Hi,
I'm new in astrophotography and as part of my upgrading for astrophotography equipment, I come across the Vixen GA-4(1.25" version). It give the following infomation for calculating the focal length of telescope or telephoto len using a guide scope of 1000mm focal length:
1st 2nd 3rd 4th 5th 6th
464mm 232mm 116mm 58mm 29mm 14.5mm
My question is that since the GA-4 will be use on the guide scope and contain a 3x barlow, does the 1000mm is the focal length after the 3x barlow or before the 3x barlow?
Thanks and have a nice day.
Question on Vixen GA-4 illuminated Guiding Adapter
Think you need Remus to answer you this. He should be familiar with Vixen's productsweixing wrote:Hi,
I'm new in astrophotography and as part of my upgrading for astrophotography equipment, I come across the Vixen GA-4(1.25" version). It give the following infomation for calculating the focal length of telescope or telephoto len using a guide scope of 1000mm focal length:
1st 2nd 3rd 4th 5th 6th
464mm 232mm 116mm 58mm 29mm 14.5mm
My question is that since the GA-4 will be use on the guide scope and contain a 3x barlow, does the 1000mm is the focal length after the 3x barlow or before the 3x barlow?
Thanks and have a nice day.
Question on Vixen GA4
Hi Weixing,
Not sure what exactly are you asking, but my knowledge of the Vixen GA4 (being a user) is this:
The built-in barlow (3X) can be used together in your guiding system. However, sometimes I feel that the 3X is a tad too high for my f.l. instrument. You can remove the module out. And it will act like a star-diagonal in place, together with any eyepiece.
However, with the 3X barlow in place: Let's say if you are using a guiding scope of f.l. 500mm and you insert a 20mm eyepiece into the GA4 WITH the 3X barlow module in place, you get a magnificaiton of 500/20X3 = 75 times. Or effectively, your scope has a "new" f.l. of 1500mm. So putting a 20mm eyepiece will yield an effective magnification of 1500/20=75 times.
Thus, you could see it either way, but essentially, resulting magnification would be similar.
Without the 3X barlow module, the calculation is even simpler, just use the formula directly - which is the scope f.l. divided by the eyepiece f.l.
Hope this helps you.
If you need a GA4, I can help you.
Ray
Not sure what exactly are you asking, but my knowledge of the Vixen GA4 (being a user) is this:
The built-in barlow (3X) can be used together in your guiding system. However, sometimes I feel that the 3X is a tad too high for my f.l. instrument. You can remove the module out. And it will act like a star-diagonal in place, together with any eyepiece.
However, with the 3X barlow in place: Let's say if you are using a guiding scope of f.l. 500mm and you insert a 20mm eyepiece into the GA4 WITH the 3X barlow module in place, you get a magnificaiton of 500/20X3 = 75 times. Or effectively, your scope has a "new" f.l. of 1500mm. So putting a 20mm eyepiece will yield an effective magnification of 1500/20=75 times.
Thus, you could see it either way, but essentially, resulting magnification would be similar.
Without the 3X barlow module, the calculation is even simpler, just use the formula directly - which is the scope f.l. divided by the eyepiece f.l.
Hope this helps you.
If you need a GA4, I can help you.
Ray
-
weixing
- Super Moderator
- Posts: 4708
- Joined: Wed Oct 01, 2003 12:22 am
- Favourite scope: Vixen R200SS & Celestron 6" F5 Achro Refractor
- Location: (Tampines) Earth of Solar System in Orion Arm of Milky Way Galaxy in Local Group Galaxies Cluster
Hi,
May be my message is not clear. The Vixen GA-4 instruction sheet indicate that if you use the GA-4 with a 1000mm focal length guide scope, you must guide the guide star in the:
1st circle if you use a 464mm telescope/telephoto len on your camera.
2nd circle if you use a 232mm telescope/telephoto len on your camera.
3rd circle if you use a 116mm telescope/telephoto len on your camera.
4th circle if you use a 58mm telescope/telephoto len on your camera.
....
But it never indicate whether the 1000mm focal length is the focal length of the GUIDE SCOPE ITSELF or is the effective focal length of the GUIDE SCOPE AND THE 3X BARLOW. If the 1000mm is the actual focal length of the guide scope, then I require a guide scope of 1000mm focal length. But if the 1000mm is the effective focal length of the guide scope and the 3x barlow, then I only require a 1000/3=333mm focal length guide scope. See what I mean....
