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Help needed with reducer / flattener


Dam

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Hi There Good People,

Hope everyone is well and safe. 

Not sure if this is correct place to post in since am newbie here and it is my first post so i hope it is ok.

I have a question or two that i need a help / advice with and i can't figure it out for the life of me which focal reducer/flattener will i need with my setup? I have recently bought Brassier AR152S/760 f/5 telescope with HEQ5Pro mount and few other accessories so i can do visual observing and at some point, do AP. However, was told that having a reducer/flattener in my optical train is a must have for AP and have since been browsing internet for more info on what is the correct focal reducer / flattener but i can't seem to figure out which one would work best with my refractor. I have Altair HyperCam 183c and some people suggested that i don’t need the flattener with my setup some people do and am confused now which one i would need lol. 

I hope that someone here can shed some help and advice and set things straight for me lol, i would appreciate that very much. 

Mnay thanks in advance and looking forward to repsonse. 

Best, 

Dam

 

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Hi and welcome to SGL.

You should wait with field flattener / focal reducer for your scope until you start doing AP.

You might not need it at all. Scope that you have is actually 4 lens design and that means it could have flat field as is.

According to TS website - in fact, this is modified Petzval design and it has flat field:

image.png.1bc0bddd045865302b5579afff02a839.png

source: https://www.teleskop-express.de/shop/product_info.php/language/en/info/p7786_Bresser-4852760---152-mm-Refraktor--f-760-mm--OTA.html

Bigger problem will be chromatic aberration, but you won't know until you start imaging with said scope - again, here 4 lens design is meant to reduce chromatic aberration usually associated with refractors.

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Hi Both, 

Thank you for the welcome and thank you for the prompt replies and advice. 

@Dave thanks for the link i have seen that on the FLO website but i wasn't sure if that was the correct for the telescope, thanks for clearing that up :)  

@Vlaiv thank you for welcome and many thanks for reply. Yes, it does have 4 lens actually and i also have got semi apo filter which i hope it will cancel some CA  😐  . I have taken a picture of the Moon other day and have noticed some faint blue around the Moon but i wasn't using filter and took image with my mobile phone thur eyepiece only. Could that be the reason maybe why (sorry i am not an expert and just trying to figure things out)? Visually i couldn't notice any CA. 

Would you guys recommend any other accessories / equipment that i might need to purchase :(  that will be needed in a long run?

Mnay thanks... 

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16 hours ago, Dam said:

@Vlaiv thank you for welcome and many thanks for reply. Yes, it does have 4 lens actually and i also have got semi apo filter which i hope it will cancel some CA  😐  . I have taken a picture of the Moon other day and have noticed some faint blue around the Moon but i wasn't using filter and took image with my mobile phone thur eyepiece only. Could that be the reason maybe why (sorry i am not an expert and just trying to figure things out)? Visually i couldn't notice any CA. 

Would you guys recommend any other accessories / equipment that i might need to purchase :(  that will be needed in a long run?

Semi apo filter is nice addition. I have F/10 4" achromat and use Baader Contrast Booster as minus violet filter. It does work.

Out of focus blue and violet are much easier to spot in images than with eye - which is good for visual but bad for astro photography. This is because camera sensor is more sensitive to blue part of spectrum than human eye is. Human eye is most sensitive in green part of the spectrum.

As for accessories, I don't think that you should worry about it now. Part of the fun (for me at least) is discovering what you need and anticipation and the thrill when you get new piece of kit and wait for a chance to test it out.

Here is something that you should consider, but like I said, it will depend on your interests / taste. I would consider 2" diagonal mirror if you don't already have one and wide field eyepiece. 6" refractor is very good scope for wide field viewing of Milky way and large open clusters.

If you are serious about astrophotography - consider getting autoguiding kit at some point. That means guide scope (you can turn your finder scope into a guide scope or add separate guide scope instead of finder scope) and planetary camera. Planetary cameras can be a lot of fun. You can take better images of the Moon and the Planets with them and even some Deep Sky imaging - that is how I started into AP.

Scope you have is not well suited for AP because it is achromatic design (it is a bit better corrected than regular achromat, but as you have seen - bright objects will have that blue / purple halo around them). Luckily there is something that you can do about it.

- You can use Semi APO filter or even regular #8 Wratten yellow filter. Very good filter for that is also 495 long pass filter (from baader - it is deep yellow filter). Problem with yellow filters is that it will skew your color balance, but that can be corrected in processing phase

- Another thing that you can add is aperture mask. That is something that you can't purchase, but you can make one, even out of cardboard. With nowadays 3d printing - it is very easy to print one to suit your needs. Aperture mask is just a mask with smaller aperture than original aperture of telescope. Chromatic blur of achromat telescope depends on clear aperture size. Reducing this size removes some of chromatic blur.

Don't think that your scope won't be able to produce good images - it is just a bit more complicated and appropriate technique needs to be used. Here is example of what F/5 achromat (2 lens simple design with a lot of CA) can do with planetary type camera:

m42.png

Of course, to get better images - you will need DSLR camera and adapter (T2 ring) for that camera. Canon cameras are probably easiest to work with for astrophotograpy because of software support and being able to shoot raw images (without any in camera processing).

