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What size threads for front of 3.5" Questar?

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#1 Erinsimaging

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Posted 26 May 2012 - 08:50 PM

Anyone know what the specificans/size/threads are for the front of the 3.5" Questar? I was thinking about purchasing the correct size UV photo filter to screw into the front and using it as a bit of added protection when I take my Field model out birding.

Erin

#2 Suk Lee

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Posted 26 May 2012 - 09:39 PM

Don't know the thread count, but highly doubt you can find a UV filter of the right gigantic size. Questar offers a protective flat with the requisite flatness but it isn't cheap. Ironically flat is harder to make than curved...

#3 Terry_McComas

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Posted 31 July 2012 - 08:18 PM

I don't have a clue as to the thread specifications, either, but wanted a UV filter for my 50th Anniversary model, as I do a lot of public outreach and am nervous about little fingers, covered in peanut-butter. The 95 mm. filter, from B + W, fits perfectly, is superb quality has a bomb-proof coating and costs considerably less than Mother's flat. (I ordered mine from B + H, in New York.)

#4 MBoys

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Posted 18 February 2014 - 01:30 PM

My 1975 3.5" Questar front threads are 95mm... so any 95mm filter should work.

#5 munirocks

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Posted 03 March 2014 - 03:12 PM

I don't have my Questar yet but I purchased the B+W filter ahead of time, for public outreach events, based on the testimony from others that the fit was good. There are several reasons that I opted for the B+W:

The optics are very good, and they use high-quality Schott glass.

There are two types of coatings available: a single coating (SC) and a multi-resistant coating (MRC). Make sure you buy the one you want. The MRC is actually harder than glass so is very scratch resistant, and you don't need to clean the MRC very often anyway because it is water-repellent. Good for dewy nights and wet days.

The B+W ring is made of brass, so is very strong and resistant to corrosion/oxidation.

The brass won’t cold-weld to steel or aluminium, so won’t get stuck on the scope if it gets left on for a long time.

The brass makes it easy to spot a fake. There are apparently cheap Chinese fake filters in the market made of aluminium, so buyer beware! Buy from a reputable source. The brass is noticeably heavy in the hand (the filter weighs 68 grams) and you can see the yellow brass colour coming through the fine laser-etched lettering on the front of the filter ring.

The B+W filter is also available in both Clear MRC and UV MRC versions. I got the UV MRC because I’ll usually be looking through miles of atmosphere (either in daytime or nighttime). If I was going to use it mostly as a long-distance microscope or other short-range activities I’d consider the Clear version.

#6 ChristianG

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Posted 04 March 2014 - 03:05 PM

I got the UV MRC because I’ll usually be looking through miles of atmosphere (either in daytime or nighttime). If I was going to use it mostly as a long-distance microscope or other short-range activities I’d consider the Clear version.


Well, you made the good choice!

In a Questar 3.5 with R1 secondary, the light passes twice through a thick meniscus lens made out of glass that is a good UV absorber.

So I doubt there would be any difference between the 362$ Clear MRC 'filter' (it does not filter anything) and the 325$ Clear UV Haze MRC filter.

But you are saving 37$ with the UV Haze, since it is made of regular glass which absorbs UV like the meniscus lens, and the Clear one has to be made of some special glass so as not to filter out UV...

But both are nice filters, no question.

--Christian

#7 munirocks

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Posted 09 January 2015 - 05:27 AM

Update: Now that I actually have my new Questar and can try the filter on, I can confirm that the 95 mm filter from B+W is NOT a good fit. It is a terrible fit. The filter is so under-sized compared to the threads on the Q that if I wiggle the filter I can get it all the way to the bottom of the threads without even rotating it. It is then difficult to get off.

 

I don't think the issue is with the Q threads, as the Questar screw-on lens cover and Questar full-aperture solar filter both fit perfectly.

 

A quick calculation shows that 95 mm is 3.74016 inches. This is suspiciously close to 3.7500 inches, which is what I think the Questar threads are. As the Q was designed in America in the 1940s I would be surprised if there were any metric threads on it anywhere.

 

3.7500 inches equates to 95.2500 mm, which is a quarter of a millimeter larger than 95.0000. When it comes to filter threads, 0.25 mm is a very large numerical discrepancy.

 

Nice filter. Shame it doesn't fit. With this new information I would not recommend any 95mm filter for the Q.

If you want a protective flat then I suggest that you go wiith the Questar version, which probably has 3.75 inch threads.


Edited by munirocks, 09 January 2015 - 05:30 AM.


#8 munirocks

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Posted 09 January 2015 - 12:56 PM

FYI, my vernier calipers made the following measurements:

 

 

Inside of the Q threads: 94.6 mm

 

Outside of the 95 mm B+W filter threads: 94.6 mm - so it won't mesh.

 

Outside of the Q lens cover threads: 95.0 mm

 

Outside of the Q full-aperture solar filter threads: 95.0 mm

 

 

If a thread is "95 mm", what is this actually supposed to mean? The inner diameter of the outer threads? The outer diameter of the inner threads? The average diameter at which the threads are supposed to mesh? 



#9 ChristianG

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Posted 09 January 2015 - 09:28 PM

Hi.

 

Metric threads are measured, on the male part, as the outside diameter of the threads. Or, the equivalent clearance hole diameter the threaded bolt would fit in. So for instance, my Celestron C80ED main tube is threaded (male) with M90X1 threads, and the diameter of the whole thing is 90 mm. An M6X1 bolt is 6 mm in diameter.

 

For the front of the Questar 1973 that I have next to me, the outside diameter of threaded part of the cap (or solar filter) is 95.0 mm. So that would indicate M95 threads.

 

Hop this helps.

 

--Christian



#10 Billydee

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Posted 09 January 2015 - 11:23 PM

I do not believe that Questar uses or has ever used metric items.  The OTA is 3.5 inches so I would think that the proper filter would stated in inches.  I agree with Munirocks as stated above.

 

Bill


Edited by Billydee, 09 January 2015 - 11:31 PM.


#11 93711dude

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Posted 10 January 2015 - 12:03 AM

try a Hoya 95mm - works for me



#12 ChristianG

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Posted 10 January 2015 - 06:24 PM

Well, 95.0 mm is very close to 3.74" but I doubt thread is 3.75". Photographic filter threads have been in mm for as long as the Questar has been in production...

 

--Chrisitan



#13 Billydee

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Posted 10 January 2015 - 07:44 PM

Chrisitan,

 

This is not a normal filter.  The opening is occupied by the Lens Cap and Questar does supply a Protective Flat , Full Aperture for the 3.5 that is coated with Magnesium Fluoride (last time I priced it it was $380).  What people want is an item that costs less.  My Lens Cap threads measure 3.744 inches, they also measure 95.00 mm (my digital caliper is dual scaled).  This brings up the thread spacing and pitch as the problem and with just two threads on the Lens Cap it is impossible to measure these.

 

Bill



#14 ChristianG

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Posted 12 January 2015 - 03:38 PM

Hi.

 

Well, I have been trying to find the front thread size for a while. The Questar 700 camera lens had a 95 mm front thread, see

 

https://www.cameraqu...com/questar.htm

 

"Front filter size is that ever popular and economical favorite, 95mm."

 

I seem to remember reading somewhere (Yahoo Questar group?) that the telescopes also had 95 mm threads. Or at least, that the usual 95 mm filters fit on the front. I have a full aperture solar filter at home, I can measure it too, it may have more than 2 thread turns...

 

Anyway, good luck.

 

--Christian




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