Slow telescope focal ratios


Kent Blackwell
 

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


jimcoble2000
 

You are absolutely right about the focus differences. May also be related to the view that fast mirrors are more critical to collimate accurately than slower focal ratio mirrors. But..... you will need a 60mm eyepiece to achieve anything less than 1000000xEmojiEmoji

On Thursday, January 13, 2022, 11:12:44 AM EST, Kent Blackwell <kent@...> wrote:


Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


jimcoble2000
 

Speaking of collimation. From Nasa's update on Webb

The primary mirror wings are now fully deployed and latched into place, but the individual mirror segments remain in their launch configuration. This operation is a multi-day, multi-step activity to activate and move each of the 18 primary mirror segments and the secondary mirror from their stowed launch configuration to a deployed position ready for alignment.

The 18 primary mirror segments and secondary mirror are adjustable via six actuators that are attached to the back of each mirror. The primary mirror segments also have an additional actuator at its center that adjusts its curvature. The telescope's tertiary mirror remains stationary. The primary and secondary mirror segments will move a total of 12.5mm, in small increments, over the course of ~10 days to complete each segment's deployment.

After all individual mirror segment deployments are completed, the detailed optical mirror alignment process begins which is about a 3 month process. In parallel, as temperatures cool enough, instrument teams will turn on their instruments and begin each instrument's commissioning process.



On Thursday, January 13, 2022, 11:12:44 AM EST, Kent Blackwell <kent@...> wrote:


Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


Stu Beaber
 

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


jimcoble2000
 

Thank you Stu. That sums it up. No one who has seen my hands can fail to understand I don't deal in microns when focusing but point taken and well stated. My f 5.6 refractor is famous for being either in focus or not. No in between. If you are not sure, you are notEmoji

By the theory, my 4 inch Televue is 68 microns. The 5 inch is 107.8           63% smaller.


On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


Stu Beaber
 

Understand the hands thing...none of us "advanced age people" are as agile as we once were! That's why I use an electric stepper motor on all my focusers...

Stu


On Thu, Jan 13, 2022 at 2:28 PM jimcoble2000 via groups.io <jimcoble2000=yahoo.com@groups.io> wrote:
Thank you Stu. That sums it up. No one who has seen my hands can fail to understand I don't deal in microns when focusing but point taken and well stated. My f 5.6 refractor is famous for being either in focus or not. No in between. If you are not sure, you are notEmoji

By the theory, my 4 inch Televue is 68 microns. The 5 inch is 107.8           63% smaller.


On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


Kent Blackwell
 
Edited

There is another POSITIVE issue with "slow" systems such as f/15. With my 60 year old Unitron f/15 achromat refractor I can use a $600 modern eyepiece or a simple $30 Kellner and, except for the wider field of view of the modern eyepiece images are every bit as sharp with either. The Unitron produces great images even with a plastic magnifying glass I got in a box of Cracker Jacks years ago. Well, maybe I'm exaggerating a little.


jimcoble2000
 

He is more machine than man.....................Obi Wan

On Thursday, January 13, 2022, 02:52:14 PM EST, Stu Beaber <wd4sel@...> wrote:


Understand the hands thing...none of us "advanced age people" are as agile as we once were! That's why I use an electric stepper motor on all my focusers...

Stu

On Thu, Jan 13, 2022 at 2:28 PM jimcoble2000 via groups.io <jimcoble2000=yahoo.com@groups.io> wrote:
Thank you Stu. That sums it up. No one who has seen my hands can fail to understand I don't deal in microns when focusing but point taken and well stated. My f 5.6 refractor is famous for being either in focus or not. No in between. If you are not sure, you are notEmoji

By the theory, my 4 inch Televue is 68 microns. The 5 inch is 107.8           63% smaller.


On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


jimcoble2000
 

Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


Ted Forte
 

And I suspect it’s a bit more complex for visual astronomy given the vulgarities of the human eye – just consider how different that sweet focus spot can be between different observers.

 

Ted

 

From: BackBayAstro@groups.io <BackBayAstro@groups.io> On Behalf Of jimcoble2000 via groups.io
Sent: Friday, January 14, 2022 7:44 AM
To: BackBayAstro@groups.io
Subject: Re: [BackBayAstro] Slow telescope focal ratios

 

Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

 

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:

 

 

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

 

Stu

 

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


Roy Diffrient
 

No hope for my 28” f/3.63 even with a FeatherTouch focuser: 0.001”.

Roy



On Jan 14, 2022, at 9:46 AM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:


Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


charles jagow
 

I agree there is no hope.  You might as well just get rid of the 28”.  Go ahead and send it to the below listed address and I will take care of its disposition for you.

 

          Chuck Jagow

          310 S. 5th St.

          Westcliffe, CO 81252

 

 

From: <BackBayAstro@groups.io> on behalf of Roy Diffrient <mail@...>
Reply-To: <BackBayAstro@groups.io>
Date: Friday, January 14, 2022 at 9:14 AM
To: <BackBayAstro@groups.io>
Subject: Re: [BackBayAstro] Slow telescope focal ratios

 

No hope for my 28” f/3.63 even with a FeatherTouch focuser: 0.001”.

 

Roy





On Jan 14, 2022, at 9:46 AM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:



Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

 

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:

 

 

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

 

Stu

 

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 



Roy Diffrient
 

Gee, thanks Chuck.

Roy



On Jan 14, 2022, at 11:44 AM, charles jagow <chuck@...> wrote:



I agree there is no hope.  You might as well just get rid of the 28”.  Go ahead and send it to the below listed address and I will take care of its disposition for you.

