Modding a 6" starblast. Any advice ?

[DanyalAG]

Hello, everybody. I have a starblast 6 (6" f/5), and I'm looking to modify it in several different ways-namely:

1. when I first recieved the scope, I was a complete newbie, and so didn't know anything about star-testing and optical quality. However, after a week or so of searching online, I learned how to star-test, and quickly figured out that the scope has (mild) astigmatism. A little more research, and after rotating the primary mirror several times in its cell, I was able to confirm that the primary actually has excellent optics, but the secondary mirror suffers from astigmatism.Now, I am also not quite happy with the focuser on this scope. So, I want the change the secondary mirror and the focuser for a low-profile one. The secondary mirror has a minor axis of 47 mm(1.85 inches). After some searching and measuring the scope, I came up with these measurements:

I. Primary mirror to secondary mirror distance = 570 mm (22.44 inches)

II.Secondary mirror to focal plane distance = 180 mm (7.08 inches)

I reckon that if I reduce the secondary-focal plane distance to around 115 mm(4.5 inches), I can get away with a secondary mirror size of 35 mm(1.3 inches) for a fully illuminated field of around 0.4 degrees and an edge magnitude drop-off of 0.3 magnitudes. However this will reduce the obstruction percentage  from 31% to 23% by diameter, and from 9% to around 5% by area. My question is, will I be able to detect the better contrast of this smaller secondary mirror or not? And if yes, how much of a difference will it actually make?

2. I am also going to modify the mount, as the azimuth motions have gotten a bit iffy. Any good links, or has anyone done this before?

Any help regarding these issues would be greatly appreciated.

[Ricochet]

2 hours ago, DanyalAG said:

I. Primary mirror to secondary mirror distance = 570 mm (22.44 inches)

II.Secondary mirror to focal plane distance = 180 mm (7.08 inches)

I reckon that if I reduce the secondary-focal plane distance to around 115 mm(4.5 inches)

Are you aware that the focal length of the scope is fixed and a function of the primary mirror? If you reduce the secondary - focal plane distance by 65mm then you will have to increase the primary - secondary distance by 65mm. This means that you will have to move both the focuser and secondary spider supports up the tube. I don't think that the tube is long enough to support such a movement unless you also plan on extending/replacing the tube as well. I think the most cost effective solution for this scope is to get a replacement mirror that is sized for the current position of the secondary. Depending on the length of time you have had the scope and the warranty provided in your country, a direct replacement from the manufacturer might be available (if you bought it new).

[DanyalAG]

48 minutes ago, Ricochet said:

Are you aware that the focal length of the scope is fixed and a function of the primary mirror? If you reduce the secondary - focal plane distance by 65mm then you will have to increase the primary - secondary distance by 65mm. This means that you will have to move both the focuser and secondary spider supports up the tube. I don't think that the tube is long enough to support such a movement unless you also plan on extending/replacing the tube as well. I think the most cost effective solution for this scope is to get a replacement mirror that is sized for the current position of the secondary. Depending on the length of time you have had the scope and the warranty provided in your country, a direct replacement from the manufacturer might be available (if you bought it new).

Yes, I am aware of that fact, but I think that 6.5 cm of space is available at the front of the tube. if it's not, then maybe I'll extend the tube(with PVC).I really want to optimize the scope, as I think that planetary views are a bit too low on contrast.Anyways, thanks a lot.