Thanks and have a nice day.
=========
Yang Weixing
May be my message is not clear. The Vixen GA-4 instruction sheet indicate that if you use the GA-4 with a 1000mm focal length guide scope, you must guide the guide star in the:
1st circle if you use a 464mm telescope/telephoto len on your camera.
2nd circle if you use a 232mm telescope/telephoto len on your camera.
3rd circle if you use a 116mm telescope/telephoto len on your camera.
4th circle if you use a 58mm telescope/telephoto len on your camera.
....
But it never indicate whether the 1000mm focal length is the focal length of the GUIDE SCOPE ITSELF or is the effective focal length of the GUIDE SCOPE AND THE 3X BARLOW. If the 1000mm is the actual focal length of the guide scope, then I require a guide scope of 1000mm focal length. But if the 1000mm is the effective focal length of the guide scope and the 3x barlow, then I only require a 1000/3=333mm focal length guide scope. See what I mean....
Thanks and have a nice day.
=========
Yang Weixing
Hi Weixing,
Ah, that is clearer now. Nope, 1000mm refers to the inherent f.l. of the guidescope. 333mm is too little. Anyway, my basis for guiding tolerance is more simplistic which I would recommend you in (say) choosing a suitable guidescope for your needs, and the guiding magnification that you would require for pinpoint stars:
Would calculate the required guiding tolerance (in arc seconds) for a given f.l. imaging instrument say a 500mm scope:
Using the formula-> G.Tolerance (arc seconds) = 2arctan (1/40F) where F is the f.l. of the imaging scope, then the G.T would be 20 arc seconds. Note: using DEG mode on the calculator and then multiply by 3600 to give G.T. in arc seconds.
The above formula assums a 1/40mm "blur" on the film to be acceptable.
To translate the above information into how "tight" or "loose" one should guide, for example., at the celestial equator, stars move at 15arc-seconds per second. In order to determine the apparent distance of 15 arc-seconds in your guiding eyepiece, switch off your drive and time 1 second of drift and the extent the star has moved across the field....the amount moved will thus be 15 arc seconds, and if we assume the above f.l. scope example of 500mm, with G.T. of 20arc-seconds, your drift of 1 second in the field equals the uncertainty allowed in guiding. If you are using a 1000mm f.l. scope then the G.T. would be twice as critical, ie G.T = 10 arc-seconds.
Hope the above is useful information to you and to the rest.
Ray
Ah, that is clearer now. Nope, 1000mm refers to the inherent f.l. of the guidescope. 333mm is too little. Anyway, my basis for guiding tolerance is more simplistic which I would recommend you in (say) choosing a suitable guidescope for your needs, and the guiding magnification that you would require for pinpoint stars:
Would calculate the required guiding tolerance (in arc seconds) for a given f.l. imaging instrument say a 500mm scope:
Using the formula-> G.Tolerance (arc seconds) = 2arctan (1/40F) where F is the f.l. of the imaging scope, then the G.T would be 20 arc seconds. Note: using DEG mode on the calculator and then multiply by 3600 to give G.T. in arc seconds.
The above formula assums a 1/40mm "blur" on the film to be acceptable.
To translate the above information into how "tight" or "loose" one should guide, for example., at the celestial equator, stars move at 15arc-seconds per second. In order to determine the apparent distance of 15 arc-seconds in your guiding eyepiece, switch off your drive and time 1 second of drift and the extent the star has moved across the field....the amount moved will thus be 15 arc seconds, and if we assume the above f.l. scope example of 500mm, with G.T. of 20arc-seconds, your drift of 1 second in the field equals the uncertainty allowed in guiding. If you are using a 1000mm f.l. scope then the G.T. would be twice as critical, ie G.T = 10 arc-seconds.
Hope the above is useful information to you and to the rest.
Ray
-
weixing
- Super Moderator
- Posts: 4708
- Joined: Wed Oct 01, 2003 12:22 am
- Favourite scope: Vixen R200SS & Celestron 6" F5 Achro Refractor
- Location: (Tampines) Earth of Solar System in Orion Arm of Milky Way Galaxy in Local Group Galaxies Cluster
Hi,
Thanks for your reply. Also, will it be better/easier if I use off-axis guiding?
