29 replies to this topic

[gordtulloch]

When I got an NEQ6 mount for a song it included a 10" F/4.7 Skywatcher 250PDS newtonian, not something I needed but hey, it was essentially free. I have the scope on the NEQ6 now but it's pretty heavy for the mount, and a tad long so manuvering it around in the micro-observatory is kind of hazardous. I would like to reduce the focal length as much as I feel capable of (not my first mirror, I've done 8" and 9" mirrors of moderate focal lengths) and reduce the tube length so it's lighter and more easily fits in the observatory. 

 

I'd also like to increase the size of the secondary mirror and get a bit more backfocus so I can use a rotator, right now with a mono camera and filter wheel I'm pretty close to bottoming out when coming to focus.  I am thinking I'd like to grind the mirror down to F/3.3 (I think this is as short as I can go without needing a Paracorr, I already have a GSO coma corrector that works well on my 16" F/4.5) and cut the tube down to about 35" from the current 50" or so. I'd also like to replace the vixen dovetail with a CGE dovetail I already have, which just requires a new adapter puck on the NEQ6.

 

Next step is to take the scope apart and see what the quality on the other components are to decide whether they are reusable or should be replaced.

 

Comments welcome! One initial question is that I want to use my ASI183MM Pro with the scope but upgrade eventually to APS-C sized sensors so I would like to illuminate an entire 23.5mm x 15.7 mm frame so choosing a secondary that does so is important.  An 88mm secondary provides (per Newt) a 20.81mm 100% Illuminated Diameter which is pretty close (I still need to firm up some measurements). It's a 35% central obstruction but for an astrograph fully illuminating the sensor seems to be important, no? Any comments on designing an astrograph would be appreciated. This scope will never be used visually.

 

 

[Garyth64]

If the 10" f/4.7 mirror is good, don't regrind it.  Get another blank and a tool, and grind that into a f/3.3.  Sell the 10" f/4.7.

 

To fully illuminate a 23.5mm x 15.7mm sensor, you'll have to illuminate a 1-1/8" diameter image plane.  And if your secondary to image plane distance is 9", then using an 88mm secondary should be just about right. (-per Texereau)

Edited by Garyth64, 24 March 2023 - 07:53 PM.

[gordtulloch]

If the 10" f/4.7 mirror is good, don't regrind it.  Get another blank and a tool, and grind that into a f/3.3.  Sell the 10" f/4.7.

 

Could do that I guess - looking at the classifieds looks like I could get maybe $350-400 for it, the only place I found that seems to cater to amatuers is https://firsthanddiscovery.com/ which sell a blank for $249, which with shipping will probably be $300... seems like more effort than it's worth, unless I'm undervaluing the Skywatcher mirror?
 

Thanks for the confirmation on the secondary!

[MeridianStarGazer]

Could do that I guess - looking at the classifieds looks like I could get maybe $350-400 for it, the only place I found that seems to cater to amatuers is https://firsthanddiscovery.com/ which sell a blank for $249, which with shipping will probably be $300... seems like more effort than it's worth, unless I'm undervaluing the Skywatcher mirror?

Thanks for the confirmation on the secondary!

$300 shipping for a 10"? Thanks for telling us. Is only their office address is domestic?

[gordtulloch]

So after having a look at possible revenue from selling the OTA (some few as high as $750 but most $250-300) and optics going for less than that I'm just going to regrind what I have, no cash out the door and no messing around with shipping.

 

I took the scope off the mount (to be replaced with an 8" SCT for the time being) and took the mirror cell off:

 

 

Pretty heavy! And looks like a 3 point support with some "reef-em-down-real-good" mirror clips. I'm pretty certain I'll replace it but given the tube diameter is pretty small (and I do want to reuse the tube) options might be limited. I'll probably end up just reinstalling the new mirror on the Skywatcher cell and do some testing on how the mirror works in that cell.

 

Next up: Strip the mirror and check for strain, cast a tile tool, and start hogging down to a 4.8175mm sagitta.

Edited by gordtulloch, 06 April 2023 - 11:10 AM.

