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Collimation & cooling

balticbirch
 

Hi folks,

I recently picked up a Starmaster 20" f3.7 and was hoping to get some advice re collimation and cooling:

Collimation: I have a 1.25" sighttube/chesire, a 1.25" non-barlowed laser and a 2" auto-collimator and use them in that order. I've read many how-to docs on the internet but have little practical experience using them. If there is an experienced hand that wouldn't mind spending 10 minutes or so with me at the next starparty to validate my technique, I'd greatly appreciate it. Collimation seems quite critical at this f ratio.

Cooling: Any idea what size fan I need attach to my tailgate to cool a mirror of this size?

Thanks,
Tony.
Bob Olson
 

Hi Tony,

A 20 inch Starmaster!! I am not too jealous. I suppose you have a complete set of Ethos eyepieces to go with it!

There are many members of this list who can help you with collimation but Attilla gave me my lessons and explained all the steps clearly and without too much sarcasm about my lack of knowledge. See if you can con him into a collimation demonstration.

Have a great day.
Bob
Attilla
 

I collimate my scope every time I set it up. So i've had lots of practice.

But I can use a sight-tube. My eyes cant focus on the mirror and the cross hairs at the same time. I've used a chesire, but i find them to be a pain. I have an auto collimator, but I don't use it. It's extremely sensitive to focuser tilt, which for a visual scope is not a critical adjustment.

I can however show you how to collimate with a barlowed laser (mine). It's the quickest method I know. I've compared it to cheshire collimation and find the barlowed laser to be just as accurate.

Come to the next Carp star party. And I'll show you how I collimate my scope and you can try the method with yours.



As for fan cooling:

if you're blowing against the back of the mirror a 120mm computer fan will suffice. But you should turn it off when doing planetary observations. I would also recommend filtering the fan intake. (You will be amazed how quickly the filter gets filthy.)

If you're sucking air out of the tube, I suggest a big a fan (or as many fans) as you can get. The key in this case is to seal the bottom of the tube so the fan(s) dosent(dont) use suction.

If you have a choice sucking is probably better than blowing. But it it is harder.

The very best fan arangement is fans that blow straigh across the front of the mirror. This is the mist effective arrangement as it produced instant results (the key is to thermaly-homogenize the air infront of hte mirror. do that, and you dont actually need to cool the mirror). However it's the hardest to mechanically implement. It hink filtering is especially important in this case.

I use a filtered-back blowing fan on my 25" because I havent figured out how to arrange surface scrubbing fans on my 25" whithout hacking the nice wood to bits.

-ad

--- In OAFs@..., "balticbirch" <tonykourlas@...> wrote:

Hi folks,

I recently picked up a Starmaster 20" f3.7 and was hoping to get some advice re collimation and cooling:

Collimation: I have a 1.25" sighttube/chesire, a 1.25" non-barlowed laser and a 2" auto-collimator and use them in that order. I've read many how-to docs on the internet but have little practical experience using them. If there is an experienced hand that wouldn't mind spending 10 minutes or so with me at the next starparty to validate my technique, I'd greatly appreciate it. Collimation seems quite critical at this f ratio.

Cooling: Any idea what size fan I need attach to my tailgate to cool a mirror of this size?

Thanks,
Tony.
Attilla
 

Idiot at the keyboard. Right. Here are some corrected sentences:

--- In OAFs@..., "Attilla" <attilladanko@...> wrote:

But I can use a sight-tube.
But I *can't use a sight-tube.




If you're sucking air out of the tube, I suggest a big a fan (or as >many fans) as you can get. The key in this case is to seal the >bottom of the tube so the fan(s) dosent(dont) use suction.
bottom of the tube so the fans don't *loose* suction.


And I should have added:

You need to suck a heck of a lot of air out of the bottom of a tube to have same affect against blowing at the back of the mirror.

However, sucking cools the mirror without distorting its figure. (Back-blowing fans can cause a local cool spot). Also, enough suction will suck some of the turbulent boundary layer off the front of the mirror making them almost as good as boundry-layer fans.



It hink filtering is especially important in this case.
I *think* filtering is especially important int his case (of boundary layer fans blowing across the mirror face).