By the way, I know that the GA-4 got some internal reflection when the illuminated reticle is turn on too bright, but will there have any internal reflection if I turn off the reticle and point to an bright object, such as Mar, jupiter or a bright star? I mean will a bright object also cause internal reflection in the GA-4 (illuminated reticle turn off) ?
Anyway, I may want to buy a GA-4. I got a few quote, but it is quite expensive... anyone got a good "loban"??
Thanks again and have a nice day.
=========
Yang Weixing
Thanks for your reply. Also, will it be better/easier if I use off-axis guiding?
By the way, I know that the GA-4 got some internal reflection when the illuminated reticle is turn on too bright, but will there have any internal reflection if I turn off the reticle and point to an bright object, such as Mar, jupiter or a bright star? I mean will a bright object also cause internal reflection in the GA-4 (illuminated reticle turn off) ?
Anyway, I may want to buy a GA-4. I got a few quote, but it is quite expensive... anyone got a good "loban"??
Thanks again and have a nice day.
=========
Yang Weixing
Hi Weixing,
Off axis guiding is ok but you have to be aware of the following:
1)Choice of guide stars will be limited.
2)Possibiility of introducing internal flares into the image due to the illuminance of the guiding reticle.
3)Little or no effects due to p.mirror slippage.
4)Balance might be an issue.
As for the GA4, there is certainly a presence in internal reflections of the guiding reticle concentric graduations when the reticle is turned too bright. But normally, when I do guiding with the GA4, I avoid turning it too bright for the following reasons:
1)Like to choose moderately bright stars (or even dim ones) for guiding. Coz bright stars flicker more during nights of poor seeing, and it is hard to "catch up" with the star when guiding. Dimmer stars have a lower tendency of doing this.
2)Turning it too bright, will overwhelm the brightness of the guide star. You do not want this to happen.
Curious to know, why you are concerned with the presence of internal reflections when the reticle is switched off entirely. Frankly, I have not noticed this before, but I suspect there will be some mild reflections present.
All in all, the GA4 is better than fixed-graduation reticles becuase you have the freedom to choose any guide star available in the field of view without the constant need to centerise the guide star (motor control) until the reticle graduation. You just move the graduation to the guide star - this I felt was one of the coolest feature available in the GA4, and is time-saving. I do not use the 3X barlow most of the time though.
I am also using the Takahashi LG5 fixed reticle for guiding, but sometimes prefer the above novel feature of the GA4 over the LG5. However, the reticle line thickness of the LG5 is thinner than those on the GA4, which I guess is typical of TAKAHASHI products. Nonetheless, the price of the LG5 is considerably more costly.
Have you seen and use the GA4 before, I can show it to you when I next meet you, and see whether it suits your liking.
Remus
Off axis guiding is ok but you have to be aware of the following:
1)Choice of guide stars will be limited.
2)Possibiility of introducing internal flares into the image due to the illuminance of the guiding reticle.
3)Little or no effects due to p.mirror slippage.
4)Balance might be an issue.
As for the GA4, there is certainly a presence in internal reflections of the guiding reticle concentric graduations when the reticle is turned too bright. But normally, when I do guiding with the GA4, I avoid turning it too bright for the following reasons:
1)Like to choose moderately bright stars (or even dim ones) for guiding. Coz bright stars flicker more during nights of poor seeing, and it is hard to "catch up" with the star when guiding. Dimmer stars have a lower tendency of doing this.
2)Turning it too bright, will overwhelm the brightness of the guide star. You do not want this to happen.
Curious to know, why you are concerned with the presence of internal reflections when the reticle is switched off entirely. Frankly, I have not noticed this before, but I suspect there will be some mild reflections present.
All in all, the GA4 is better than fixed-graduation reticles becuase you have the freedom to choose any guide star available in the field of view without the constant need to centerise the guide star (motor control) until the reticle graduation. You just move the graduation to the guide star - this I felt was one of the coolest feature available in the GA4, and is time-saving. I do not use the 3X barlow most of the time though.
I am also using the Takahashi LG5 fixed reticle for guiding, but sometimes prefer the above novel feature of the GA4 over the LG5. However, the reticle line thickness of the LG5 is thinner than those on the GA4, which I guess is typical of TAKAHASHI products. Nonetheless, the price of the LG5 is considerably more costly.
Have you seen and use the GA4 before, I can show it to you when I next meet you, and see whether it suits your liking.
Remus