[MitchAlsup]

I was at TSP one year talking to a famous astrophotographer and his wife.

Tony said that he had converted his 25" F/5 Obsession into a 25" F/4 himself.

His wife told the other part of the story::

That almost every component of the telescope had to be re-ATMed and that it took 2 years.

[ccaissie]

Seems a shame to un-do the f/ 4.7 mirror, but I re-did a 10" f/6 to an  f/8 and didn't cry a bit. Project sounds good.

 

Hey! Wait!  Just checked my junk pile and I have a 10" full thickness Pyrex "mirror" i would love to swap.

It's fine ground all around, and has an 8" f/1.6 concave in the middle.  It would require you to grind it out by working the outer area, as it's too deep, 14.5" f.l. for the 8" aperture.  Sagitta is too deep, but how much?

 

P.M.me if you'd swap.  

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Edited by ccaissie, 06 April 2023 - 03:50 PM.

[Stephen Kennedy]

The problem with trying to change the focal ratio of a perfectly good existing mirror is that by grinding, you are destroying a perfectly good mirror.  You will have to start again from rough grinding to fine grinding, polishing, figuring and coating with no guarantees that the new mirror you created will be better, or even as good, as the one you have now.   Better to keep or sell the mirror you have and start with a fresh blank to make a mirror with a different focal ratio.

[gordtulloch]

Hey! Wait!  Just checked my junk pile and I have a 10" full thickness Pyrex "mirror" i would love to swap.

No thanks I have no interest in shipping mirrors around.

Edited by gordtulloch, 06 April 2023 - 04:15 PM.

[gordtulloch]

The problem with trying to change the focal ratio of a perfectly good existing mirror is that by grinding, you are destroying a perfectly good mirror.  You will have to start again from rough grinding to fine grinding, polishing, figuring and coating with no guarantees that the new mirror you created will be better, or even as good, as the one you have now.   Better to keep or sell the mirror you have and start with a fresh blank to make a mirror with a different focal ratio.

My new mirror will be better because 1. It'll be the focal length I want/need and 2. It'll be tested and crafted to be the highest quality mirror I can make, not a factory mirror and 3. every time I image with it I'll know it was my own creation.

 

 

[Steve Dodds]

No thanks I have no interest in shipping mirrors around.

You are going to be shipping it when it comes time for coating, unless you are going to spray silver it.

[gordtulloch]

I have a few of mirror projects on the go I want to get grinding through, so getting into spray silvering is a given.

 

Honestly, if people are just going to argue with me about reusing my existing glass, no need to post anything further. I thought it might be helpful to some beginners to share a project like this, and no doubt I'll have things I would like input on. The substrate decision is made.

Edited by gordtulloch, 07 April 2023 - 10:27 AM.

[ccaissie]

Not arguing, just offering a suggestion in support of your project, and hoping to gain some advantage for myself.  We here on CN are an annoying bunch, I know.  8^)

[gordtulloch]

We here on CN are an annoying bunch, I know.  8^)

Yeah I know. I'm annoying myself, that's why I felt I had to make it clear if this thread was going to be three pages of opining on my substrate decison lets just not do that

[gordtulloch]

Stripped the mirror this weekend with FeCl. Took a full 24 hours but this FeCl has been reused a few times so it might not be working at 100%. Interestingly it seemed to attack the coating via pinholes in the surface which eventually joined up and left the clean surface except for one stubborn spot that took another application to clear. Fresh solution zapped it inside an hour. Always use latex gloves with FeCl it's messy stuff! Almost like an oil, rub all you want the blank is going to have an orange coating til washed off with fresh water.

 

 

On the third image note the largish bubble in the substrate at about 2 o'clock midway to the edge. There are a few but they are midway through the disk so they won't surface taking the blank down to F/3.3.

Edited by gordtulloch, 11 April 2023 - 04:32 PM.

