Question about mirror size and f ratio
Posted 24 January 2004 - 08:10 PM
Posted 24 January 2004 - 08:20 PM
Posted 24 January 2004 - 09:34 PM
Posted 24 January 2004 - 11:14 PM
Posted 25 January 2004 - 01:18 AM
I'd put my money on Dan's explanation and add that I'll bet this sort of thing is happening very frequently with the Chinese imports and that most likely the price you paid reflects what you got rather than what you thought before that you had gotten. BTW, focal length is often variable to several mm, so it's hard to say what your f-ratio is exactly without measuring the focal length of the mirror precisely.
Posted 25 January 2004 - 04:36 AM
Posted 25 January 2004 - 10:05 AM
Oh no! After reading my posts, is it possible I could be slipping to the dard side!
Posted 25 January 2004 - 11:14 AM
Posted 25 January 2004 - 12:03 PM
Step 2: hopefully Hardin will refuse to change your mirror or refund you for 4mm.
Step 3: sue for consumer neglect and misinformation and moral prejudice
Step 4: say to cameras it 'wasn't about the money' as you walk out of a store with a CGE14 with full range of TV eyepieces and TOA130 riding piggyback.
Posted 25 January 2004 - 12:36 PM
Then again, if you have plenty of focuser travel now, it should not really make a difference.
So if I understand correctly, as long as my focuser has enough travel, then I should be ok. Is this correct? Also, is there any way to accurately measure my focal length.
I would be curious to know from other commercial dob owners out there if they have ever measured their primary mirrors and if so, were they accurate as to what was advertised. e.g. 8" mirror being 203mm or just 200mm, etc.
Posted 25 January 2004 - 01:19 PM
With a 10"/254mm primary at f-5, you'd need an addtional 20mm of outward travel on your focuser (assuming the focal ratio is really f-5), or you could move the mirror a bit. So, as long as you have enough focuser out travel, or can move the mirror away from the secondary a little, you should be okay. Also, keep in mind that US mirror makers may use foreign-sourced (i.e., metric) blanks. So you may still get a 250mm 10" mirror anyway. But the difference in area between a 250mm and 254mm is only about 3.5%, so it's not a critical difference in light-gathering ability.
My 12.5" dob had the focal length engraved on the side of the mirror. It was a 12.5" f-6, and had a focal length of 74.6" instead of a true 75", so it was off 10mm. A faster scope might be off more as the mirrors are harder to grind accurately. Of course, if you order a mirror, somebody might be able to pick a slightly shorter one for you out of stock, which would alleviate any focal length issue to start with. So you'd want, say, a 1250mm focal length f-4.921 10" mirror instead of a 1270mm 10" f-5. But, like I said, it's not really that big of a deal.
I don't know how to measure the focal length of a mirror.
Posted 25 January 2004 - 04:16 PM
1. Pointing your telescope at the moon.
2. Putting a piece of tissue or some such over the focuser's eyepiece holder.
3. Focus the moon on the tissue.
4. Measure the current focuser height above the tube = A.
5. Measure the tube thickness = B.
6. Measure the distance from the inner wall of the tube to the center of the secondary holder = C.
7. Measure the distance from the mirror face to the middle of the focuser hole = D.
The focal length is approximately A+B+C+D.
Posted 26 January 2004 - 12:42 AM
That should be f/4.7 for your 254 mm 10" recheck that when you can I could be wrong.
Posted 26 January 2004 - 05:21 PM
I would call Hardin and ask them about it.
I think they probably rounded in the advertisement to be consistent with everybody else. This doesn't make sense since everything else is measured in mms.
Posted 26 January 2004 - 09:44 PM
If you go to Hardin's website and look at the specs for their 8" dob and run the numbers, you will see that your primary couldn't be a true 8" (203mm). If it were, with a 1200mm focal length, your focal ratio would be F5.9 not F6 as is stated in their specs. However, if your primary is 200mm then everything works out precisely. The next time you get a chance, measure your mirror and I would be willing to bet that it is only 200mm. One of the posts above probably explained it best. They are Chinese made mirrors and they just rounded them off to a nice even number and Hardin decided to call it a 8" or 10" scope just to keep everything standardized. In reality, the slight difference would be completely unnoticeable.
Please don't take this like I'm bashing Hardin. To the contrary, I am very pleased with my scope. I didn't purchase this scope expecting perfection for $495. I was just curious about whether this is industry standard for sizing mirrors, that's all.
This is what happens when it's cloudy for weeks on end!
Posted 27 January 2004 - 12:02 PM
I think that in a field where everything is mathematical and engineers are abound it makes sense to me that you questioned the difference.
If eyepieces and focal lengths are measured in mm then I don't know why the mirror wouldn't be.
My father is a retired electrical engineer and a professional photographer. He would have worked out everything on paper then called the CEO of Hardin to let him know that the "jig is up" so to speak.
I like my scope also. I am so new to this field, and unfortunately, I am mathematically impaired. If you did a heat scan on my brain while giving me a math problem, that area would be black. It just shuts down.
(I can plug things into a formula.)
I would think JCrilly would be the man to ask about this subject.
If you find out whats the what then please post it.
Posted 27 January 2004 - 12:06 PM
Posted 27 January 2004 - 04:56 PM
Also, mirrors will have a ground bevel of at least a few mm around the outside of the reflective surface which will reduce the reflective surface area also vs the actual OD of the glass blank.
Mirror making is more an art than a science, your going to get variations in FL. There is a human factor here folks.
We are not talking MIL specs here or building a NASA telescope for orbit. Hey even they have tolorences +/-.
I guess what im saying is you should expect variations on telescope specs, within reason. Most manufacturers will tell you what that is 1% or 2% etc. The variation listed earlier is not unreasonable. Dont sweat it.
Posted 27 January 2004 - 05:44 PM
Posted 28 January 2004 - 04:05 PM