller is really overpriced. I do take advantage of the extra ports on it for additional dew heater outlets and power outputs, so I make good use of it. And, honestly, the thermistor/heater combination really works wonderfully together and as I said in my initial comments, I turn it on and forget it knowing I won't get any dew or frost on the corrector plate. So, yes, very expensive but at least it works well (except for the spikey star problem).

That's true. Seeing is a big limiting factor and once you cross a certain threshold of aperture, seeing becomes the primary constriction. We are at a point in technological development where the scopes and cameras we use can typically excel the seeing conditions. But if one does imaging in areas of exceptional seeing, aspects such as resolving power of the optics becomes more relevant. Since in I live in an area that can easily reach Bortle 1.5 on a good night, and seeing and transparency can coincide, it matters for me. If someone were shooting from a typical suburban Bortle 5-7 setting, and maybe struggle with seeing and transparency, it would matter less. An astronomer once said: The better the seeing, the more the optics matter. But Starizona techs have a great article on this that summarizes that if your goal is a great image, sometimes you have to toss the math and just think this way: Big object, low focal length; small, distant object, high focal length; and more aperture is always better. That's a paraphrase lol, but the article is well worth a read. If you want to read it, google "Starizona" and cross reference to "Harold Nyquist". Brilliant article.

The first of that series of five videos will come out on April 11.

It is easy to collect them, isn't it.

Not really. I just made a starfield shot with a planetarium and screen composited it over the image.

I'm glad that went smoothly. What mount are you going to put it on? OAGs are not as hard as is sometimes implied. At least, I didn't find them to be any big deal. Main thing, make sure the widest side of the guide cam sensor faces the body of the scope, keep the crystal outside of the line of sight of the imaging cam's sensor, and do your initial focusing by day. Also, use an OAG with a focus ring to avoid lots of intense frustration.

@SKYST0RY The only part I messed up was loosening the fork without the Dec motor BEFORE marking it in several spots. But that should only be an issue if I'd want to re-fork it for some reason. It'll be riding on a ZWO AM5. I rust received the Player One FHD OAG Mini, and will be pairing it with a Player One Sedna-M. I would like to eventually build a Dark Sky Geek EAF for OAGs, but I don't have 3D-print capabilities yet. I may be buying a Pegasus Astro SCT bracket kit or the ZWO one, to see if they will work for my LX200. Unfortunately, my LX200 didn't come with the Zero Image Shift Autofocuser, so I will have to retrofit another EAF. I haven't found a ton of good info on this older OTA yet, so there may be lots of trial, error, & fabrication involved!

There is a good discussion on Cloudy Nights about RC vs SCT. Look up "choosing-between-a-rc-or-sct-optical-design". I haven't used a RC scope, but from what I understand, they are excellent for photometry with scientific purposes, less so for imaging. You may also want to consider a good Newtonian. They can be very affordable and give great results.

​@@SKYST0RY I have an RC and to be honest actually its designed for imaging most than anything, yeah definitely bigger ones are even used for scientific purposes, tho mine performs fantastically and i love it, i neither have an extraordinary mount, i actually have an Ioptron gem28 and my whole rig weights 8kg, tho rms is very stable, usually under 0.4" so i don't complain, love every single buck i put into it

You may find this useful when setting up your plate solving on NINA with a high FL scope. Run a test image through Astrometry.net and see what it shows as your actual FL. I was having a devil of a time getting my images to plate solve till I did this. The problem was the math showed the scope shooting at 1240 mm, but Astronometry said it was actually shooting at 1207 mm with the reducer/corrector. I think that is a result of camera placement in my image train, but I am not sure. Anyway, a difference of a few mm is enough to throw off plate solving through ASTAP, but you can sort it out quickly by letting Astrometry.net tell you what your actual FL is and then programming that answer into the NINA plate solving options. Then ASTAP works much more smoothly.

But this whole collimation process is such a nightmare 😭

It's a bit tedious but that bad, as long as one doesn't become perfectionistic about it. With an SCT collimation is something you'd only need to do rarely, anyway. Once or twice a year if not on a permanent pier. If on a permanent pier, probably less.

I’m in the market for one 😮‍💨

That's a lot of mirror, and a lot of FL. I am envious. You are definitely across the threshold where seeing is likely to be your major limiting factor, which is good. You should be able to use it to resolve great images of remote galaxies, small nebula and structures like planetary nebulae. Put an IR sensitive camera on there and the possibilities are endless.

@@thugg_nuggets I know someone in Chicago area that is selling his Meade 14" LX200 ACF FOR $4000 ... if you're interested?

Definitely. A properly imaged Iris would be much better. What is going to work for you depends a lot on your photographic goal (and not doing something dumb like I did when I deleted my dithering entry in the sequencer).

I definitely agree that last image of the Iris was not the best comparison. As noted in the video, it was flawed (mea culpa). However, I even examined images from persons who had gotten better but lower aperture and FL images and, upon cropping in, the detail softens. That is always inescapable due to the information chokepoint created by real estate usage on camera sensors along with some other factors. I plan to reshoot the Iris this summer with the 81 mm and run a new comparison and contrast.