I need to hire a proof reader
 

Hmmmmmmm.....who is the guy with the $gazzillion Astro-Physics mount? I can't remember his name BOB.
On 2011-09-12, at 5:46 PM, "Bob Olson" <r.olson@...> wrote:

Hi Tony,

A 20 inch Starmaster!! I am not too jealous. I suppose you have a complete set of Ethos eyepieces to go with it!

There are many members of this list who can help you with collimation but Attilla gave me my lessons and explained all the steps clearly and without too much sarcasm about my lack of knowledge. See if you can con him into a collimation demonstration.

Have a great day.
Bob




[Non-text portions of this message have been removed]
 

.....at least you didn't type "poop reader"....LOL
On 2011-09-12, at 7:16 PM, "Attilla" <attilladanko@...> wrote:

Idiot at the keyboard. Right. Here are some corrected sentences

I need to hire a proof reader


[Non-text portions of this message have been removed]
balticbirch
 

Thanks for the great info, Attilla. I'll try to make it to Carp on the 17th.

-Tony.

--- In OAFs@..., "Attilla" <attilladanko@...> wrote:

Idiot at the keyboard. Right. Here are some corrected sentences:

--- In OAFs@..., "Attilla" <attilladanko@> wrote:

But I can use a sight-tube.
But I *can't use a sight-tube.




If you're sucking air out of the tube, I suggest a big a fan (or as >many fans) as you can get. The key in this case is to seal the >bottom of the tube so the fan(s) dosent(dont) use suction.
bottom of the tube so the fans don't *loose* suction.


And I should have added:

You need to suck a heck of a lot of air out of the bottom of a tube to have same affect against blowing at the back of the mirror.

However, sucking cools the mirror without distorting its figure. (Back-blowing fans can cause a local cool spot). Also, enough suction will suck some of the turbulent boundary layer off the front of the mirror making them almost as good as boundry-layer fans.



It hink filtering is especially important in this case.
I *think* filtering is especially important int his case (of boundary layer fans blowing across the mirror face).

I need to hire a proof reader
balticbirch
 

Thanks, Bob. Attilla was kind enough to offer to help me as well.

-Tony.

--- In OAFs@..., "Bob Olson" <r.olson@...> wrote:

Hi Tony,

A 20 inch Starmaster!! I am not too jealous. I suppose you have a complete set of Ethos eyepieces to go with it!

There are many members of this list who can help you with collimation but Attilla gave me my lessons and explained all the steps clearly and without too much sarcasm about my lack of knowledge. See if you can con him into a collimation demonstration.

Have a great day.
Bob

[Non-text portions of this message have been removed]
 

Hi Attila,

Noticed your thread on fans blowing against mirror (see below). You mentioned it was hard to implement.?

Is this simply because securing fans in place inside a mirror box is a pain, or because the air flow from four corners needs to be perfectly balanced and directed?

Thanks,
-Tony.
 

Greetings Tony,

securing fans in place inside a mirror box is a pain
Only on some scopes.

air flow from four corners needs to be perfectly balanced and directed
yes.

But that's not the main problem. Blowing air against the back middle
of a mirror can distort the figure of the mirror while it is cooling.
That is the reason why Rick Singmaster refuesed to put cooling fans on
any Starmaster dob.

I discus some alternatives in this video:
<>
from to 31:38 to 33:46

Clear skies.

attilla danko, , danko@...,
attilladanko@...

On Fri, Apr 21, 2023 at 10:18?AM Tony Kourlas via groups.io
<tonykourlas@...> wrote:

Hi Attila,

Noticed your thread on fans blowing against mirror (see below). You mentioned it was hard to implement.

Is this simply because securing fans in place inside a mirror box is a pain, or because the air flow from four corners needs to be perfectly balanced and directed?

Thanks,
-Tony.
 

Thanks for sharing that. Great presentation btw.

Blowing air across top of mirror from back to front makes a lot of sense. I've seen designs where they place a fan in each corner of the mirror box and blow air towards the center of the mirror... seemed to me this would just cause a different kind of turbulence. And while your suggested approach does involve some serious hacking of the pristine starmaster furniture... its better than waiting 2 hours for mirror to cool with a back-blowing fan. I'll probably give it I go when I retire next year... trick will be to eliminate any vibrations.
 

Vibrations
There are at least 3 ways

- expensive anti vibration fans. example
<>

- rubbery gromets, "vibration damping bushings" around the mounting
screws. example: <>

- mount fan switch near eyepiece and turn off while observing at high
power. Turn back on while not looking or observing at low power.

