Telescope Diagonals

Whenever I'm observing the night sky (as opposed to imaging the night sky), I use a diagonal placed into my focuser.  The diagonal reflects the light that is streaming through the telescope to a 90° angle making it easier to view through the telescope.  Some diagonals are at 45° but those are mostly used for terrestrial observing where the telescope is mostly horizontal or level.  The eyepiece is placed into the diagonal and, due to the diagonal reflecting the light 90°, the top of the eyepiece is now pointing upward making it easier to look down into the eyepiece.  This is the purpose of the diagonal...  to direct the light to the eyepiece at a position that is easier for viewing.

If you had no diagonal to change the direction of the light, you would need to crane your neck to view straight through the telescope.  Sometimes you would even need to be sitting or lying on the ground to view the sky through the telescope if not using a diagonal.  A diagonal makes it easier to view through your telescope especially when the telescope is pointed upward toward the sky so it really is a necessary part of observing.

Diagonals come in different sizes.  Some are larger in diameter, some are longer while others are smaller and shorter.  Naturally, when you put a diagonal between the telescope and an eyepiece, it takes up space.  Some diagonals take up more space than others and this often will create problems since there is only so much space between a telescope and its focal point.  Each telescope comes to a focal point at some distance behind the focuser.  Sometimes this is an ample longer distance and sometimes this is a rather short distance.  Consequently, you need to use the appropriately sized diagonal with your telescope.  Additionally, prism diagonals are better with slower telescopes (f7 and up) while mirror diagonals can be used with any telescope requiring a diagonal.

Each telescope comes to a focal point at some distance from the focuser.  It is within this free space distance between the focuser and the focus point of the telescope that any accessories must be placed including the diagonal.  Some telescopes have a lot of space here while others have very little space.  If you have very little space or you need to fit a few accessories into this space, then you must use a smaller diagonal with a shorter optical length.  

Sometimes you might want to add something else in the optical path with that diagonal such as binoviewers.  Binoviewers also take up space in the optical path.  The more space you eat up in the optical path, the more back-focus you need on your telescope.  Back-focus is the ability to move your focuser into and toward your telescope.  Binoviewers eat up 100mm (four inches) of optical path so understanding the optical length of all of your gear is important.  Diagonals eat up anywhere from 60mm to 110mm so it is good to know the length of each when figuring out the configuration of your telescope. 

I recently purchased a binoviewer so knowing the optical length of my diagonals is quite important.  Understanding the optical lengths of my diagonals will make it easier to adapt each telescope for use with my binoviewer.  Yesterday, I finally put aside some time to actually measure and calculate the true specifications of each of my diagonals and some of the results surprised me.

This first diagonal (pictured below) was the biggest surprise.  This diagonal is, by far, the absolute smallest diagonal I own.  It is only a 45° diagonal and it even appears to have a much shorter optical length than all my other diagonals.  What we see, however, is very deceiving because this tiny diagonal is actually the diagonal with one of the longest optical lengths.  The light must bounce around on three sides of the prism making the optical length a very deceivingly long 89mm!

This tiny 45° diagonal came packaged with a crappy Orion Mak-Cass telescope.  I don't like Orion in the least but I thought that this little diagonal would come in handy because I figured its optical length would be very short and short diagonals come in handy in certain circumstances.  I clearly was wrong in my thoughts about this diagonal but I suppose I should not be surprised by anything that comes from Orion...  I simply do not like Orion and probably never will.  For a few very valid reasons unrelated to this blog entry, I'll never purchase another thing from them again.  Putting aside their ridiculous business practices, I feel they are making things far more difficult for amateur astronomers with their selfish and frivolous lawsuits.  This Orion telescope I purchased, as well as this diagonal that came with the telescope, is yet another disappointing product from a company whose nonsensical store policies and absurd lawsuits baffle and frustrate me.  Enough said about this particular diagonal so let's move on to better products and the intended subject of this blog entry.  

This next diagonal, shown below, was also quite a surprise but, this time, it was a pleasant surprise.  

