The OP is referring to our discussion under the thread that he mentions. Now that there is a (another) new thread to discuss sizing the secondary I’ll provide a little more detail as well as some additional comments on the topic since my post a few weeks ago.
I’ve built a number of Dob’s and my biggest struggle when collimating comes when there is very little room around the edge of the primary reflection in the secondary. The tighter the distance the more difficult it is to do correctly.
Admittedly, the OP in the previous thread has real concerns regarding secondary size since the price difference between a 4” and 4 ½” is pretty steep. I don’t fault anyone for wanting to really dig deep to determine if it’s worth it or not.
I purchased a 22” F/4.16 primary and 4” secondary as a set, a few years ago. The primary mirror was made by John Lightholder, the secondary was made by Galaxy Optics. I had no reason to doubt the original owner’s ability to size the secondary mirror and so proceeded with specifying the appropriate secondary holder from AstroSystems.
I chose to use the Starlight Instruments SIPS focuser on this telescope. I don’t own a P2 and have only one other telescope that I have no plans on changing the focuser on to use with the SIPS. For me it was the right choice. I also decided to purchase a 31 Nagler 5 to use as my low power eyepiece with this scope. Combined with the SIPS it yields a 6.5 exit pupil but also requires more in-focus than many other eyepieces.
Getting back to the secondary holder, since I was also going to have AstroSystems “build in” the offset to the holder itself I needed to calculate it. Along with the primary mirror specifications, I plugged the FIF value and intercept distance into the Dakin formula to size the mirror and determine the offset and that’s when I noticed that I needed to take a closer look.
For those who are not familiar with using this formula, for visual use it is suggested to use a value of 0.5” for the diameter of the Fully Illuminated Field (FIF). You would use a larger value if sizing the secondary for photographic use, and a smaller value if you’re interested mainly in planetary or other high magnification / narrow field of view work. If you use a FIF of 0” you will calculate the smallest possible secondary you can use in your scope and good luck trying to collimate!
For my initial calculation I used the 0.5” Diameter Fully Illuminated Field value and an intercept distance of 14.75” which resulted in a secondary mirror size of 4.03”. Since the AstroSystems holder has a lip, my clear aperture would really only be about 3.8”. I wouldn’t get the 0.5” FIF value I was looking for with the 4”.
I ran the numbers again, using a secondary size of 3.8” to see what the FIF would be and the result was 0.25”. Since I was building the scope to provide views at all magnifications ranges, very low to very high I was concerned about the diameter of the FIF being smaller than the “tried and true” value. At this point, I began to sweat. What does the difference between a 0.5” FIF and 0.25” FIF look like? Running the numbers through the Bartels calculator seemed to show that I had nothing to worry about.
I made a quick line drawing in CAD using the 4” mirror and there was still a little room between the edge of the primary and the secondary holder – not a lot, but a bit. Since it came as a package with the primary I decided to proceed with using it in the design. As a backup though I sourced a 4.5” secondary – just in case.
Once the structure was complete I tested the optical system to make sure it focused with all of my eyepieces. I then brought the structure back into the lighted work area and it is at this point that observed that I could not see the entire primary reflection in the secondary mirror. It could have been a case where some modification of the lip that holds the mirror in place - I didn’t measure it – would have been enough. Having less metal around the edge of the secondary holding it in place was not an option that I would feel comfortable with. This aspect, along with having a smaller FIF than I wanted made making the decision to replace it with something bigger a non issue for me.
Needless to say, the 4.5” solved the problem nicely.
After posting a few weeks ago I thought about this a bit more. Neither my CAD drawing nor the Dakin formula result showed that the 4” would not work. But visually it didn’t. Looking at the SIPS installed in the optical system shows the focal plane lying inside of the SIPS body, about 1.75” below the top of the focuser. If you installed a 2” low profile focuser in place of the SIPS, you would bring the top of the focuser 2.25” closer to the mounting board and probably allow the user to see the entire primary mirror reflection in the secondary.
So, if you’re planning on installing the SIPS into a system where you are trying to use the smallest possible secondary mirror you should look at all aspects thoroughly before making a decision.
Edited by andreww71, 15 April 2019 - 06:20 AM.