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Question about mirror size and f ratio

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#1 Mark233

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Posted 24 January 2004 - 08:10 PM

I bought a Hardin Optical DSH-10 dob about 4 months ago and the other night I decided to pull the primary mirror to add a center mark on it to make it easier to collimate. I measured the mirror and found that it is only 250 mm edge to edge. Now I may not be the sharpest knife in the drawer, but I do know that 10" should be 254 mm. Did I get screwed on my scope or do they measure mirrors like they do lumber e.g. a 2 X 4 is actually only 1.5 X 3.5. Or are all 10" mirrors 250 mm? If that is the case, it changes the focal ratio from 4.9 (using 254 mm) to 5.0 (250 mm). I know this is probably splitting hairs but things like this just bug me!

#2 Tom L

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Posted 24 January 2004 - 08:20 PM

If it makes you feel better, stick with the lumber measuring system! You gots what you gots! Send it back if you don't like the view. f/5 is better than f/4.9! ;) :D

#3 Mark233

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Posted 24 January 2004 - 09:34 PM

The view is fine. I am very happy with the quality of the optics. I was just curious about how they measure these things.

#4 Guest_**DONOTDELETE**_*

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Posted 24 January 2004 - 11:14 PM

{WAG} The chinese company made it to an even metric value, and the US marketdroids decided that because it sounded sloppy and required more ink to print in the adds that 9.84" = 10" {/WAG}

#5 Mike Hosea

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Posted 25 January 2004 - 01:18 AM

Mark,

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.


#6 matt

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Posted 25 January 2004 - 04:36 AM

On the other hand, not a few Europeans are wondering why schmidt-cassegrains come in 203mm diameter and refractors in 102! Even a lot of European made telescopes have inch-related diameters.

#7 Mark233

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Posted 25 January 2004 - 10:05 AM

OK, if that is the reason, what would happen if I decided to replace my primary mirror with a high quality mirror made in the USA and specified that I wanted it to be 10" and F5 (which is what my current scope is). Would I then in fact get a true 10" (254 mm) mirror and if so, 1.) would it fit my holder, and 2.) would I be able to get it to focus properly because it would have been ground as an F5 but in reality, with a 254 mm mirror, my scope would be F4.9 (4.921 to be exact). Would this slight change make a noticeable difference? Would a slightly larger mirror mean that my secondary would no longer be big enough? :question: The reason I ask all of this is because I do plan on eventually replacing all of the optics in an effort to turn a good quality scope into a great one.

Oh no! After reading my posts, is it possible I could be slipping to the dard side! :help:

#8 Guest_**DONOTDELETE**_*

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Posted 25 January 2004 - 11:14 AM

Whether the primary can be easily replaced depends on your cell. Some cells are fully adjustable, and can handle a narrow range of cell diameters. Many are not. The difference in focal length between a 250mm f5 and a 254mm f5 primary, assuming they are ground accurately (most likely the final figure is off a few milimeters, as noted my Mike, above), is only 20mm, so you may have to move the cell 20mm away from the spider, or move the spider and focuser 20mm away from the cell to keep the focusing positions of your various eyepieces the same. Then again, if you have plenty of focuser travel now, it should not really make a difference. It is likely that you would have to replace the cell, and the primary mirror. I'd not worry about increasing the size of the secondary. Your fully-illuminated field of view will be a bit smaller, but I doubt you'd notice. Most reflectors don't have 100% edge-to-edge illumination anyway as it requires an excessively large secondary to do it with fast newts.

#9 matt

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Posted 25 January 2004 - 12:03 PM

Step 1: call Hardin
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.

#10 Mark233

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Posted 25 January 2004 - 12:36 PM

Then again, if you have plenty of focuser travel now, it should not really make a difference.


Warpd,

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.

#11 Guest_**DONOTDELETE**_*

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Posted 25 January 2004 - 01:19 PM

Mark223,
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.

#12 Mike Hosea

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Posted 25 January 2004 - 04:16 PM

You should be able to get a good approximate focal length by

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.
--
Mike

#13 jack45

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Posted 26 January 2004 - 12:42 AM

Mark

That should be f/4.7 for your 254 mm 10" recheck that when you can I could be wrong.

Clear Skies!

#14 Harvest Moon

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Posted 26 January 2004 - 05:21 PM

That is interesting about your mirror. I have the 8 inch, and I am pretty sure it measured 8 inches across.

I would call Hardin and ask them about it.
I think they probably rounded in the advertisement to be consistent with everybody else. :question: This doesn't make sense since everything else is measured in mms.

#15 Mark233

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Posted 26 January 2004 - 09:44 PM

Carolyn,

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!

#16 Harvest Moon

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Posted 27 January 2004 - 12:02 PM

I suspected rounding also when I read your post. Next time I take the mirror out I will measure it more carefully out of curiosity.

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. :shrug:

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.

#17 LivingNDixie

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Posted 27 January 2004 - 12:06 PM

Well I can't speak for Hardin but I know my Discovery mirror is not exactly the same as the specs mention. Every mirror is going to have some variation. I imagine the mirror has the actual specs somewhere written on it, maybe in ink or in the case of my Discovery, on a piece of tape LOL

LivingNDixie

#18 Guest_**DONOTDELETE**_*

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Posted 27 January 2004 - 04:56 PM

When mirror blanks are ordered from glass suppliers there is always a +/- spec. its is rare you will get a mirror diameter spot on. Different glass can have different specs as can suppliers/molds

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.

=D

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.

#19 Guest_**DONOTDELETE**_*

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Posted 27 January 2004 - 05:44 PM

If I was to choose between an f/4.9 254cm or f/5 250cm dobsonion I would take the f/5 250cm; the numbers are so much nicer!

#20 Harvest Moon

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Posted 27 January 2004 - 11:15 PM

Didn't think about possible tolerance +/-. Thanks.


#21 Mark233

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Posted 27 January 2004 - 11:54 PM

These are the things that make you go hmmmmmm! :question:

#22 Guest_**DONOTDELETE**_*

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Posted 28 January 2004 - 04:05 PM

Hubble was designed to a +-250 atom tolerance, and then they botched it with enough spherical aberation that the center of the field focused at a point 4" away from the outside edge.


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