Advice about reflector Astrographs

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Jose Urias

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Mar 18, 2025, 3:07:41 PM3/18/25
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Good afternoon HAL,

I wanted to ask for some advice about Newtonian Astrographs.

I currently have an apertura CarbonStar 150 which is a native 150mm aperture (6”) and 600mm focal length which makes it an f4.
Additionally I use the 0.95xfocal reducer/ field flattener which roughly brings it down to 585mm focal length.

This telescope is fantastic and I have taken amazing pictures with it, but I have wanted to take some planetary and lunar pictures and the aperture in my telescope should allow me to get some decent detail, however, because this telescope gathers light fast, all I get when I point it to the moon with the shortest exposure time and 0 gain from my camera is an overexposed bulb with no shape, and when I point to planets, I see a bright star with refraction spikes and maybe their moons but no detail as it is so bright.

I know there is no telescope that can work for all types of astrophotography, but I am wondering if using a barlow lens would “slow” or “dim” down the objects. Or maybe some type of filter (I have used a neutral density filter for the moon but it just doesn’t look as good).

I saw someone else using a UHC  filter for the moon with a similar setup and I am curious to try that, but I wanted to look for other recommendations first.

Thank you so much in advance!

johnjhudak

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Mar 18, 2025, 3:35:45 PM3/18/25
to Jose Urias, Howard Astronomical League
Jose,

Yes, using a 2x Barlow will increase the f value from f4 to f8. Since it magnifies the object by a factor of two, it will reduce the brightness of the object by 4. It's the same amount of light spread over 4 times the area.
Clear Skies,
JOHN

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Garry Ingle

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Mar 18, 2025, 4:43:56 PM3/18/25
to johnjhudak, Jose Urias, Howard Astronomical League
I don’t believe using a barlow will change your F ratio unless you’re using a reducer!
Regardless of your eyepiece or Barlow F10 will still be F10 and F8 will still be F8!  

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From: howar...@googlegroups.com <howar...@googlegroups.com> on behalf of johnjhudak <johnj...@gmail.com>
Sent: Tuesday, March 18, 2025 3:35:07 PM
To: Jose Urias <juria...@gmail.com>
Cc: Howard Astronomical League <howar...@googlegroups.com>
Subject: Re: [howardastro] Advice about reflector Astrographs
 

Kurt Bauch

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Mar 18, 2025, 4:50:44 PM3/18/25
to Garry Ingle, johnjhudak, Jose Urias, Howard Astronomical League
I thought that a 2x Barlow would double the F ratio, but my friend at ChatGPT confirmed that it doesn’t. I learned something new!

Kurt Bauch


Garry Ingle

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Mar 18, 2025, 4:55:36 PM3/18/25
to Kurt Bauch, johnjhudak, Jose Urias, Howard Astronomical League
No it will only magnify your image times two depending on what eyepiece you’re using! 

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From: Kurt Bauch <kfbf...@gmail.com>
Sent: Tuesday, March 18, 2025 4:50:29 PM
To: Garry Ingle <garr...@hotmail.com>
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Gene Handler

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Mar 18, 2025, 5:24:04 PM3/18/25
to Kurt Bauch, Jose Urias, Howard Astronomical League
I sure thought a 2x barlow would double the effective focal length. What does ChatGPT say?

j...@tomney.com

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Mar 18, 2025, 5:32:18 PM3/18/25
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Yep, the Barlow should increase the effective focal length. As John indicated, using a Barlow is probably a good first step to decrease the brightness.

 

Jim T

Garry Ingle

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Mar 18, 2025, 5:37:32 PM3/18/25
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 No! No! No !! noooooo!

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From: howar...@googlegroups.com <howar...@googlegroups.com> on behalf of j...@tomney.com <j...@tomney.com>
Sent: Tuesday, March 18, 2025 5:30:48 PM

Cc: 'Howard Astronomical League' <howar...@googlegroups.com>
Subject: RE: [howardastro] Advice about reflector Astrographs
 

Garry Ingle

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Mar 18, 2025, 5:37:51 PM3/18/25
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Where are you people getting your information from?

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Sent: Tuesday, March 18, 2025 5:37:28 PM
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Jim Johnson

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Mar 18, 2025, 5:57:27 PM3/18/25
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I use a 1.5” x 2.5x PowerMate, which increases the effective focal length and f/ ratio of the optical system by 2.5x. It does stop the light down and increases magnification. I also use a planetary cam for planetary and lunar imaging, which also helps with the light. You should be able to use a low gain and very fast exposure times (just a few ms), both of which will help with the light “problem” contribute to the image quality.

 

TeleVue also makes a 4.0x PowerMate in the 1.5” format, which will help even more, but I don’t know how well it will work with your 6” aperture.