[Shed9]

Yeah I know. I'm annoying myself, that's why I felt I had to make it clear if this thread was going to be three pages of opining on my substrate decison lets just not do that

I get where you are coming from on you doing what you want to do however to be totally fair, those comments you elicited came across as genuine advice to me, advice bore out of experience. That's just the risk you run in sharing your project on a public forum. My point is that people are going to chime in at other points in this focal length reduction operation again. No need to take offence at it and besides, sometimes that advice, whilst possibly not relevant to you, may be to others reading the thread, again it's just a function of a public forum. I don't know anyone who starts a thread who can dictate how many pages are filled with what content or what direction it takes - take it as a compliment that people are engaged.

 

I'll be watching this one, I'm really interested in the work and effort involved in regrinding a factory mirror, in particular the home silvering aspect.

[star acres]

I see it differently. You should procure a lighter telescope for your mount. Your Sky Watcher? If you don't love it, sell it to someone who will care for it. Taking something good apart to make something isn't creating. Telescopes don't shorten, anyway. 

[totvos]

Stripped the mirror this weekend with FeCl. Took a full 24 hours but this FeCl has been reused a few times so it might not be working at 100%. Interestingly it seemed to attack the coating via pinholes in the surface which eventually joined up and left the clean surface except for one stubborn spot that took another application to clear. Fresh solution zapped it inside an hour. Always use latex gloves with FeCl it's messy stuff! Almost like an oil, rub all you want the blank is going to have an orange coating til washed off with fresh water.

 

IMG_2946.jpgIMG_2947.jpgIMG_2953.jpg

 

On the third image note the largish bubble in the substrate at about 2 o'clock midway to the edge. There are a few but they are midway through the disk so they won't surface taking the blank down to F/3.3.

FeCl is gross stuff, smelly and stains like all get out. On the plus side, it wipes silver in minutes. It was probably your overcoat that slowed it down for you, but just guessing at that.

[gordtulloch]

I see it differently. You should procure a lighter telescope for your mount. Your Sky Watcher? If you don't love it, sell it to someone who will care for it. Taking something good apart to make something isn't creating. Telescopes don't shorten, anyway. 

Sorry, that's nonsense.

Edited by gordtulloch, 13 April 2023 - 11:26 PM.

[gordtulloch]

I get where you are coming from on you doing what you want to do however to be totally fair, those comments you elicited came across as genuine advice to me, advice bore out of experience. That's just the risk you run in sharing your project on a public forum. My point is that people are going to chime in at other points in this focal length reduction operation again. No need to take offence at it and besides, sometimes that advice, whilst possibly not relevant to you, may be to others reading the thread, again it's just a function of a public forum. I don't know anyone who starts a thread who can dictate how many pages are filled with what content or what direction it takes - take it as a compliment that people are engaged.

 

I'll be watching this one, I'm really interested in the work and effort involved in regrinding a factory mirror, in particular the home silvering aspect.

I'm not taking offense just trying to keep the thread on topic.

[Shed9]

I'm not taking offense just trying to keep the thread on topic.

Appreciate that, my point is that the more you dictate what people can and cannot say on this thread, then the less inclined anyone will care to say anything at all. As above, you are not the only beneficiary to this discussion, it will go off on tangents that are useful to others whilst possibly useless to yourself, just the outcome of an open discussion on an open platform is all. The thread will always have the capacity to predominantly concentrate or come back to the original post but tangential discussion is not a bad thing, nor something you can realistically control without disengaging your audience from your actual original intended discussion. I say this as considered (respectful) feedback and in anticipation of more interesting discussion.

[gordtulloch]

I took a few minutes to strain test the mirror. Normally I use a DSLR in front of my big screen TV but with the smallish mirror I just took the polarizer off of my DSLR lens, put the mirror in front of a computer monitor showing a white Word document, and rotated the polarizer til the screen was as dark as possible, and snapped a photo with my phone. This picture was the result.

