What is Backfocus? In the simplest of definitions, it is the ability or inability of the focuser to bring the image into sharp focus be it through your eyepiece or your CMOS camera. In other words, you rack the focuser all the in or all the way back but can not acquire sharp focus. Adding extensions tubes before the diagonal can correct situations where you need more backward focus, but for situations where you need to move the focuser closer to the OTA for proper focus is a tougher nut to crack. It is not like you can just loop off and inch of your OTA to make your telescope shorter. This article offers methods to assist in achieving inward focus.
I have a Stellarvue SV80 Access Telescope that I use for my nighttime wide-field visual and imaging activities but I also use it for my Solar observing and imaging routines as well - so this Telescopes serves me at night as well as daytime. If you have not had an opportunity to read my comments about the Stellarvue SV80 Access Telescope, please take a few minutes a give it a read, the link is below. This article covers my journey from conception, research, purchase, assembly, trial, and error, and finally reaching a solution for each of my visual / imaging situations that worked.
This article is to share a few ideas to anyone who happens upon this site and was seeking help on trying to achieve more in-focus. If you find something useful - great, share it with others. This article is not to discuss whether you should buy a pre-canned Solar HA system costing thousands or simply trying your OTA with a Daystar Quark, Solar Wedge, or film for a different experience. I can tell you, Solar observing and imaging are rewarding and challenging.
Using a Telescope at night for visuals is rather quite simple with fewer challenges because telescopes are basically designed to deliver great visuals of the night sky using any number of eyepieces you can fit into the diagonal. In theory, nighttime astronomers and astrophotographers normally require more back focus rather than in-focus, and from my experience adding back focus is less frustrating than acquiring more "in Focus" which most-Solar Astronomers require due to specialized filters. Achieving more back focus can be as simple as buying a set of good extension tubes and using them accordingly. In this article I will share some techniques to acquire more in-focus, as that seems to be the biggest challenge for people - I included.
There is nothing more frustrating to a hobbyist than gathering up all your gear, setting up all your equipment, and be frustrated when you have to admit to yourself you can not get enough in-focus - you stand there perplexed at what to do next.
If you are on the edge or just about ready to start your journey in Solar imaging or observing I would recommend before you make purchases carefully consider
what combinations of cameras, filters work right out of the box for the OTA you are considering. You can do a lot of that research through Astronomy forums such as Cloudy Nights, or troll through the internet looking for others who solved the mystery with their set up. Let's assume we are starting out with some great white light adventures - this will allow you to gain experience in imaging, processing the results, and eventually, be ready for the pinnacle of HA viewing and imaging. Regardless of where you start you will be faced with the dreaded "not enough" in-focus challenge.
As an example say you wanted a great white light setup and just maybe, your number one consideration might be a good quality solar wedge, and you went with the Baader Herschel Wedge over the Lunt Solar Wedge; why? the reason has nothing to do with any significant visuals differences between the two just that your research showed the Baader solution allows you to completely remove the nose piece holder and filter assembly making it easy to adapt those low profile 1 1/4 eyepiece holder or direct mount your camera to the solar wedge itself and shave at least 2 inches on the optical path. Using a Lunt Solar wedge you are pretty much stuck with their design. Lunt is a great visual wedge but the design is not optimized for photographers where the camera sensor needs to be closer - not further. Read my comparison of Lunt vs Baader - link below.
Your Telescope: I was lucky choosing Stellarvue SV80 because Stellarvue offers an optional "low profile" backplate which gives me 1/2' more inward focus movement. For Solar White Light and maybe eventually using a Daystar Quarks, you should carefully consider that OTA choice, Keep in mind if a Daystar would be in your line up, they have recommendations on the size and f stop for their Quarks + you may desire full solar disk view vs a small portion of the sun. My current solar set up uses a Stellarvue SV80 Telescope as my choice of OTA - why? Stellarvue makes excellent Telescopes and has high-quality assurance checks before they reach your hands, each telescope is inspected and pass a QC before being shipped + they are USA based company and the best thing is - there is a human you can call and speak with. I would highly recommend you take time to select your telescope - some are more suited and more easily adapted to perform both tasks, but sometimes we have what we have.
Telescopes choices vary depending on your preference either Solar or nighttime. It would be better but not cheaper to have one telescope for day and one for the night. Solar Telescopes can be less expensive OTAs since you do not really require highly color corrected optics using rare-earth glass. You just need a good, solid, sharp refractor and the size depends on your budget. I personally like refactor that delivers close to a full solar disk when I use my Quarks, so my 80mm F/7 @ 560mm is a bit too long even when using a focal reducer on the Quark. However, it is excellent for White Light Continuum or Calcium H line and or Nighttime wide-field observing. 4 out of 5 attributes I can live with. But none of this matters if you cant focus the subject - right? If you are gathering up your shopping list a short tube OTA might be worth a look.
