Focal reducer and back-focus for prime focus photography?

[Akarsh Simha]

Hi

Some of you might be aware that I had problems achieving focus on my
telescope with a T-ring adapter, since I don't have enough back-focus.
Of course, one way to solve the problem is to shorten the truss poles,
but that will result in an undersized secondary and other mechanical
issues. I understand for the most part that dob astrophotography is a
lost cause (even the slightest wind will blow things out etc, Hemant
and I have tried this on a hazy night at TSP), but I still cannot help
drool at the pictures that Tom Osopowski showed off at GSSP and want
to give it another shot. I'm wondering if a cheap focal reducer (like
this one https://www.amazon.com/dp/B00VBF3O6K) will help increase the
backfocus. A barlow certainly helps achieve backfocus, but stabilizing
an image at 4100 mm is too hard. I understand that coma will be a
problem, since my telescope is an f/4.5, but producing good
photographs is not a concern. The aim is to just produce something :P

Regards
Akarsh

[Karthik Subramanian]

On Monday, 11 July 2016 12:17:58 UTC+5:30, Akarsh Simha wrote:

 I'm wondering if a cheap focal reducer (like this one https://www.amazon.com/dp/B00VBF3O6K)

will help increase the backfocus. 

I'll wait for the AP gurus to confirm, but I'd think yes.

I was suffering from lack of  back-focus in my Mak (FL 1590mm), and a cheap focal

reducer like the one you linked to helped a lot. The caveat is that I was using an analog

video camera, and not a DSLR - but I don't think that should make too much of a difference.

Oh, and you should definitely trawl through aliexpress for the focal reducer - you often get

better deals than on amazon, and I hear that their US shipping time is far faster than their

India shipping time.

K.

[sathya kumar Prasanna]

I have seen many users of Coronado PST solar telescope use a Barlow to bring the focus outside so that the DSLR will obtain an image. This sounds contradictory to Akarsh's problem and the solution of using a focal reducer. Infact, I am certain of the case where it is a common practice of using a 0.63x focal reducer/Field Flatteer for the Celestron C8 schmidt cassegrain to bring the focal ratio to F/6.3. This however results in shortening of back focus distance from roughly 150mm to 110mm. This is all my recollection from memory.

Thanks and Regards

Mr. Sathyakumar Sharma,

Scientific Officer,

Karnataka Science and Technology Promotion Society,

DST, Govt of Karnataka,

Banashankari 2nd Stage,

Bangalore.

 
Phone: +91 8095626184

http://sathya-uvacha.in

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[keerthi kiran]

Akarsh,

How do you connect your camera to the telescope?

Do you scew in the T-ring directly to the focuser or use a 1.25" adapter on t-ring and then insert the setup into the focuser like a normal eyepiece?

Regards,

Keerthi

[Akarsh Simha]

Keerthi: I have a 2" focuser barrel, so I need to use the 1.25"
adapter since my T-ring is only 1.25". Do you think that means I will
have a field stop somewhere in the light path?

Sathya: I don't think it will be as easy as Barlow and Focal Reducer
being opposites, unless they really operate using the opposite kind of
lens. For example, depending on whether I place a convex lens before
the focal plane of the primary or after the focal plane of the
primary, I can either bring the focus out or take it in. Similarly, I
could place a concave lens before the focal plane and it would slow
the convergence of the beam bringing the focus out. But you are in
general correct that bringing the focus out is associated with
increasing the focal length, not reducing it -- but I don't think that
is true in all cases.

Karthik: What kind of lensing element did it have? Do you know exactly
which brand etc?

Regards
Akarsh

[Karthik Subramanian]

On Monday, 11 July 2016 13:15:59 UTC+5:30, Akarsh Simha wrote:

Karthik: What kind of lensing element did it have? Do you know exactly
which brand etc?

Akarsh, I'll have to look this up when I get home. Will check it out and get back 

to you later this evening.

K. 

[Akarsh Simha]

Thank you, Karthik!

Regards
Akarsh

[Suresh Mohan Neelmegh]

Akarsh u need a low profile focusser and 2 you need to the the exact back focus of focal reducer

Sent from my iPhone

[Karthik Subramanian]

Hi Akarsh,

This is what I ordered:

http://www.aliexpress.com/item/New-C-Mount-Adapter-for-Telescope-0-5X-Focal-Reducer-adapter-w-0-5x-focal-reducer/32253964264.html?spm=2114.13010608.0.54.0y9d7A

I primarily got it for the C-mount adapter (since the analog video camera board I have comes with a C-mount).