 

 

My interpretation of this image (based on Mike Lockwood's article on testing mirror strain (here) is this mirror is not completely without internal strain but it is minor. A similar image from Mike's site indicated:

 

"Now let's see what a little bit of strain looks like - note the lighter regions near the edges of the blank.  Stress often shows up in glass as a four-sided symmetry, and the "gray" portion of the mirror looks vaguely square.  As it gets worse, it looks more like the dreaded "cross" that some opticians speak of.  Some inclusions within the glass again show up as bright spots, one of which has four-sided symmetry itself, though again none are large enough to be of concern.  Again, faint arcs from the generating are visible, but not of concern. This blank made a very good telescope mirror."

 

The blank has some stresses extending down from the right at 1 o'clock down to towards just right of centre as well as at 4 o'clock. Overall however these appear minor and shouldn't impact whether the mirror can hold a figure. Certainly if you compare this image with my 24" blank that's sitting in a crate awaiting re-annealing it's night and day!  I'm proceeding with grinding this blank to F/3.3.

[gordtulloch]

In case you didn't see it THIS is what severe strain looks like in my 24" blank... current plans are to grind this blank to F/3.3 and use it to cast a form that can be used in a pop-up kiln as a form to melt it (to "heal" damage to the mirror) and then re-anneal it so it is usable. It's looking like a long term project LOL

 

Edited by gordtulloch, 15 April 2023 - 12:23 PM.

[PrestonE]

I took a few minutes to strain test the mirror. Normally I use a DSLR in front of my big screen TV but with the smallish mirror I just took the polarizer off of my DSLR lens, put the mirror in front of a computer monitor showing a white Word document, and rotated the polarizer til the screen was as dark as possible, and snapped a photo with my phone. This picture was the result.

 

IMG_2975.jpg

 

My interpretation of this image (based on Mike Lockwood's article on testing mirror strain (here) is this mirror is not completely without internal strain but it is minor. A similar image from Mike's site indicated:

 

"Now let's see what a little bit of strain looks like - note the lighter regions near the edges of the blank.  Stress often shows up in glass as a four-sided symmetry, and the "gray" portion of the mirror looks vaguely square.  As it gets worse, it looks more like the dreaded "cross" that some opticians speak of.  Some inclusions within the glass again show up as bright spots, one of which has four-sided symmetry itself, though again none are large enough to be of concern.  Again, faint arcs from the generating are visible, but not of concern. This blank made a very good telescope mirror."

 

The blank has some stresses extending down from the right at 1 o'clock down to towards just right of centre as well as at 4 o'clock. Overall however these appear minor and shouldn't impact whether the mirror can hold a figure. Certainly if you compare this image with my 24" blank that's sitting in a crate awaiting re-annealing it's night and day!  I'm proceeding with grinding this blank to F/3.3.

Our first mirror a 12.5 inch that we were making at F4.5 with Carl Zambuto's help online gave us fits,

until we did a similar Strain Test.

 

Our pattern was just about like yours.  The Figure would move depending on the temperature in the shop

that we were doing the Figuring.  A 10 deg F temperature change totally ruined the figure.  Carl sent me

another Blank that was Fine Annealed and we were able to make a Fine Mirror that we later compared to

one of Carl's and it was as good.

 

At F3 your Envelope of Tolerances is Much Tighter than our F4.5 and as such, I would not recommend spending

all of the time that it will require to produce a good mirror with that amount of Strain.

 

Best Regards,

 

Preston

[gordtulloch]

Our first mirror a 12.5 inch that we were making at F4.5 with Carl Zambuto's help online gave us fits,

until we did a similar Strain Test.

 

Our pattern was just about like yours.  The Figure would move depending on the temperature in the shop

that we were doing the Figuring.  A 10 deg F temperature change totally ruined the figure.  Carl sent me

another Blank that was Fine Annealed and we were able to make a Fine Mirror that we later compared to

one of Carl's and it was as good.

 

At F3 your Envelope of Tolerances is Much Tighter than our F4.5 and as such, I would not recommend spending

all of the time that it will require to produce a good mirror with that amount of Strain.

 

Best Regards,

 

Preston

You're referring to the first image or the second? The first image doesn't appear much different than Lockwood's example... certainly the second image of my 24" shows it to be unusable but the 10" doesn't look bad.