 

As a point of reference, James Willinghan images at about 7,500 mm focal length with a Meade 12” f/10 and a 2.5x PowerMate. I image at about 6,000 mm focal length with a Meade  12” f/8.

 

A quick search on line backs up the focal length and f/ increase:

 

AI Overview

Learn more

Watch Your Back Focus! – Tele Vue Optics, Inc. Blog

Yes, a Tele Vue Powermate 2.5x increases both the effective focal length and the f/ratio (or slows down the f/ratio) of a telescope. 

Here's a more detailed explanation: 

·        Focal Length Increase:

The Powermate amplifies the image, effectively lengthening the telescope's focal length by a factor of 2.5x.

·        F/ratio Increase (Slowing Down):

Since the focal length increases, the f/ratio (which is the focal length divided by the aperture) also increases, meaning the telescope's focal ratio slows down.

·        Example:

If you have a telescope with a focal length of 600mm and an f/ratio of f/7, using a 2.5x Powermate will result in an effective focal length of 1500mm (600mm * 2.5) and an f/ratio of f/17.5 (1500mm / 85mm).

 

 

~Jim Johnson

Dietmar Tietz

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Mar 18, 2025, 6:17:39 PM3/18/25
to Jose Urias, howar...@googlegroups.com
I used Perplexity.ai to check out about how to adjust a CarbonStar 150 for Moon photography. The answer was to use a neutral density (ND) or moon filter and use the lowest ISO value and shortest exposure time. There are also variable ND (variable polarizing ) filters for moon photography. They also recommended blackening the inside tube to cut down on reflections.

ChatGPT confirms that your telescope setup should allow insertion of a filter (1.25" or 2") in the imaging path.

Dietmar

From: "Jose Urias" <juria...@gmail.com>
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Sent: Tuesday, March 18, 2025 3:07:26 PM
Subject: [howardastro] Advice about reflector Astrographs

Mike Ficco

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Mar 18, 2025, 7:06:48 PM3/18/25
to Dietmar Tietz, Jose Urias, howar...@googlegroups.com
Garry I’m confused but willing to learn something new. How does a Barlow increase the magnification of an eyepiece if it doesn’t lengthen the focal length?


Sent from my iPhone

On Mar 18, 2025, at 6:17 PM, Dietmar Tietz <d...@his.com> wrote:



Garry Ingle

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Mar 18, 2025, 7:20:32 PM3/18/25
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Blackening means to flock the interior of the OTA. I offer that service if you are interested.

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From: 'Mike Ficco' via Howard Astronomical League <howar...@googlegroups.com>
Sent: Tuesday, March 18, 2025 7:06:18 PM
To: Dietmar Tietz <d...@his.com>
Cc: Jose Urias <juria...@gmail.com>; howar...@googlegroups.com <howar...@googlegroups.com>
Subject: Re: [howardastro] Advice about reflector Astrographs
 

Jeffrey Pleines

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Mar 18, 2025, 7:42:54 PM3/18/25
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Do you do mak-newt collimation?

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From: howar...@googlegroups.com <howar...@googlegroups.com> on behalf of Garry Ingle <garr...@hotmail.com>
Sent: Tuesday, March 18, 2025 7:20:27 PM
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Garry Ingle

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Mar 18, 2025, 7:50:36 PM3/18/25
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Yes 

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From: Jeffrey Pleines <jple...@verizon.net>
Sent: Tuesday, March 18, 2025 7:42:45 PM
To: Garry Ingle <garr...@hotmail.com>; Mike Ficco <mfi...@comcast.net>; Dietmar Tietz <d...@his.com>

Dietmar Tietz

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Mar 18, 2025, 9:54:05 PM3/18/25
to howardastro, Jose Urias, Mike Ficco
Let me try to define some telescope and viewing parameters:
  • The telescope’s aperture (its physical opening) stays the same regardless of what you attach on the viewing side.
  • Magnification is calculated by dividing the telescope’s focal length by the eyepiece’s focal length. For example, a telescope with a 600 mm focal length and a 6 mm eyepiece provides 600 ÷ 6 = 100× magnification.
  • Both a Barlow lens and a PowerMate effectively increase the telescope’s focal length. If a 600 mm focal length is extended to 1200 mm, using the same 6 mm eyepiece now yields 1200 ÷ 6 = 200× magnification.
  • The telescope’s focal ratio, F, is the focal length divided by its aperture. For instance, a 600 mm focal length and a 150 mm aperture correspond to an F-ratio of 600 ÷ 150 = 4 (often written as f/4).
  • When a Barlow doubles the focal length from 600 mm to 1200 mm, the focal ratio increases from f/4 to f/8. As a result, although the magnification is doubled, only one-quarter of the original light intensity reaches your eye or camera sensor because the image is spread over a larger area.
Dietmar