Unfortunately, the link above is short on the actual details. Elsewhere, I remember digging up more

about this focal reducer - I'll look for it and post it here. I was put on to this model via a post in Cloudy Nights.

K.

[Akarsh Simha]

Karthik, are you sure it allowed you to focus when your problem was
that you had to have the camera go deeper into the tube than it would
allow? From reading most of the internet posts, I think Sathya is
right that a focal reducer pulls the focal plane deeper in, so if
going deeper in is your problem, a barlow / increasing your focal
length is the only solution. I don't want to spend money on something
and realize it won't help my issue.

Keerthi, I did some jugaad to have the same effect as this:
http://www.scopestuff.com/ss_npea.htm -- and I don't think that even
then I got enough backfocus.

Thinking about it, if one places a concave lens of long focal length
(I calculated about 38m!) in a draw-tube sufficiently deep into the
focuser, the concave lens needs to add only about 100mm to the focal
length (so it would be a 1.05x focal extender), which is not bad (way
better than using a 2x Barlow). Unfortunately, there seems to be no
such product readily available?

Regards
Akarsh

[Karthik Subramanian]

On Monday, 11 July 2016 22:52:43 UTC+5:30, Akarsh Simha wrote:

Karthik, are you sure it allowed you to focus when your problem was
that you had to have the camera go deeper into the tube than it would
allow? From reading most of the internet posts, I think Sathya is
right that a focal reducer pulls the focal plane deeper in, so if
going deeper in is your problem, a barlow / increasing your focal
length is the only solution. I don't want to spend money on something
and realize it won't help my issue.

Hi Akarsh,

It did help me focus ... but I forgot to mention something, which is rather

stupid in retrospect :)

I had to reverse the lens in the focal reducer to make it work.

The funny part was that I'd earlier tried a 2X barlow, which didn't help.

I'm not really sure what's going on, but I didn't spend a lot of time figuring

this out either. 

K.

  

[Akarsh Simha]

Hi Karthik

> Hi Akarsh,
>
> It did help me focus ... but I forgot to mention something, which is rather
> stupid in retrospect :)
>
> I had to reverse the lens in the focal reducer to make it work.
>
> The funny part was that I'd earlier tried a 2X barlow, which didn't help.
>
> I'm not really sure what's going on, but I didn't spend a lot of time
> figuring
> this out either.
>

That is a funny thing, because a single lens should work exactly the
same way if you flip it when it comes to focusing, except from the
aberrations point-of-view, there is some merit to putting a
plano-convex lens one way as opposed to the other.

Regards
Akarsh

[Karthik Subramanian]

On Monday, 11 July 2016 23:21:06 UTC+5:30, Akarsh Simha wrote:

That is a funny thing, because a single lens should work exactly the
same way if you flip it when it comes to focusing, except from the
aberrations point-of-view, there is some merit to putting a
plano-convex lens one way as opposed to the other.

I guess I'll have to try out the camera without the focal reducer

and confirm that the problem I had (have) is indeed lack of back-focus.

I know what I'm doing this weekend!

 

K. 

 

[Karthik Subramanian]

I didn't wait for the weekend. What follows is fairly obvious, but here it is for the sake

of completeness.

My scope/camera setup was suffering from a lack of *back-travel*, not back-focus.

If I had stopped to think a little before I posted, or spent a little more time reading Akarsh's

post properly, I could've avoided a fair bit of confusion.

Sorry about the noise,

K.

[Akarsh Simha]

Karthik,

Thank you so much for trying it out and clarifying it for me. BAS, as
usual, has never disappointed me as one of the most erudite mailing
lists I've seen.

So I'm basically hosed -- my only solutions are to either to move the
mirror forward, or shorten the trusses; or use a barlow (which is
sometihng I don't want to do if I can avoid it). I'm trying to take
the easy way out here, not sure what it might be.

Next question: Does anyone know if a very fractional Barlow, like may
be a 1.1x Barlow is a) available, b) will help me achieve focus?

Regards
Akarsh

[sathya kumar Prasanna]

Akarsh, I for one am one of those who would not like to make any physical changes to any equipment, but rather prefer to procure a separate system altogether for any said purpose. I would propose you refer cloudynights forum classified ads and purchase a small astrophotography setup. The reasoning is that while you setup the system to take photos of whatever you may fancy, you can still use your monster dob for visual observations of said target or others. :P

Thanks and Regards

Mr. Sathyakumar Sharma,

Scientific Officer,

Karnataka Science and Technology Promotion Society,

DST, Govt of Karnataka,

Banashankari 2nd Stage,

Bangalore.