Solar Observing and Infocus: Technically the quarks should allow for great solar observing because they are designed as an "eyepiece" that drops into a diagonal. However in real life that sometimes is not the case and you need another 1/2" more in focus to get the sun to snap into view. I have found that some eyepieces work better with the Quark than others, not just because of better contrast, etc but some seem to allow the focus to be achieved while others need even more in-focus. So, try a few different eyepieces first if that is the fix - you are in luck.
Some of the options in the illustration are shown that I went through to get my telescope, Quarks, and White Light Wedge to reach in-focus. What is not shown is my other Baader Diagonal that like my Baader Solar Wedge allows the eyepiece holders to be removed and attaching a low profile holder, shaving off about 1/2 inch, and every mm count. In practice using a good diagonal you can re-configure it using shorter 1.25 eyepiece holders and attach your camera direct to the diagonal. Adapters are the key -You may require an M43 to M44 male to female adapter to be able to remove the eyepiece altogether- You basically are trying to remove anything that can be un-threaded not needed that is in the optical path.
Solar Imaging Infocus: From my experience, most Solar imaging rigs require more in focus when using accessories such as Daystar Quarks. The first step in solving your focus problem is determining if you need more in focus or more back focus, and looking through an eyepiece with a big fuzzy object will not instantly tell you which way you need to go. One simple technique I use for my Solar is to first pop in an eyepiece and look at the disk - is it Hugh and fuzzy? While looking slowly rack the focuser inward - Does the disk get smaller an tighter? if so you need more in focus. If the disk gets fuzzier or larger, then you need to rack your focus back. Once you know which travel direction is required - normally inward, and you can see a blurry object in the eyepiece, I throw on a camera, set the exposure until it is white/over-exposed then I slowly move the focuser in if the disk gets smaller and more defined - confirms you need more in focus. But how to achieve that when your focuser is hitting the back of the telescope? Reduce the metal parts!
This is the challenge, because your OTA is physically in the way of the focuser's inward travel and bottoms out before achieving sharp focus, if so you have a few possible solutions that you can apply to help solve the issue.
- If possible, try switching to a 1 1/4 eyepiece diagonal that switch will normally cut about 5mm off the optical path.
- Another possibility is to direct mount your camera to the telescope; bypassing the diagonal. My ZWO camera will direct mount with an adapter.
- A possible solution if finances allow is to acquire a better "modular" diagonal - see images. One that can be broken down and reconfigured.
- Does your telescope use replaceable visual backs? if so you may be able to purchase a low profile visual back? This alone may be enough to acquire focus.
- Can the entire focuser be replaced with a low profile one? If you are desperate give moonlight a call, link below.
- An extreme measure if your telescope has a replaceable focuser - consider a visit to a machine shop to chop off some of that OTA length.
- Finally, you may have to admit defeat and look around for another Telescope - I suggest a short tube version - maybe.
There is one thing I should mention, even though you may not be able to achieve enough in focus for visuals, at times the camera when mounted in the right configuration will have focus and you can at least get a few images while you try to solve the other challenge. This happened to me, and I eventually took my old OTA to a machine shop and cut off 2 inches and had it re-threaded. You do what you got to do.
|- Original Setup -
||- Optimized Setup -
Real life Practice - My Baader Herschel wedge and camera setup would not achieve in focus as it was originally configured when purchased, I was about 1/2 inch shy of pure visual greatness. My solution was to swap out the 2" eyepiece holder with a low profile 1 1/4 one along with replacing my Stellarvue 40mm visual back with a 20mm low profile visual back - Problem solved both for imaging and observing - perfect!
Real Life Practice - My Daystar Quark in focus problem was solved by using the low profile visual back I purchased from Stellarvue along with the 1 1/4 eyepiece holder solved my visual and imaging problems.
Real Life Challenge: My new Daystar Calcium H Line gave me fits for visual because I could not get enough in-focus using an eyepiece, however I could direct mount my ZWO camera and get perfect focus with almost 98% full disk views for imaging. To solve my visual observing challenge I bought one male to the female adapter and un-threaded the nose from the eyepiece, used the adapter, and mounted it directly to the back of the Quark Calcium H - now I can observe using the Quark Calcium H. The pitfall to this set up comes when you want to change eyepieces - you need to un-thread it from the Quark and re-attach the other eyepiece vice drop in. A good zoom eyepiece will help avoid having to switch out the eyepieces for a different view, just use the zoom.
In Summary: I was having trouble acquiring enough inward focus travel to obtain proper focus using cameras or eyepieces. My approach to solving this was to make a couple of purchases that help increase the inward focus travel. One purchase was to replace my 40mm visual back with a low profile 20mm version, secondly, I made a couple of purchases of adapters that allowed me to direct mount and swap out the 2" holders as necessary. Overall, after a few weeks of troubleshooting - I am now happy with my setup and ease of use. Also, I simplified the change from white light, calcium H line, or H-Alpha using the Quark with a fairly simple and quick system, and to make the task even more pleasurable I now carry all my supplies out in the field in one Pelican Air Case shown to the left here. Having everything organized and at the ready makes set-up simple and less frustrating, and that is exactly what I need. Simple with no frustration! Now, get out there and enjoy some Astronomy,...