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How to Add and Reduce Focal Length correctly?

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#1 Ryan1776

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Posted 23 September 2021 - 06:01 AM

Not sure how to phrase that question. 

I have a Meade LXD55 8" 812mm SN. 

294MC pro. 

Is there a good way to reduce FL to half or less? So I can get, lets say the Veil Nebula in one shot? 

Conversely what about a good barlow to increase FL for galaxies, Ring Neb, Dumbbell? Etc. 

 

If I understand correctly, I'll lose some speed from my scope, that's an F4 to begin with, so might not be awful.

 

Thank you!

 

Ryan


Edited by Ryan1776, 23 September 2021 - 06:03 AM.


#2 PiotrM

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Posted 23 September 2021 - 06:53 AM

There are Boren-Simon corrector-reducers for Newtonians but that may be quite tricky to use with a SN (unknown field quality, backfocus).

 

Galaxies tend to have low surface brightness so imaging at longer FL may be even more tricky (plus you will need good guiding anyway). Any "good" brand Barlow should work, but I would start with something less extreeme - like the GSO ones that you can unscrew the lens setup and screw it directly into a camera nosepiece for 1.5X instead of 2X.



#3 imtl

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Posted 23 September 2021 - 06:56 AM

Just be aware that for a x2 barlow you will need x4 the exposure. And that will give you a serious challenge. In AP barlow is not used and for good reasons.


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#4 Ryan1776

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Posted 23 September 2021 - 07:54 AM

There are Boren-Simon corrector-reducers for Newtonians but that may be quite tricky to use with a SN (unknown field quality, backfocus).

 

Galaxies tend to have low surface brightness so imaging at longer FL may be even more tricky (plus you will need good guiding anyway). Any "good" brand Barlow should work, but I would start with something less extreeme - like the GSO ones that you can unscrew the lens setup and screw it directly into a camera nosepiece for 1.5X instead of 2X.

I couldn't find a Boren-Simon Reducer. Found full scopes though. 

Yes, good call on the guiding. 
I didn't mention I have an Atlas EQG with ASI Air and guide scope/cam. I've taken 10min subs without any trailing. 

 

 

Just be aware that for a x2 barlow you will need x4 the exposure. And that will give you a serious challenge. In AP barlow is not used and for good reasons.

I couldn't remember that number thank you. 

Sheesh! hahaha 

What about the other way?

 

Reducing the FL?



#5 jonnybravo0311

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Posted 23 September 2021 - 07:59 AM

Focal reducers are used all the time in deep sky imaging. I've got a 0.8x reducer/flattener on my GT81. The Celestron EdgeHD series have a 0.7x reducer. Non-Edge Celestron tubes have a 0.63x reducer. I have no idea if Meade (or some third party) manufactures one for that 8" SN, though.



#6 Alex McConahay

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Posted 23 September 2021 - 08:03 AM

CN's sponsor Astronomics (among others) sells a .5 Focal Reducer that will cut your focal length in half. Just go to their website, search for ".5 focal reducer" and take it from there. $24.95. 

 

Realize, though, that whenever you add more glass between your primary and your sensor, you are running the risk of losing something. I should think it is worth a try at $24.95. It will certainly change the Field of View. Whether you will still be able to cover your sensor completely, whether you suffer from some abberration or another because of the additional optic, is something you will have to see. (Or maybe it is covered in a review someplace. 

 

The important thing to remember, though, is that you have other options. One of course is to use mosaics and cropping to change the composition on any given target. Another is to get different tubes for different targets. A third is to simply NOT do targets that are inappropriate. For now, until you go to another tube, simply don't go after those targets that do not fit your scope. 

 

Alex


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#7 Ryan1776

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Posted 23 September 2021 - 08:06 AM

Focal reducers are used all the time in deep sky imaging. I've got a 0.8x reducer/flattener on my GT81. The Celestron EdgeHD series have a 0.7x reducer. Non-Edge Celestron tubes have a 0.63x reducer. I have no idea if Meade (or some third party) manufactures one for that 8" SN, though.