 
Phone: +91 8095626184

http://sathya-uvacha.in

[Akarsh Simha]

Sathya,

There are a few reasons I don't want to do that at this stage. 1)
Financial, 2) Space and hassle of having a lot of equipment, 3) There
is a charm to astrophotography with a big dob -- here's an f/4.5
camera at 2000mm, which is really hard to get! Or if I could use a
focal reducer, it would be an f/2.25 at 1000mm! That's a degree field
of view on APS-C Canon. So there's certain advantages with it that are
not easy to get elsewhere, although the image quality may be crappy.

Regards
Akarsh

[keerthi kiran]

Hi Akarsh,

When you say your T-ring is 1.25", it has two parts. One, standard T-ring with 42mm thread, another adapter on top of that which has 1.25" tube.

No, you should be able to take out the 1.25" adapter and directly screw in the T-ring to your focuser. That is how it works in Skywatcher and other telescopes. Please check whether it is possible in your scope.

By doing this, you are pushing your camera front, so should be able to get the back focus.

Another way is to push the primary mirror up. If you are missing the back focus by a few cm, you can try to turn all your alignment screws and push the mirror up (but I guess you would have already tried this).

Can you please post a close up photo of your focuser?

Thanks and Regards,

Keerthi

Regards
Akarsh

[Akarsh Simha]

Wait, what? Wow, I did not know that the T-ring connects directly to
the focuser! I'll try it next time I have access to my telescope (it's
tucked away in storage) -- that's also when I can click a close-up of
the focuser. But I think it's a JMI, 2" Crayford without fine focus /
motorization.

I have tried moving the mirror up, but I was stupid enough not to know
that a T-ring is supposed to screw onto the focuser. That might solve
all the problems. I'll try it out next time I have a chance.

Regards
Akarsh

[Santhosh S]

Hey,

My 5" has the same problem. So I always use a barlow. This will increase the magnification(Good for Planetary and  Surface of moon imaging). Why don't you try that.. ?
Also DONT take still images... Just let a planet drift in the eyepiece while shooting a video. Since you have a platform the presence of the planet will be there for a longer time. Shoot many such videos with ~ a minute or two of total presence of the planet in the Field... Use PIPP to merge the videos and use Autostakkert!2 to stack... You can use Registax6 and Autostakkert!2 on linux through wine.. You can give a nice competition to renowned planetary photographers with your setup!!! :)

With an 18" dob, if you have a nice video camera you can image some Globulars and HSB galaxies... 

Regards,

Santhosh S  

[Akarsh Simha]

That's a good point about doing video, but I don't know if I can image
deep-sky that way. I should try that some time. I think my Barlow is
in Bangalore for use with the 17.5", but let me see what I can do (I
have the Barlow that came with the 6" Dob). If Keerthi's suggestion
works, I might not need the barlow...

Regards
Akarsh

[Santhosh S]

On Wednesday, July 13, 2016 at 10:50:29 PM UTC+5:30, Akarsh Simha wrote:

That's a good point about doing video, but I don't know if I can image
deep-sky that way.

 

You can shoot the bright objects with videos, only thing is you should increase the sensitivity to get maximum data with a reasonable SN ratio. Even 2 second exposure might leave a trail in the long 18" scope...!
 

I should try that some time. I think my Barlow is
in Bangalore for use with the 17.5", but let me see what I can do (I
have the Barlow that came with the 6" Dob).

 

You dont have the 'Big Barlow' there..  ? An Orion barlow will be good enough for planets and moon though...
 

If Keerthi's suggestion
works, I might not need the barlow...

Regards
Akarsh

https://www.facebook.com/marco.guidi.54?fref=ts    (Couldn't find other sources)
Just go through his profile. He has given an amazing competition to Damian Peach in planetary imaging. He uses a big DOB... 

Thanks and Regards,
Santhosh S

[Akarsh Simha]

Hi Keerthi

I just brought my telescope home after a long time, and this is what the focuser looks like (attached).

It doesn't look like this one:
http://www.xraymachines.info/article/469874314/1-25-filters-on-a-2-focuser-and-focusing-a-dslr-in-prime-focus/

I don't seem to have the T-ring threads; at least I can't find any -- the drawtube is smooth.

I think my 6" may have it, but the entire point of using an 18" for astrophotography is lost.

Do you know of any other ideas?

Regards
Akarsh

[Suresh Mohan Neelmegh]

Scopestuff sells these adapters , your question isn't clear , I don't see any focal reducer , the reducers have the t thread for your t ring

Sent from my iPhone

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