This cheap little Celestron prism diagonal came packaged with a Celestron telescope I purchased a few years ago primarily for solar astronomy.  This telescope is my Celestron XLT 120mm Omni refractor telescope.  I threw this little diagonal in a drawer figuring it's quality was lacking since it seemed to be a cheaply made kit diagonal.  I recently pulled it out of the drawer to use with my new binoviewer because I had heard that it had a short optical length.  

I was wrong about this little prism diagonal.  I thought it was a cheap piece of junk but this time I was pleasantly surprised.  This Celestron Prism diagonal has a very short optical length which makes them great for observing with my binoviewers!  The eyepiece holder on this diagonal was of rather poor quality so I replaced the eyepiece holder on this prism diagonal with a better one.  I removed a better eyepiece holder from another diagonal to use on this prism diagonal.  This better eyepiece holder has a compression ring for holding eyepieces in place so I am now very happy with the quality of this little prism diagonal too.  When there is only a setscrew and no compression ring, the setscrew mars the nosepiece of the eyepiece so I definitely prefer compression rings.

At the top of the line among diagonals are the Baader Prism diagonals, shown below.  The prism in this little diagonal is made my Zeiss so it is an outstanding quality.  The whole diagonal oozes quality.  It is a solid little piece of hardware with outstanding optics and fine precision parts.

This particular Baader Prism diagonal came with a helical fine focus eyepiece holder.  This focusing mechanism is a nice little feature to have in certain circumstances but I didn't need that for binoviewing.  Not only didn't I need this feature but this helical fine focus eyepiece holder is taller than most which makes the optical length of this diagonal a little longer than necessary and even longer than advertised.  It is definitely still on the very short end of the spectrum when it comes to 1.25 inch diagonals but I knew it could be shorter if I could replace that helical fine focus eyepiece holder (see the photo following this photo below for the 'Ultra Short' eyepiece holder on the Baader Prism diagonal). 

This photo, below, shows the 1.25" Baader Prism diagonal with an Ultra Short eyepiece holder in place of the helical fine focusing eyepiece holder.  This cuts the physical measured optical length of this diagonal by approximately 9mm bringing it down to approximately 60mm in total optical length.  Realistically, however, very few things will seat down fully into this ultra short eyepiece holder because it is so short which means the optical length is not as short as physically measured.  

Almost all eyepiece nosepieces are longer than this ultra short eyepiece holder so I think I'm going to add a 3mm extension to this ultra short eyepiece holder which will make the diagonal's total optical length approximately 63mm which will make this a bit more useful, I believe.  The quality and flexibility of this Baader prism diagonal is outstanding (thanks to its standard T2 connections) so adapting it to be more useful for me has worked out very well.

EDIT:  I did add the 3mm extension to this ultra short eyepiece holder (between the prism and the ultra short eyepiece holder) and now all my eyepieces seat fully into this ultra short eyepiece holder.  At this point, the total optical length of this prism diagonal configured this way is at 64mm.  That is a very short length for a diagonal!  

Additionally, I realized that if I change the standard 1.25" nosepiece on this diagonal to a larger 2" nosepiece, then I can remove the 2" to 1.25" adapter from the focuser which saves me an additional 10mm of space!  Being able to eliminate this adapter when using this diagonal provides an 'equivalent' optical length of only 54mm as compared to all my other diagonals since my other diagonals need to have this 2" to 1.25" adapter in the optical train and the adapter eats up 10mm of length.

This William Optics Prism diagonal, below, has always been one of my favorites.  It is a bit longer than the two other prism diagonals I own (above) but most of that extra length is due to the very nicely dampened helical fine focus eyepiece holder.  It is worth noting that this William Optics prism diagonal is still shorter than all my mirror diagonals even with this longer than average eyepiece holder.  