It's true. You can't be the limitations imposed by the sky. The FL you choose ultimately comes down to your local factors as well as your personal choice in what you want to accomplish. I have a friend in the RASC who has taken some breathtaking images with nothing but a DSLR and 115 mm lens on a Star Adventurer.

Reminds me of my old days chasing the astronomy bug, when Carl Sagan was my hero. I tried strapping an X-15 camera onto a mount that I could turn with wire attached wheels. Never worked out great. Things have come a long way. If you attach a 2x Barlow to a 750 mm Newt, you will get 1500 mm of FL, but remember you'll cut your F ratio in half (which actually means double the F stop rating). But that's the price one pays to go after those small astronomical structures.

I think that's the general consensus--reflectors of all types take a bit more work whereas refractors are much more point-and-shoot. Mainly, the extra work with reflectors is just coma correction and occasional collimation. But for that extra work, you get so much more focal length, aperture and light gathering ability at little extra cost. Sometimes (like with Newtons) reflectors are even cheaper than comparative refractors. My Celestron C8, for example, has about three times the aperture and focal length, and six times the light gathering ability, of my 81 mm refractor, yet it costs only $300 more.

@@SKYST0RY In my experience, when someone says light gather ability, it is usually referring to brightness. This subject can quickly go down a deep rabbit hole. When you said in the video and your reply, "light gathering ability", are you referring to brightness? I think the general consensus for brightness is that for visual observation aperture is the ruling metric and that your statement of the SCT having six times the light gathering ability "brightness" of your WO81 is acceptable with the modifiers being magnification/exit pupil. But in astrophotography, light gathering ability "brightness" is based on focal ratio. When I use my SCT vs my Redcat71, I expect to get the benefit of greater resolution and magnification but will have to spend considerably more integration time to achieve the same brightness and therefore the same SNR compared to my Redcat71. Thoughts on this? PS. You have become one of my favorite astrophotography channels, and I really appreciate your different approach to the subject.

​@@Hubaround1 Thank you, Hubaround1! I think the concept of "light gathering ability" refers generally to aperture but can have various implications depending on the context of the discussion. Example: Light gathering ability (aperture) and focal length create F ratio. Or light gathering ability (also as in aperture) can apply to resolving detail. With a given aperture, you can reduce focal length and get a faster F ratio, but that comes at the expense of decreased magnification, meaning larger regions are captured on fewer pixels of a sensor (thus resolution is irretrievably lost). You can easily see this by trying to crop into an image of something like a distant galaxy shot with a low FL scope. It doesn't matter how good the scope was, or how long the exposure was, the image will become softer as you crop. This is because cropping is just spreading out a limited amount of information. The restriction wasn't created by the F ratio, it was created by the lack of focal length. If a distant galaxy occupied only 10% of a camera's sensor on a low FL scope, cropping is just spreading out that 10% on the view screen. On the other hand, you can increase focal length and gain magnification, meaning your field of view will narrow but your target will fill more of your sensor. This spreads out less light over a wider area, so it will take longer to brighten the image and illuminate details. But this way you have more pixels recording information. This is how you get better resolution, but the cost is you will need more exposure time. Other factors not covered in this video come into play, but one of the biggest is seeing. If the seeing is bad, that imposes a serious restriction on what you can resolve regardless of your aperture and focal length. However, even then there are ways to mitigate that restriction. The image of the Horsehead that I posted was shot in poor seeing conditions. Really poor. I mitigated the restriction partly by shooting for several hours and selecting only the sharpest subs. I also shot short subs to take advantage of moments of clarity. I also only used the L filter in order to make best use of any light I captured. Still, when I shoot the Horsehead on a night of excellent seeing, the outcome will be much better. You can mitigate seeing; you can't defeat it. But these are topics for other videos. If I went into depth on this in a single video, it would be hours long. But you may enjoy this discussion of the topic on Cloudy Nights. Google: Cloudy Night and cross reference to "going-deep-is-a-large-aperture-worth-it". It is one of the best discussions I've ever read on the matter.

​@@Hubaround1 My definition of "light gathering ability" here would be "light energy gathered per time unit". For distant targets like stars that's the same as unobstructed aperture area. What this does to your photos is boost signal to noise (by the square root of the light gathering ability). Assuming the same sensor area of course. All this is independent of focal length. In other photography stuff moves so you can't shoot mosaics/panoramas with a slow, long FL lens and get a good SNR from its large aperture. That fact and the resolution benefits of a large aperture being more of an issue in astro lead make me feel that while f number is more important for other photography, in astro aperture is king.

I just did a price comparison in the USA and Canada. Among refractors, they're actually pretty on par price-wise with the better makers. What alternatives are you thinking of?

It helps to have a low and high FL scope, for sure. Gives you more options. I think something like a 8" or 10" Newt is a good compromise, if you want one scope to do it all and don't mind the extra challenges of working with a Newt. They sure are fast, so you can relatively quickly make mosaics of larger targets. But something like a 70-80 mm refractor combined with a high FL SCT or RC or Newt makes a very good combination of scopes.