Another MAK issue - actual focal length

[Avocette]

With the Moon and Venus as targets I have been experimenting with imaging with my SW SkyMax 150 Pro. I swapped out my usual 80ED refractor and tried using a Canon DSLR at prime focus on the back of the Mak which is fitted with a Crayford focuser. I calculated the FOV assuming that the focal length was 1800mm as stated in the SW specifications. But there was an obvious and non-trivial discrepancy with the actual FOV. After platesolving with Astronomy.net in Ekos, the FOV measured for the Canon EOS 600D was 36.1’ x 24.1’. A quick calculation makes the actual focal length to be 2122mm, i.e. 18% longer than expected. I appreciate that in its normal visual observation mode my Mak would have a slightly longer optical path to an eyepiece through a star diagonal, and the adjustment to the primary mirror to reach this new focus point would change the focal length, but surely not by so much? And in the case where the diagonal was attached directly to the visual back and not through a Crayford focuser add-on, surely the focal length would be rather similar to the case with the DSLR at prime focus. Any thoughts?

 

[andrew s]

The effective focal length depende very sensitively on the mirror separation and distancee between the secondary and the focal plane. These are linked quantities.

Regards Andrew 

[JamesF]

I suspect that the quoted focal lengths for the Skywatcher Maks are, near enough, what you'd expect to get with the supplied diagonal and an eyepiece.  A secondary focuser and DSLR will probably have a noticeably longer optical path than that.  Perhaps as much as 100mm longer as a minimum?  Moving the primary to get the focal plane within reach of the DSLR sensor when you need to move that much probably will have a significant effect on the effective focal length of the OTA.  The effective focal length does change quite dramatically as you move the focal plane further out.

When lunar imaging with my 450D and 127Mak I usually put the T ring straight onto the visual back which gives me a lunar disk that just fits into the full field of view of the sensor.  At that point I think I am pretty close to the quoted focal length of the OTA.  I did once try putting a 50mm extension tube between the visual back and the T ring.  With the image in focus it wasn't even remotely close to fitting on the sensor, so what you're seeing doesn't surprise me at all.

James

[Avocette]

I had to follow this up with a bit of measuring........and I was surprised to discover that the distance from the rear body of the Mak to the sensor of my Canon DSLR is almost exactly the same as the distance from the same point (where the visual back screws on) via the star diagonal to the field stop of the standard eyepiece normally supplied with the scope by SW (within about 10mm). In other words, there will be very little adjustment of the primary mirror if I were to go from my present imaging arrangement to visual observing with the standard setup. For me this suggests that the focal length of (my) SW SkyMax 150 Pro as supplied for visual observing mode is closer to 2100mm than 1800mm!

[JamesF]

I think the "optical length" of a 2" diagonal is usually around 100mm (less for prism diagonals I believe), so it's possible you're over-estimating.  The visual focal length may also depend on the design of the eyepiece, I guess.

James

[Alien 13]

I have played around with spacing but have never convinced myself as to which is best so in general I try to put my DSLR sensor at the same point as that obtained with a diagonal/eyepiece combination. I would like to know though what is likely to be the best distance to minimize any vignetting..

Initial thoughts are greater spacing=longer focal length=shallower light cone=less vignetting caused by the focus tube but then the longer focal length may cause issues because the secondary might not get fully illuminated.

Alan

Edited April 9, 2020 by Alien 13

[JamesF]

48 minutes ago, Alien 13 said:

I have played around with spacing but have never convinced myself as to which is best so in general I try to put my DSLR sensor at the same point as that obtained with a diagonal/eyepiece combination. I would like to know though what is likely to be the best distance to minimize any vignetting..

Initial thoughts are greater spacing=longer focal length=shallower light cone=less vignetting caused by the focus tube but then the longer focal length may cause issues because the secondary might not get fully illuminated.

I assume it applies to Maks as well though given the difference in design it may not hold, but there is occasional discussion that (for example) Celestron SCTs are designed to give the best corrected image at their quoted focal length.  There may perhaps be a trade-off between correction and field of view (like there's ever not a trade-off where telescopes are concerned :)

James

[Alien 13]

3 minutes ago, JamesF said:

I assume it applies to Maks as well though given the difference in design it may not hold, but there is occasional discussion that (for example) Celestron SCTs are designed to give the best corrected image at their quoted focal length.  There may perhaps be a trade-off between correction and field of view (like there's ever not a trade-off where telescopes are concerned

James

I expect the manufacturers of Maks and SCTs optimize the performance for use with the std diagonal and eyepiece spacing but then add a T2 thread on the visual back so maybe they have optimized for both visual and imaging at differing points.

Alan

[Captain Scarlet]

I noticed the same thing via a vis reduced field of view from that expected, which prompted me to do the calculations of effective focal length of my Mak 180 in combination with knowledge of the focal lengths of the individual mirrors and mirror separations etc. It turns out that the effective focal length doesn’t vary slightly, it varies A LOT.

If I add an external focuser, in my case a Baader diamond steeltrak,  FL becomes just under 3000mm, rather different  from the nominal 2700mm!

I'm still doing the calcs and will post separately in due course, but the same will apply to any moving-mirror-focus scope.

M

[JamesF]

It certainly will be interesting to see the calculations when you've done them, Magnus.

James

[Alien 13]

12 minutes ago, JamesF said:

It certainly will be interesting to see the calculations when you've done them, Magnus.

James

+1 to that..

Alan

[Louis D]

I think the quoted focal lengths are assuming the use of a 1.25" visual back and diagonal.  The fact that they supply it with two inch accessories doesn't change that.  I'll agree it's probably misleading.

[michael8554]

Here's a quote from Cloudynights:

Just to expand on your comment, my TEC6, a 6” F/12 Mak Cass, has a moving mirror, according to Yuri the F ratio varies from F/11 to F/14 as the primary is moved in and out

Michael

[Avocette]

Further practical measurements of a SkyMax 150 Pro focal length. 
So I removed the add-on Crayford focuser and reverted to the originally supplied 2” visual back. Into this I fitted a 2” to 1.25” adapter and used this with a T2 threaded eyepiece tube to mount two cameras, firstly the ASI120 and secondly the Canon EOS 600D. Based on the specified focal length of 1800mm, the FOV with the ASI120 calculates as 9’ x 6.6’ (‘=arc minutes). I chose M44 as a target hoping that Ekos and the downloaded astrometry.net files would find a match, but without success. Subsequently I swapped the ASI120 with my Canon EOS 600D, which required refocusing by 1 and a quarter clockwise turns of the knob. This time, the theoretical FOV calculation yields 42.6’ x 28.2’, and with platesolving completed, the measured FOV was given as 41.2’ x 27.4’. This suggests that the actual focal length with this primary mirror setting was 1860mm. I attach photos of the scope and camera for this configuration, and also with the ASI120 in position. My guess is that only with the prime focus position of the ASI120, or with an eyepiece placed directly without star diagonal very close to the visual back, would you achieve the 1800mm standard specification focal length for the SkyMax 150 Pro.

[michael8554]

Was the 600D fully illuminated via a 1.25" adapter?

Michael

[Avocette]

I believe so (the sensor diagonal is around 27mm). Here is a test image of M101 in that configuration stretched in KStars FITS viewer (alongside the DSS image) which shows some stars near edges and corners.