From: "'Mike Ficco' via Howard Astronomical League" <howar...@googlegroups.com>
To: "Dietmar Tietz" <d...@his.com>
Cc: "Jose Urias" <juria...@gmail.com>, "howardastro" <howar...@googlegroups.com>
Sent: Tuesday, March 18, 2025 7:06:18 PM
Subject: Re: [howardastro] Advice about reflector Astrographs

Mike Ficco

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Mar 18, 2025, 10:19:28 PM3/18/25
to Dietmar Tietz, howardastro, Jose Urias
All that matches what I thought I knew. I was trying to understand why Garry seemed so adamant that the focal length was NOT increased. Of course the ACTUAL focal length of the telescope is fixed (as is the aperture ) but the APPARENT/effective focal length is increased by a Barlow. I was trying to understand Garry’s point 


Sent from my iPhone

On Mar 18, 2025, at 9:54 PM, Dietmar Tietz <d...@his.com> wrote:



Garry Ingle

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Mar 18, 2025, 10:22:16 PM3/18/25
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Thanks for explaining that that’s easier for me to wrap my head around

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From: 'Mike Ficco' via Howard Astronomical League <howar...@googlegroups.com>
Sent: Tuesday, March 18, 2025 10:19:00 PM
To: Dietmar Tietz <d...@his.com>
Cc: howardastro <howar...@googlegroups.com>; Jose Urias <juria...@gmail.com>
Subject: Re: Defining telescope and viewing parameters -- Re: [howardastro] Advice about reflector Astrographs
 

Wayne Baggett

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Mar 18, 2025, 11:57:34 PM3/18/25
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Kurt:  Yeah, well, your friend at ChatGTP has a history of hallucinations, and I think you just experienced one.  :)  I suggest you forget that lesson.

Many people misuse ChatGPT: it is a large language model designed to write paragraphs and such.  It is NOT a search engine, and sometimes in it's zeal to provide you with a well-written response, it makes things up.  It's better to use a search engine when actually searching for information.  (From Wikipedia: "OpenAI acknowledges that ChatGPT "sometimes writes plausible-sounding but incorrect or nonsensical answers".[15] This behavior is common for large language models, and is called "hallucination"."  https://en.wikipedia.org/wiki/ChatGPT).  People use it to write code -- I have done that myself.  However, I caution you to use that code only after you have thoroughly debugged and tested it since the code is often incorrect.  ChatGPT will cheerfully "correct" it for you, but again, debug and test thoroughly!

Garry: Thanks for being receptive to Dietmar's explanation!

Finally Dietmar:  Thanks for taking the time to write up that clear explanation; it is much better than my response to Garry was going to be: "Physics".  :)   One minor extension (in italics, I hope) to your last bullet: " As a result, although the magnification is doubled, for extended objects, only one-quarter of the original light intensity reaches your eye or camera sensor because the image is spread over a larger area; visually stars are affected much less because they are point sources regardless of your achievable magnifications (imaging is a somewhat different case since you are increasing the physical size of the Airy disk relative to your pixels)."

Wayne

Garry Ingle

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Mar 18, 2025, 11:59:55 PM3/18/25
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Never heard of chatGPT before? Thanks 

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From: 'Wayne Baggett' via Howard Astronomical League <howar...@googlegroups.com>
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Victor Sanchez

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Mar 19, 2025, 12:00:57 AM3/19/25
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Dietmar,
 Nice Explanation.  There's more along with some nice graphical depictions of what's going on at the link below...


Regards,

Victor

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Sent: Tuesday, March 18, 2025 6:53 PM
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Cc: Jose Urias; Mike Ficco
Subject: Defining telescope and viewing parameters -- Re: [howardastro] Advice about reflector Astrographs

Wayne Baggett

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Mar 19, 2025, 12:10:58 AM3/19/25
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Focal reducers and Barlow lenses are opposite sides of the same coin: they are both optical elements that change the cone angle of the converging light beam.  Focal reducers make the cone angle larger, and Barlows make it smaller.  Since the focal ratio, defined as the focal length divided by the aperture, is just the inverse of the cone angle of the converging light, changing the cone angle changes the distance the cone has to extend backward to be as wide as the aperture; i.e., the focal length.  Thus, focal reducers make the cone angle larger and the focal length shorter, while a Barlow makes the cone angle smaller and the focal length longer.

Also note that it is common in optics to refer to the "focal length" of a single optical element, while "effective focal length" applies to systems of optical elements.  The secondary mirrors in Cassegrains, SCTs, Maksutovs, and RC's all act in the same way as a Barlow lens - decreasing the cone angle of the converging light from the primary mirror - thus allowing long (effective) focal length systems in short tubes.

Wayne

Garry Ingle

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Mar 19, 2025, 7:57:44 AM3/19/25
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I completely understand everything you just said thank you for that.

