All about SCT

ariefm71 · Post by **ariefm71** » Wed Jan 31, 2007 11:57 am

I used a caliper to measure my C8 rear baffle, it's 38mm:
C8 at F10: 38/2032*57.3 = 1.08 degrees
C8 at F6.3: 38/1280*57.3 = 1.7 degrees (e.g. with pentax xw 30mm)

Post by **weixing** » Wed Jan 31, 2007 12:46 pm

Hi,
May be the most accurate method of testing the maximum actual field of view is by using the drift star method:
By pointing the telescope at a star near the intersection of the celestial equator and the meridian. Took the timing with a stopwatch how long it takes for the star to drift across the field of view of each eyepiece (from edge to edge). The mathematical formula for calculating the actual field of view:
Actual field of view (deg) = time in seconds / 240 seconds

Have a nice day.

Canopus Lim · Post by **Canopus Lim** » Wed Jan 31, 2007 5:30 pm

Hi,

I did some simulations on the C5 using Zemax (defacto standard for optical design software..every optical designer use it); I know how to use the software quite well since it is part of my job. I don't have the essential parameters of the C5 like the radius of curvature for the primary and secondary mirrors, the diameters of the mirrors, the aspheric constants for the corrector plate etc. So, I did my own 'rough' simulation using the software (using known parameters like the primary mirror about f/2, baffle size, aperture, focal length) to check if it is possible for the C5 to achieve 2 degree field without significant vignetting.

My answer is YES. It is possible to achieve 2 degree and how much vignetting it has depends really on the design. There are many design parameters that can effect the performance of the C5. As said I do not have the design secrets of the C5 but from the rough simulation, the vignetting is rather gradual and it is possible to keep the vignetting to be less than 50 percent. Also, I am not going to spend weeks to design a good SCT. I am also not going to post pictures of the rough simulation too since very few will understand.

So that baffle hole of 25mm is not going to stop the C5 from achieving 2 degree true field.

Eyepieces have to follow the field stop basically it is the imaginary 'line' where the telescope focuses light and the light diverges into the eyepiece. Hence by having a stop, focused rays (these rays determine the true field) from the telescopes that fall out of the field stop will not enter the eyepiece and hence the field is limited by the field stop. For the case of the C5, focused rays are still able to go through the baffle. The baffle is meant to block stray light from entering directly into the telescope's exit hole instead of being reflected off the primary mirror.

rlow · Post by **rlow** » Wed Jan 31, 2007 6:57 pm

it is possible for the C5 to achieve 2 degree field without significant vignetting.

This tallies with my observation last night, there seems to be no significant vignetting seen in the 40XW in the C5.

Canopus Lim · Post by **Canopus Lim** » Thu Feb 01, 2007 2:24 pm

A review on the C6S SCT. The user used a 40mm 62 degree AFOV eyepiece giving 1.6 degree TFOV and did not observe vignetting visually.

http://www.astromart.com/articles/artic ... cle_id=333

A review on C5. The user used a 35mm Panoptic and could also get nearly 2 degree field too.

http://www.cloudynights.com/item.php?item_id=870

Canopus Lim · Post by **Canopus Lim** » Thu Feb 01, 2007 3:13 pm

I found an article that explains well enough on the vignetting issue in SCT.

http://www.mailbag.com/users/ragreiner/vignetting.html

I tried before C5+0.63 reducer+32mm Plossl (1.25inch) and it can achieve 2 degree fields. I have also tried C5+0.63 reducer+35mm Panoptic (2inch) and the field is very slightly bigger than the 32mm Plossl but it does not look as sharp and nice; The very slight increase in the field probably meant that the 'extra field stop' the Panoptic 35mm has been severely vignetted until I could only see a slight increase in field compared to the 32mm Plossl . The vignetting is mainly caused by the reducer and baffle of the C5 interplaying. The article tallies with what I observed that with a reducer in place, it is best to use 1.25 inch eyepieces. If one has 2 inch eyepieces, use a 2 inch diagonal with it and without the reducer. Also cheaper 2 inch eyepieces like UO MK70, Orion Optiluxe 40mm etc can be used with the SCT and gives good results for achieving the largest field possible without significant vignetting.

Al Nagler also recommends using 40mm 70 degree type eyepieces to achieving maximum field of 1.3 degree for a C8.

Post by **weixing** » Thu Feb 01, 2007 4:10 pm

Hi,

A review on C5. The user used a 35mm Panoptic and could also get nearly 2 degree field too.

http://www.cloudynights.com/item.php?item_id=870

Also saw this article at sometime ago... I think may be my eye got stamp when I do my test...

Anyway, that's a nice article on the SCT vignetting issue. I also notice the vignetting getting more serious on my DSLR when I install the F6.3 focal reducer.

By the way, since SCT achieve focus by moving the primary mirror toward and away from secondary mirror, so that the focal plane will move inward and outward of the visual back. This also mean that it is possible that at certain position onward, you won't get a fully illuminated field if the SCT didn't design properly. Hmm... now I seem to understand why Meade SCT use an oversize primary mirror.

Have a nice day.