Weasner's Stiletto Focuser Review

Stiletto Focuser

Last updated: 16 October 2007

What follows is a preliminary review (posted 4 July 2007). I will update my comments as I use it more during future trips to Oracle Observatory.

I have been discussing focusing challenges with my Nikon D70 DSLR, whether doing prime focus astrophotography (with or without the Meade Off-Axis Guider) or eyepiece projection astrophotography, even when using the Nikon Eyepiece Magnifier.

Richard Shell at Stellar Technologies International heard about this and was kind enough to send me one of their "Series IV Stiletto" Pro Model Precision Focusers. The device comes in a very nice small foam-padded case:

It includes everything (well, almost everything; more about that shortly) you need to get started. Your first impression is that this is a QUALITY product (and to not keep you in suspense, it IS a QUALITY product!). The manual is on one of those small non-standard shaped CD-ROMs. These can jam or destroy some CD-ROM drives, typically slot-loading drives. Since all of my computers have slot-loaders and the manufacturer says to NOT use non-standard (i.e, not 5.25" circular) CD-ROMs, I went to the Stellar Technologies web site and looked for a manual there. They do have a manual online but some users may experience problems trying to get the file; I did. Their web designers have coded the site for Windows as opposed to Internet Standards. This is unfortunate; they had a chance to provide 100% of buyers a good web experience but instead have chosen to limit the access to the manual to only Windows users. However, emailing them and reporting the difficulty will get you a copy of the 25 page manual via return email.

With the manual in hand you will learn about the technology used in the Stiletto and how to use it for achieving precise focus. The technology is a ronchi screen (grating) with 180 lines/inch (standard model, no case) or 300 lines/inch (pro model with case). A True Knife-Edge is included with the Pro model Stiletto as an alternative focusing method. According to Stellar Technologies, "for those operating at above about f/8, which is most of the ETX users, the Standard model will perform superbly." Also, included was a nice pen with an illuminated tip:

I had never used one of these type pens before and found it very handy when making notes of my testing in the dark. Thanks Stellar Technologies for including one!

A 25mm Plossl eyepiece is supplied for use with the Stiletto. Other 1.25" eyepieces can be used but some adjustments in the calibration might need to be made for optimum use. I stuck with the ronchi screen and supplied eyepiece for my initial tests. The system was evidently designed for maximum flexibility and allows for many orientations when mounted on the telescope. When ordering you only have to specify your camera or CCD model.

The Stiletto can be used at prime focus, with or with an off-axis guider, or with eyepiece projection. It attaches to your telescope via the same T-Ring Adapter that you would use with your camera and at the same location as well. It can even be used for piggyback astrophotography by removing the camera lens from the camera, attaching it to the "bayonet/Stiletto M42 assembly" (the round device on the left in the second photo below). I haven't yet tried this or tested the Stiletto with Eyepiece Projection. Both will have to wait for a future trip to Oracle Observatory. But if you do detach the M42 assembly use caution so that you don't lose the rubber O-ring that is on it. If you do lose it, don't panic. According to Stellar Technologies, it is not absolutely necessary except to prevent overtightening and not having it doesn't affect backfocus or basic functioning. If you change cameras your Stiletto can be user-recalibrated using a procedure described in the manual (not tested) or sent to Stellar Technologies for a free (plus shipping) recalibration at their factory. The Stiletto is warranted for one year.

So how do you use the Stiletto to focus the image from the telescope for your camera? The manual describes the steps in good detail. Basically you find a bright star that is near the object you plan to photograph, Magnitude 2.0 or brighter is suggested (I used Spica for my initial testing) and center it in your telescope. You attach the Stiletto using the same T-Ring that you use with your camera. What you attach is the complete device as seen in the top photo here:

You first look into the Stiletto with the eyepiece removed to center and roughly focus the star. You then insert the supplied (or other) eyepiece and do the final focusing. You will likely see several parallel bands of alternating white and dark lines. You then adjust the telescope's focus to eliminate the bands and see a uniformly gray area in the eyepiece. Achieving this "sweet spot" of precise focus is fairly easy but any telescope movement or atmospheric turbulence (including a telescope that has not reached "thermal equilibrium") will make it more challenging. With my LXD75-8"SC I noticed that as I got really close to focus (only seeing one or two bands) it was best to make a small focus adjustment and then let go of the focus knob to let the telescope settle down. I would repeat this process, perhaps going past the infocus position to the other side and then reverse direction. If you have an electric focuser you likely won't have to let the telescope settle down. Once you have achieved the proper focus position you remove the Stiletto from the T-Ring (leaving the T-Ring attached to your telescope), attach your camera, and take your images.

Yep, that is all that is required. So is it really that simple and does it work?

My first test was at prime focus using my Scopetronix Prime Focus T-Adapter. After just a few minutes of practice, moving in and out of either side of the infocus position, it was easy to see where that "sweet spot" was. Once I was convinced I had it, I went to remove the Stiletto from the T-Ring. Oops, the Stiletto wouldn't turn to release the T-Ring but the telescope moved while I was fiddling with it. So I removed the Stiletto with the T-Ring and T-Adapter still attached to see what was wrong. What was wrong is that the manual didn't mention that there is a metal "latch" that you have to depress to release the T-Ring lock. When the latch was depressed the T-Ring could be rotated and removed. After determining this I reattached the whole shebang back on the telescope, recentered Spica, refocused by watching the bands, and then tried to release the T-Ring. Hum, still wouldn't turn. After some more fiddling I was finally able to get that latch to depress enough to release the T-Ring. This was actually the hardest part of using the Stiletto! Unfortunately, the pressure required was such that the telescope would move and so the object was no longer centered. I hope that as I gain more experience I'll be able to release the lock more easily.

Here's an image of Spica that was taken at 1/30sec at ISO 1600. There is some slight image movement due to the camera's mirror movement and it is slightly overexposed but the image appears infocus!

I also did a test with the Stiletto mounted on my Meade #777 Off-Axis Guider. It was even easier to achieve focus, possibly due to the fact that there was no "star diagonal" in the light path as there was when I did the prime focus tests. Here's Spica, again at 1/30sec, ISO 1600. Focus looks good.

The Series IV Stiletto Pro model costs $279, with other models and options available starting at $179. If you are serious about astrophotography with a DSLR (or film SLR) or CCD you should consider this a worthwhile addition to your box of accessories.

I am very pleased with these preliminary tests on my LXD75-8"SC (f/10) and I look forward to using the Stiletto more with both my LXD75-8"SC and ETX telescopes, including Eyepiece Projection and piggyback astrophotography, on future trips to Oracle Observatory. I will report back here with the results.

[10/16/07]
I was able to do more testing on 12 October 2007 during a visit to Oracle Observatory. See that report for more information and test photographs taken with my Nikon D70 DSLR. As you can see from those photographs, using the Stiletto Focuser really helps achieve improved focusing. I did find it occasionally necessary to reattach the Stiletto to redo the focus as the SC focus will shift depending on the telescope's orientation. I plan to get a Focal Reducer for my LXD75-8"SC and look forward to trying the Stiletto with a combination of the Focal Reducer plus Off-Axis Guider. Stay tuned for more updates.