I've used all three methods -- except for the switch near the eyepice.
All 3 work. Though running down the ladder to flip the fan switch on
my mirror box was annoying.


attilla danko, , danko@...,
attilladanko@...

On Fri, Apr 21, 2023 at 10:56?AM Tony Kourlas via groups.io
<tonykourlas@...> wrote:

Thanks for sharing that. Great presentation btw.

Blowing air across top of mirror from back to front makes a lot of sense. I've seen designs where they place a fan in each corner of the mirror box and blow air towards the center of the mirror... seemed to me this would just cause a different kind of turbulence. And while your suggested approach does involve some serious hacking of the pristine starmaster furniture... its better than waiting 2 hours for mirror to cool with a back-blowing fan. I'll probably give it I go when I retire next year... trick will be to eliminate any vibrations.
 

I've built a number of computers throughout the years and I always recommend Noctua fans. They also have anti-vibration mounts that work very well and are easy to install if you don't want to go with the McMaster Carr grommet route. I'm sure they could also be doubled-up with the grommets for extra vibration dampening.?




Richard?

On Fri, Apr 21, 2023 at 11:15?AM Attilla Danko <danko@...> wrote:
>Vibrations

There are at least 3 ways

- expensive anti vibration fans. example
<>

- rubbery gromets, "vibration damping bushings" around the mounting
screws. example: <>

- mount fan switch near eyepiece and turn off while observing at high
power. Turn back on while not looking or observing at low power.

I've used all three methods -- except for the switch near the eyepice.
All 3 work. Though running down the ladder to flip the fan switch on
my mirror box was annoying.


attilla danko, ,? danko@...,
attilladanko@...

On Fri, Apr 21, 2023 at 10:56?AM Tony Kourlas via
<tonykourlas=[email protected]> wrote:
>
> Thanks for sharing that. Great presentation btw.
>
> Blowing air across top of mirror from back to front makes a lot of sense. I've seen designs where they place a fan in each corner of the mirror box and blow air towards the center of the mirror... seemed to me this would just cause a different kind of turbulence. And while your suggested approach does involve some serious hacking of the pristine starmaster furniture... its better than waiting 2 hours for mirror to cool with a back-blowing fan. I'll probably give it I go when I retire next year... trick will be to eliminate any vibrations.
>





 

I always opened my observatory roof an hour before sunset and used a large floor standing box fan blow on the back of the mirror, for the front I would let mother nature do the work by uncovering the mirror and letting the breeze blow on the front. With my metal/aluminum Starstructure? I? do the same but find that the structure of the scope itself acts as a heat sink to cool the primary. Once you get the optic close to ambient really any image degradation you see at the ep is 98% seeing, nothing we can do about that!

On Fri, Apr 21, 2023 at 11:34?AM Richard Hum <hum.richard@...> wrote:
I've built a number of computers throughout the years and I always recommend Noctua fans. They also have anti-vibration mounts that work very well and are easy to install if you don't want to go with the McMaster Carr grommet route. I'm sure they could also be doubled-up with the grommets for extra vibration dampening.?




Richard?

On Fri, Apr 21, 2023 at 11:15?AM Attilla Danko <danko@...> wrote:
>Vibrations

There are at least 3 ways

- expensive anti vibration fans. example
<>

- rubbery gromets, "vibration damping bushings" around the mounting
screws. example: <>

- mount fan switch near eyepiece and turn off while observing at high
power. Turn back on while not looking or observing at low power.

I've used all three methods -- except for the switch near the eyepice.
All 3 work. Though running down the ladder to flip the fan switch on
my mirror box was annoying.


attilla danko, ,? danko@...,
attilladanko@...

On Fri, Apr 21, 2023 at 10:56?AM Tony Kourlas via
<tonykourlas=[email protected]> wrote:
>
> Thanks for sharing that. Great presentation btw.
>
> Blowing air across top of mirror from back to front makes a lot of sense. I've seen designs where they place a fan in each corner of the mirror box and blow air towards the center of the mirror... seemed to me this would just cause a different kind of turbulence. And while your suggested approach does involve some serious hacking of the pristine starmaster furniture... its better than waiting 2 hours for mirror to cool with a back-blowing fan. I'll probably give it I go when I retire next year... trick will be to eliminate any vibrations.
>





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