It is obvious just by looking at it that this eyepiece holder is a bit taller than most.  Still...  this is a very nice prism diagonal.  This helical focuser on this prism diagonal comes in handy when I use it on my Celestron 70mm refractor because this telescope only has a single speed coarse focus focuser.  The nicely dampened helical fine focus of the prism diagonal comes to the rescue when used with this telescope.  Plus, prism diagonals work very well in slower optics like this f10 refractor.  I really like using this diagonal with this old refractor telescope.  Now if I can just find a metal focuser for this telescope I would be exceptionally happy with this inexpensive beginner refractor!

This 1.25 inch High Point mirror diagonal, below, has always been a sort of utilitarian type of diagonal for me.  I have it because it was nice to have a decent diagonal on hand for setting up an extra telescope in the yard.  This diagonal is yet a bit longer than the William Optics prism but still a good length and a decent quality.

This 1.25 inch Apertura mirror diagonal, below, is another one of my favorites.  This one is used solely with my Skywatcher ED 72mm refractor and it resides in that telescope's case.  I have no complaints about this sharp-looking diagonal but I was surprised to find that, when it comes to optical length, it is on the longer end of my diagonals.  Still...  it works very well in my Skywatcher refractor whereas 2 inch diagonals do not work well in this telescope's 2 inch focuser.  The optical length of 2 inch diagonals is too long for this particular telescope so I strictly use this 1.25 inch Apertura mirror diagonal for this telescope and it works very well.

I should point out that before purchasing this Apertura diagonal for use with my Skywatcher ED 72mm refractor, I tried using my William Optics prism diagonal on that telescope.  I quickly found out why prism diagonals are not recommended for "fast" telescopes (my Skywatcher ED 72mm refractor is an f5.8 so it is considered relatively fast).  The prism diagonal introduced some chromatic aberration to my views yet this telescope is perfectly corrected when using a mirror diagonal.  This prompted me to purchase the 1.25 inch Apertura mirror diagonal and I use that diagonal exclusively with this refractor now.  

I also have two much larger 2 inch mirror diagonals so I calculated the optical lengths of these two diagonals while I was taking measurements and making calculations.

First, I found that my 2 inch Explore Scientific mirror diagonal was a solid 100mm in optical length.  This is a very nice diagonal although I'm not too fond of the tapered nosepiece.  I'd like to replace it with a straight nosepiece but the thread is an odd thread so I cannot find a replacement nosepiece.  This is actually a bit of nitpicking because this diagonal really is a very nice diagonal.

Ironically, this tapered nosepiece does not sit well in my Explore Scientific 102mm APO refractor.  I say "ironically" because both are Explore Scientific products and this diagonal came packaged with this telescope yet they don't fit well together.  You can see a long scratch on the nosepiece in this photo below.  This scratch is due to how this diagonal sits in the focuser on my Explore Scientific 102mm APO refractor.  I do use this diagonal exclusively with that particular telescope but I would like to find a better nosepiece for this nice diagonal.  

My diagonal with the longest optical length is my 2 inch High Point mirror diagonal.  This one has a length of 107mm.  Honestly, I don't use this one often as it is relegated as a spare.

I put together a chart last night making it a bit easier to see the comparison of optical lengths.  On a sidenote, I haven't created a chart or graph in a word processor in decades so I had some learning to do again in order to create this chart!  

 

I do have a few solar wedge filters which are a type of diagonal but I didn't measure any of those.  First off, I would need a sunny day to measure and calculate the specifications of my solar wedges and it was not sunny today.  Next, I simply did not have the time to add the solar wedges to this batch of diagonals.  Although I chose to skip these types of diagonals, I really should do this sometime and write about the results also.  

These solar wedges have a significantly longer optical length than the standard nighttime diagonal.  One of the reasons for this is because they have much longer eyepiece holders.  Another reason could be due to the type of prisms in solar wedges.  These eyepiece holders also house one or two extra filters in them so extra room is necessary for the filters so the eyepiece holders are noticeably larger and longer than on standard diagonals.  I think the optical length of my 2 inch solar wedge is around 120mm or so.  I'm going to guess that my 1.25" solar wedges have an optical length in the 90mm+ range.  

This was an enlightening exercise.  I now have a much better understanding of the quality and usefulness of each of my diagonals!