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I agree with Vlaiv. Try it first. You certainly won't be wanting to make it any faster because a 6 inch refractor at F5 is already screamingly fast. (Keeping the F ratio low without colour fringing gets harder and harder as aperture increases.)

Olly

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19 hours ago, Dam said:

couldn't notice any CA.

Hi

The 152s has a f10 doublet as well as the correction elements in the focus tube. To correct any residual colour we used a #8 yellow. Best to try with a camera to see if it's acceptable to you. 

Visually, it gave the best views down by Scorpio and Sagittarius I've ever seen.

HTH

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On 12/06/2020 at 10:32, vlaiv said:

 

As for accessories, I don't think that you should worry about it now. Part of the fun (for me at least) is discovering what you need and anticipation and the thrill when you get new piece of kit and wait for a chance to test it out.

Here is something that you should consider, but like I said, it will depend on your interests / taste. I would consider 2" diagonal mirror if you don't already have one and wide field eyepiece. 6" refractor is very good scope for wide field viewing of Milky way and large open clusters.

If you are serious about astrophotography - consider getting autoguiding kit at some point. That means guide scope (you can turn your finder scope into a guide scope or add separate guide scope instead of finder scope) and planetary camera. Planetary cameras can be a lot of fun. You can take better images of the Moon and the Planets with them and even some Deep Sky imaging - that is how I started into AP.

 You can use Semi APO filter or even regular #8 Wratten yellow filter. Very good filter for that is also 495 long pass filter (from baader - it is deep yellow filter). Problem with yellow filters is that it will skew your color balance, but that can be corrected in processing phase

- Another thing that you can add is aperture mask. That is something that you can't purchase, but you can make one, even out of cardboard. With nowadays 3d printing - it is very easy to print one to suit your needs. Aperture mask is just a mask with smaller aperture than original aperture of telescope. Chromatic blur of achromat telescope depends on clear aperture size. Reducing this size removes some of chromatic blur.

Don't think that your scope won't be able to produce good images - it is just a bit more complicated and appropriate technique needs to be used. Here is example of what F/5 achromat (2 lens simple design with a lot of CA) can do with planetary type camera:

 

Hi Vlaiv, many thanks for the pointers much appreciated, note taken of all of them. Really appreciate time you taken to get this down. This indeed will help me to get on the right leg and start in right direction. One thing am sure it certainly is very expensive learning curve i would say but very rewarding too . I think i have a lot there to start with so hopefully get some clear skies soon :) as it been constantly cloudy and rainy  :( . Once again, many thanks.

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On 12/06/2020 at 13:00, ollypenrice said:

 (Keeping the F ratio low without colour fringing gets harder and harder as aperture increases.)

Olly

Many thanks Ollypenrice , that i didn't know  thanks for pointing that out.

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If it is a quadruplet then you most likely do NOT need a field flattener. That is because the fourth element IS the field flattener. Do not worry about reducing the focal length quite yet as well. I would recommend going to https://astronomy.tools/calculators/field_of_view/ , click on imaging and then input the details of your scope and camera. This will give you and idea of your FOV. type in targets you are interested in capturing and then determine feom there if you would want to reduce your fov. right now I am no reducing my fov, however a benefit of doing so is that your scope becomes faster as well. 

Edited by Danield06
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  • 8 months later...

Nice to see that someone has one of these scopes.  I first looked through a 6" short tube several years ago.  Just a normal 2 element achromat, but it knocked my socks off!  To be fair I do have good Apo triplets (as you see in my signature) but that 6" glass has always fascinated me for visual observing - wide field, good light grasp and contrast with no central obstruction.  By chance I recently spotted this scope on a couple of web sites, this one and here  And it would appear that the manufacturers/designers have done their best to minimise chromatic aberration and supply a flat field all at a very reasonable price.  Temptation!   Unfortunately I don't do much visual these days because of deteriorating eyesight - Glaucoma and cataracts.  Can't do anything about the glaucoma which fortunately is only in one eye, but I hope (once the Covid hold ups clear a bit) to get the cataracts sorted.  Then we shall see!  Pun intentional 😉

So if you, 'Dam' have now got used to the scope, your comments would be very much appreciated.  As would any other reviews that people might know about.

Cheers,

Peter

Edited by petevasey
Corrected faulty link
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I have a very different telescope, but with essentially the same specs in terms of aperture and focal length: a Meade 6" F/5 Schmidt-Newton, and I combine that with a camera with the same chip as yours (mine is the ASI183MC (non cooled)), so to give an idea of the kind of field of view you would have, here is an example of what I managed after four hours on M101, from Bortle 4-5 skies

M101-14560.0s-3.thumb.jpg.8e483539ecc3a44edd97492a72e8eff3.jpg

or 13 hours on M27 with L-eNhance filter

M27-46950.0s-NR-x_1.0_LZ3-NS-ref-qua-add-sc_BWMV_nor-AAD-RE-MBB10_4crop.thumb.jpg.84855d3877c4412e275621d294e35e80.jpg

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