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Sent: Wednesday, March 19, 2025 12:10:52 AM
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Dietmar Tietz

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Mar 19, 2025, 8:14:17 AM3/19/25
to howardastro, Wayne Baggett

I posted to ChatGPT the question, "How does a 2x Barlow affect the F-ratio of a telescope?"

ChatGPT's answer is attached as a pdf file.


Why did someone not get the correct answer from ChatGPT? Perhaps, because the question was not framed precisely.


F-ratio can mean different things, e.g.:

The F-ratio, or F-statistic, is a ratio of two variances used in statistical analysis, particularly in ANOVA (Analysis of Variance), to determine if there are statistically significant differences between group means.


A Barlow will not affect this.


I agree, like humans, AI can make mistakes.


Dietmar




From: "'Wayne Baggett' via Howard Astronomical League" <howar...@googlegroups.com>
To: "howardastro" <howar...@googlegroups.com>
Sent: Tuesday, March 18, 2025 11:57:28 PM
2x Barlow F-ratio Effect.pdf

Bob Bunge

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Mar 19, 2025, 8:37:07 AM3/19/25
to Dietmar Tietz, howardastro, Wayne Baggett
Interesting conservation.  To add one additional bit of information that might be useful regarding the original post, is the use of barlow will increase the image scale at the focal plan, which means the non-point object, like the Moon and a planet will be larger on the sensor.  There is science behind imaging the planet - ideally, the image scale is matched to the size of pixels of your sensor, which is often done using barlows, as well as a good bit of process and art when it comes to collecting and processing the resulting data.

Nutshell, put a barlow in that little astrograph, point it to the Moon and experiment!

Bob


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Stephen Gauss

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Mar 19, 2025, 1:16:52 PM3/19/25
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For Sale:
Celestron 150mm (6-inch) Newtonian Reflector on Advanced VX computerized mount.
The mount has the ADM saddle to allow both Losmandy and Vixen dovetails.
Excellent condition, stored indoors and used only for occasional star parties.
Contact me for questions and pictures.
Asking $800.

Dale Ghent

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Mar 24, 2025, 9:37:04 AM3/24/25
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This is so factual wrong it's actually pathetic.

A barlow, which is a negative lens, will increase the focal length by the barlow's multiplier and therefore also the focal ratio. Why? Because the aperture of the telescope doesn't change; it's a constant. The formula is elementary:

focal_length = aperture * focal_ratio

If you have a 150mm aperture telescope at f/4, its focal length will be 150*4=600mm

A 2x barlow would multiply that product by 2. The aperture is still 150mm; it does not change. Therefore the focal ratio also doubles, giving you a 1200mm focal length at f/8:

focal_ratio = focal_length / aperture

1200/150=8

The fact that chatGPT got this most basic concept of optics egregiously wrong is a good example of not taking everything it spews at face value.


> On Mar 18, 2025, at 16:50, Kurt Bauch <kfbf...@gmail.com> wrote:
>
> I thought that a 2x Barlow would double the F ratio, but my friend at ChatGPT confirmed that it doesn’t. I learned something new!
>
> Kurt Bauch
>
>
> On Tue, Mar 18, 2025 at 4:43 PM Garry Ingle <garr...@hotmail.com> wrote:
> I don’t believe using a barlow will change your F ratio unless you’re using a reducer!
> Regardless of your eyepiece or Barlow F10 will still be F10 and F8 will still be F8!
>
> “Clear Skies” Keep looking up
> Telescope Repairs/Upgrades
> Technical support
>
> TPO 14” Truss RC
> Stellarvue SVX 80MM TR-LE APO Triplet F6 CF
> Stellarvue SVX 102MM TR APO Triplet F7 CF
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> www.Meademods.com Garry Ingle Jr
> 443-604-7302
>
> To view this discussion visit https://groups.google.com/d/msgid/howardastro/CAKGcFaYT7ddYR%2BnMdw%3DHTeYQ4Dk__D%2B0%2B%3DKss37iSoB4SeKfeA%40mail.gmail.com.

Kurt Bauch

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Mar 24, 2025, 9:51:16 AM3/24/25
to Dale Ghent, Garry Ingle, johnjhudak, Jose Urias, Howard Astronomical League
Definitely! I’ve had a few other run-ins with ChatGPT. A little disconcerting. 

Kurt Bauch

Lee Brown

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Mar 24, 2025, 9:54:08 AM3/24/25
to Kurt Bauch, Dale Ghent, Garry Ingle, johnjhudak, Jose Urias, Howard Astronomical League
Greetings!

ChatGPT is still bound by Cole's Law.  Cabbage in, cabbage out.

Best Regards,

Lee E. Brown
Senior Mechanical Engineer

Aerolab LLC
8291 Patuxent Range Road, Suite 1200
Jessup, MD 20794 USA

Office:               301-776-6585 Ext 1010
Shoe Phone:    410-598-9188




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