 

          Chuck Jagow

          310 S. 5th St.

          Westcliffe, CO 81252

 

 

From: <BackBayAstro@groups.io> on behalf of Roy Diffrient <mail@...>
Reply-To: <BackBayAstro@groups.io>
Date: Friday, January 14, 2022 at 9:14 AM
To: <BackBayAstro@groups.io>
Subject: Re: [BackBayAstro] Slow telescope focal ratios

 

No hope for my 28” f/3.63 even with a FeatherTouch focuser: 0.001”.

 

Roy





On Jan 14, 2022, at 9:46 AM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:



Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

 

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:

 

 

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

 

Stu

 

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


--

v/r

v/r

Chuck Jagow

Member – Dark Skies of The Wet Mountain Valley

Member - Back Bay Amateur Astronomers

Member – San Diego Astronomy Association

Member – Colorado Springs Astronomy Association

Future         Verde Mont Observatory

Gone...        Rott'n Paws Observatory

 

 


jimcoble2000
 

Oh come on Roy. Are you gonna let a measly one thousandth of an inch get in the way??

Fast cars..............fast scopes.EmojiEmojiEmoji

On Friday, January 14, 2022, 11:15:02 AM EST, Roy Diffrient <mail@...> wrote:


No hope for my 28” f/3.63 even with a FeatherTouch focuser: 0.001”.

Roy



On Jan 14, 2022, at 9:46 AM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:


Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:


It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

Stu

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


jimcoble2000
 

Send it overnight 1st day. COD

On Friday, January 14, 2022, 11:58:52 AM EST, Roy Diffrient <mail@...> wrote:


Gee, thanks Chuck.

Roy



On Jan 14, 2022, at 11:44 AM, charles jagow <chuck@...> wrote:



I agree there is no hope.  You might as well just get rid of the 28”.  Go ahead and send it to the below listed address and I will take care of its disposition for you.

 

          Chuck Jagow

          310 S. 5th St.

          Westcliffe, CO 81252

 

 

From: <BackBayAstro@groups.io> on behalf of Roy Diffrient <mail@...>
Reply-To: <BackBayAstro@groups.io>
Date: Friday, January 14, 2022 at 9:14 AM
To: <BackBayAstro@groups.io>
Subject: Re: [BackBayAstro] Slow telescope focal ratios

 

No hope for my 28” f/3.63 even with a FeatherTouch focuser: 0.001”.

 

Roy





On Jan 14, 2022, at 9:46 AM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:



Of course that means also that the theoretical critical zone of focus for my telescope is 1/500  of an inch!Emoji

 

On Thursday, January 13, 2022, 01:35:33 PM EST, Stu Beaber <wd4sel@...> wrote:

 

 

It's because the critical focus zone (CFZ) is greater with slower telescopes. The CFZ is the distance one can move the focal point without any apparent change in focus, According to Ron Woraski's book "The New CCD Astronomy'', the formula to determine that is CFZ=focal ratio squared times 2.2=answer in microns. He goes on to say that the CFZ for a f/5.5 scope would be one quarter that of a f/11 scope. However real world values for the CFZ are approximately 10-30% greater than theoretical values.

 

Stu

 

On Thu, Jan 13, 2022 at 11:12 AM Kent Blackwell <kent@...> wrote:

Most of us prefer "fast" telescope ratios. With fast systems, such as f/4 to 4/6 telescope tubes can be shorter, important for transporting our instruments to dark sky locations. There are exceptions, such as folded telescope systems. Back in the late 1960's Celestron offered a Schmidt-Cassegrain 8" f/10 folded system in a very compact tube. I had one years ago and it was compact enough I even took it to a cruise to Africa on an eclipse expedition. 

I currently have a rather rare Meade 10" f/15 Maksutov design. What I've noticed with fast system such as f/4 to f/6 is stars tend to "snap" in and out of focus with just a touch of the focuser. With the above mentioned f/15 system I've noticed I can turn the focusing knob a considerable distance and the stars stay in focus. I also noticed the same with my 4" f/15 Unitron refractor. Focus becomes less critical with "slow" systems such as f/10 to f/15. 

I don't know why I brought this up, but last night I just thought it was amazing how far I could turn the focusing knob on the Meade 10" f/15 system and maintain a sharp focus. 


--

v/r

v/r

Chuck Jagow

Member – Dark Skies of The Wet Mountain Valley

Member - Back Bay Amateur Astronomers

Member – San Diego Astronomy Association

Member – Colorado Springs Astronomy Association

Future         Verde Mont Observatory

Gone...        Rott'n Paws Observatory

 

 


Roy Diffrient
 

Mark said “Fast cars..............fast scopes.”

…. Fast women???  

“Oh no, not so fast Rapid Roy!”  says Dee, “Just drink your Geritol and act your age.”

$#%¥

Roy



On Jan 14, 2022, at 3:50 PM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:


Fast cars..............fast scopes.


jimcoble2000
 

I sort of left that out cause at this stage it really does not apply.

On Friday, January 14, 2022, 05:32:09 PM EST, Roy Diffrient <mail@...> wrote:


Mark said “Fast cars..............fast scopes.”

…. Fast women???  

“Oh no, not so fast Rapid Roy!”  says Dee, “Just drink your Geritol and act your age.”

$#%¥

Roy



On Jan 14, 2022, at 3:50 PM, jimcoble2000 via groups.io <jimcoble2000@...> wrote:


Fast cars..............fast scopes.