OK cool. 

It sounds like it's specific to the scope though? I was hoping there might be one that would attach to the camera nose after the filter, before or something.

 

Sorry for the very newb question. 

 

The reason I'm digging down this hole is I don't have the time to build mosaics of large objects. 
A 3.5 year old, one on the way and work! haha.

I'd be willing to spend a good sum 3-400 on a reducer if it's good quality optics to make that happen. 

As opposed to a different scope- 250-350mm refractor. 

 

That said, I want to be able to attach and run without too much fanfare. 

Ryan 



#8 Ryan1776

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Posted 23 September 2021 - 08:10 AM

CN's sponsor Astronomics (among others) sells a .5 Focal Reducer that will cut your focal length in half. Just go to their website, search for ".5 focal reducer" and take it from there. $24.95. 

 

Realize, though, that whenever you add more glass between your primary and your sensor, you are running the risk of losing something. I should think it is worth a try at $24.95. It will certainly change the Field of View. Whether you will still be able to cover your sensor completely, whether you suffer from some abberration or another because of the additional optic, is something you will have to see. (Or maybe it is covered in a review someplace. 

 

The important thing to remember, though, is that you have other options. One of course is to use mosaics and cropping to change the composition on any given target. Another is to get different tubes for different targets. A third is to simply NOT do targets that are inappropriate. For now, until you go to another tube, simply don't go after those targets that do not fit your scope. 

 

Alex

Alex, 100% on every single one of your points! 

And a new scope is something I've been pondering, looking at. 

I would love something in the 250-300mm range? I think. The Radian Raptor has been very intriguing to me. 

 

I do agree, at 25.00, not sure how good that's really going to be. A nod to all your possible negative results. To be sure. 



#9 PiotrM

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Posted 23 September 2021 - 08:31 AM

Reducing the FL?

By using a short small APO :)

 

 

CN's sponsor Astronomics (among others) sells a .5 Focal Reducer that will cut your focal length in half. Just go to their website, search for ".5 focal reducer" and take it from there. $24.95.

Which is quite useless for astrophotography with anything than a super small sensor. And to have enough backfocus on a Newtonian it has to be mounted quite deep in the focuser - if at all.

 



#10 Ryan1776

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Posted 23 September 2021 - 08:35 AM

By using a short small APO smile.gif

 

haha I know. And that was/is in my thoughts.

My wonderings were if I could reduce the FL for half or less of a new scope, I would. 


Edited by Ryan1776, 23 September 2021 - 08:35 AM.


#11 PiotrM

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Posted 23 September 2021 - 08:39 AM

haha I know. And that was/is in my thoughts.

My wonderings were if I could reduce the FL for half or less of a new scope, I would. 

Focal reduction is super hard while wanting to maintain good field quality. Right now we have a lot of lens-alike APOs (or adapted lenses) which are cheaper than a typical APO while provide similar performance and very short focal lenght.



#12 Ryan1776

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Posted 23 September 2021 - 08:47 AM

Focal reduction is super hard while wanting to maintain good field quality. Right now we have a lot of lens-alike APOs (or adapted lenses) which are cheaper than a typical APO while provide similar performance and very short focal lenght.

I'm not sure what you mean? "lens-alike apos"?



#13 jonnybravo0311

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Posted 23 September 2021 - 08:50 AM

OK cool. 

It sounds like it's specific to the scope though? I was hoping there might be one that would attach to the camera nose after the filter, before or something.

 

Sorry for the very newb question. 

 

The reason I'm digging down this hole is I don't have the time to build mosaics of large objects. 
A 3.5 year old, one on the way and work! haha.

I'd be willing to spend a good sum 3-400 on a reducer if it's good quality optics to make that happen. 

As opposed to a different scope- 250-350mm refractor. 

