EdgeHD and SteelTrack a Marriage Not Made in Heaven

[DeepSkyMan]

Okay folks, so, for a variety of reasons, including but not least an appalling sequence of poor weather since my last post on this particular topic, I have been unable to obtain an accurate measurement of the focal length off my 8” EdgeHD incorporating Baader SteelTrack Focuser and Click-lock Diagonal.  I managed to remedy this situation over the weekend just past.

Initial measurements based on adding the optical path of each of the components together, had the complete resulting optical path at about 2400mm, however this was not bourne out in practice.  Using my 22mm T4 Nagler, at 2400mm focal length this should have given approx 45’ TFOV.  Using M81 and M82 as a yardstick (approximate separation 36’), they should easily have been visible in the same field of view, however they were not.  Extrapolating back from these values seems to be indicating a focal length somewhere  between 3000 and 3225, which makes the revised speed of the system a whopping F15 to F16.  This must be having a significant detrimental impact on the sub-aperture correction inherent in the Edge HD design, so I’m thinking of removing the Steeltrack from the system and replacing with a FeatherTouch or equivalent.  This measurement is actually consistent with a drift measurement I made last year, (instructions courtesy of Don Pensack), but discarded due to it being so far removed from the expected result nearer 2400mm.  But the proof as they say is in the pudding.  So for those considering adding a Crayford to their EdgeHD, based on this experience I would say don’t.   From feedback on this topic in CN, it is very possible that it is something to do with either the slightly faster primary/secondary combination in th EdgeHD and/or the sub-aperture correction in the baffle tube, possibly resulting in significant vignetting. (Thanks to Don Pensack and Edggie from CN, for their input).  If you are interested in reading the CN thread discussing this you can find it here:

 

https://www.cloudynights.com/topic/574002-calculating-effective-focal-length-of-an-edgehd/

 

Kind Regards

Paul.

Edited March 27, 2018 by DeepSkyMan

[Paz]

I found something similar with my maksutov and measured the effect with a microguide eyepiece. The further back the eyepiece is the longer the effective focal length of the scope becomes. I noticed this when putting on a dual speed focuser or putting a filter wheel in the path. Now if I'm measuring double stars refractors or reflectors have the advantage of having a fixed focal length so they only need calibrating once.

[DeepSkyMan]

Paz, I was aware that there would be some increase in FL as a result of introducing the SteelTrack, based on a rule of thumb for SCTs which says that FL is increased by 10mm for every 3.1mm of backfocus.  On this basis I should have been looking at approx 2400mm FL, and I was happy to wear this, even though this would have hurt the correction in the Edge design, and any potential vignetting would have been acceptable to me at this FL.  But this past weekends results show that the 3.1mm rule of thumb obviously does not hold true for the EdgeHD, and an increase to in excess of 3000mm is definitely indicating that the quality of the optical delivery has been severely compromised.

 

Paul.

Edited March 27, 2018 by DeepSkyMan

[YKSE]

I'm glad you come to the conclusion.

When I got C8 for some years ago, I did read quite a lot on CN, when Eddgie had numerous posts about the disadvantage of increase back focus with accessaries, therefore I haven't bother to add any external focuser to it.

To my understanding, increase FL with narrower TFOV and possible vignetting with max TROV eyepieces are minor fault, the biggest one the spherical aberration correction is too far away from the optimal, which has major impact on image quality.

 

[Highburymark]

Yep - that's the conclusion I came to when I owned an Edge C8 - stuck with existing focuser after reading all the threads on CN - and YKSE's expert light path calculations on SGL. Shame because the Steeltracks are lovely focusers - I have a Newtonian one on my Lunt solar scope and like it very much.