 

That said, I want to be able to attach and run without too much fanfare. 

Ryan 

No need to apologize for questions. A lot of manufacturers create their own reducers for their scopes. There are some generic ones as well that work across a number of different scopes and focal ratios. Typically you'll see something like "reducer works for scopes from f/4 to f/7".

 

Anyway, instead of trying to find a way to reduce the SN, have you considered perhaps just picking up a small APO refractor? I'm not sure what your budget is, but something like the AT60 and its flattener would run you about $500. That'll give you a 360mm focal length. Too much? They also sell a 0.8x reducer, bringing the cost up to about $550 and giving you a focal length of about 290mm.


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#14 PiotrM

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Posted 23 September 2021 - 08:50 AM

RedCat 51, Askar 180/135/ACL200 and similar small APO refractors.


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#15 Alex McConahay

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Posted 23 September 2021 - 09:20 AM

>>>>>>Which is quite useless for astrophotography with anything than a super small sensor. And to have enough backfocus on a Newtonian it has to be mounted quite deep in the focuser - if at all.

 

See, I thought there would be a review out there!!!

 

Alex


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#16 imtl

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Posted 23 September 2021 - 10:00 AM

Reducing your F/4 scope to F/2 by using a 24.94$ reducer is something you should not expect to yield good results unless your sensor is 10 pixels wide...


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#17 Ryan1776

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Posted 25 September 2021 - 11:19 AM

No need to apologize for questions. A lot of manufacturers create their own reducers for their scopes. There are some generic ones as well that work across a number of different scopes and focal ratios. Typically you'll see something like "reducer works for scopes from f/4 to f/7".

 

Anyway, instead of trying to find a way to reduce the SN, have you considered perhaps just picking up a small APO refractor? I'm not sure what your budget is, but something like the AT60 and its flattener would run you about $500. That'll give you a 360mm focal length. Too much? They also sell a 0.8x reducer, bringing the cost up to about $550 and giving you a focal length of about 290mm.

I have. I'll check out that scope. I'd like something that has good chromatic aberration control, if that's the right verbiage. 

A triplet, that is flat. 360mm would be pretty good, and the extra 50 for a reducer would be worth it. 

I was thinking the Radian Raptor to be honest. I like that it's all inclusive. 

 

I don't have a TON of time to dedicate to figuring it all out on a new scope. A night that I plan to image, I can up and running in under 20 min. 

And then tend to the family for whatever. I don't get much down time. 

So ideally, I'd want something that I can just remove my reflector, mount on the refractor and attach my camera and run it. 

 

I just read a few reviews on that scope...that might be the perfect match for me. Good price point. Sounds like it takes good images. 

 

On my way to Astrobin to check out some results. 

 

Thank you to all! 

Ryan 


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#18 jonnybravo0311

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Posted 25 September 2021 - 11:28 AM

A lot of really nice images have been created with the Raptor. There are also a lot of people who bash it as an overpriced Sharpstar 61 EDPH. If you're looking at the Raptor, the William Optics RedCat 51 is a very nice little scope as well. It's a petzval design, so no need for an additional flattener. It has a helical focuser, which can be a deal breaker for some.



#19 Ryan1776

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Posted 25 September 2021 - 11:34 AM

A lot of really nice images have been created with the Raptor. There are also a lot of people who bash it as an overpriced Sharpstar 61 EDPH. If you're looking at the Raptor, the William Optics RedCat 51 is a very nice little scope as well. It's a petzval design, so no need for an additional flattener. It has a helical focuser, which can be a deal breaker for some.

I've seen the bashing about it, but you're 100% right, there's some amazing images I've seen from it. 

 

I suppose I'm not experienced enough to know whether a helical focuser is "good" or "bad". 

I have the auto focuser as well on mine. Which just adds another level of automation. 
 

It wouldn't bother me to much to pull the motor and attach to a refactor when I wanted to run it either. 

Something to think about. 




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