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Tips for Eaa telescope

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20 replies to this topic

#1 nettdi

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Posted 29 June 2020 - 09:50 AM

Dear boys,
I have been using the eaa since 2017 with my 10 "dobson goto
and my celestron mak 127 slt.
my cam is Gpcam2 imx 224.
When I use the dobson with focal reducer the stars I have seen are very bad ...
When I always use the focal reducer with my mak the image is good but the exposures are very long ...
I'm thinking of selling one of mine objectives and to buy the new scope. I do the EAA from the city center with a high LP
You can give me an advice?
I am thinking of buying a refractor on eq
I attached the photos to you
the first image is M31;
the second image is NGC 6888
sorry for my English
I wrote to you from Italy

Attached Thumbnails

  • M31 andromeda galaxy.jpg
  • ngc 6888 crescent nebula_4.jpg

Edited by nettdi, 29 June 2020 - 05:43 PM.


#2 mclewis1

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Posted 29 June 2020 - 12:56 PM

It looks like you have a problem with the amount of spacing from the focal reducer to the camera sensor, or the quality of the focal reducer itself. Some inexpensive focal reducers have shown similar optical problems.

 

What kind of focal reducer are you using?

How is your camera and focal reducer attached?

 

For a good portable telescope to be used for EAA I think something smaller than you 10" but also faster (shorter f ratio) than your 127mm Mak would be a good idea. I would suggest something like a 150-200mm SCT (Celestron 6SE or similar) with an SCT focal reducer (so working at about f6) or a 130-150mm f4-f5 Newtonian on a similar size alt az goto mount.

 

An 80mm  refractor would be even more portable (but most 100+mm refractors may be too big) and would make sense but the larger aperture of an SCT or Newtonian would be even more useful, especially for visual work.



#3 nettdi

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Posted 30 June 2020 - 12:29 AM

I install the reducer directly on the cam.
do I have to distance it to improve?

#4 nettdi

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Posted 30 June 2020 - 10:28 AM

What do you think about it for eaa:

BRESSER 4802460 Messier Ar-102 X S/460 Hexafoc



#5 cmooney91

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Posted 30 June 2020 - 12:18 PM

I used a very similar camera (RisingCam IMX224) with a Meade Mini LightBridge 114mm F4 Newtonian.($114-$130)

 

Here is an EAA album using that combo. I used a small GEM but an Alt-Az would be fine too.

 

I really liked that combo, I think it is the ideal scope for that camera. Through convergent design, this also happens to be the exact same sensor and optics selected for the EVscope.

 

 

Positives

  • inexpensive
  • F4 is fast enough to not require a focal reducer at all
  • The IMX224 sensor is small enough to not pick up a lot of coma (no coma corrector needed)
  • The small scope has a 450m focal length, which gives a decent FOV (0.62deg 0.47deg) for the small camera (no too tight)
  • The paired resolution is 1.7"/px  which is good enough for average seeing.
  • the camera fits down into the focuser, so it can reach focus with out modification. 
  • no glass in the light path, so no chromatic aberration
  • Very small and light combination
  • comes with a dovetail.
  • Fun rich field visual scope

 

 

Negatives

  • Focuser will need re-greasing and shimmed. 
  • Focus will be shallow, a bahtinov mask will help.
  • Collimation will be critical (need cheap laser +barlow)
  • Secondary is small, will need accurate location, and flat frames for vignetting.
  • changed secondary adjustment screws to cap head vs set screw

Edited by cmooney91, 01 July 2020 - 07:00 AM.


#6 nettdi

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Posted 30 June 2020 - 02:55 PM

seems to be very interesting. can I mount it on the slt? I wanted to buy a light refractor and mount it on the goto dobson in order to have two ready configurations with a single mount. but i am not sure that my orion xt10g can maintain a 4/5 kg telescope

#7 Rickster

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Posted 30 June 2020 - 09:54 PM

I see that Celestron makes a 130SLT.  It is a 130mm F5 Newt.  If you can buy the optical tube separately you could put it on the SLT mount instead of the Mak.  The main thing is to get a Newt.  It solves many problems.



#8 nettdi

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Posted 01 July 2020 - 03:54 AM

I see that Celestron makes a 130SLT.  It is a 130mm F5 Newt.  If you can buy the optical tube separately you could put it on the SLT mount instead of the Mak.  The main thing is to get a Newt.  It solves many problems.

Your Opinion is correct!!!

Now i have to value the Mini LightBridge or 130 newton

the Mini LightBridge should be more fast then 130 newton

the other solution colud be to mount the new scope on ota dobson



#9 Clouzot

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Posted 01 July 2020 - 10:58 AM

I install the reducer directly on the cam.
do I have to distance it to improve?

Yes. Could you say which focal reducer you use exactly? The correct distance (also named "backfocus") will depend on it.

 

(you can PM me in Italian if that's easier for you)


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#10 fromEarth

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Posted 01 July 2020 - 01:06 PM

I have used a cheap (~ $12) 0.5x generic focal reducer with my RT224 and experienced the similar distortions with bright stars. My FR seems to be a generic version of a SVBONY product. The Revolution Imager uses the same FR, I guess.  

 

https://www.aliexpre...2793084618.html

 

Currently, I am thinking of upgrading my FR. Could you recommend me a higher-quality 0.5x 1.25'' FR, based on your own experience? My RT224 came with a 10mm-long c-mount adapter and a 20mm-long c-to-1.25'' nosepiece. The distance from sensor to FR is ~50mm.

 

fromEarth 


Edited by fromEarth, 02 July 2020 - 09:26 AM.


#11 nettdi

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Posted 01 July 2020 - 03:44 PM

Yes. Could you say which focal reducer you use exactly? The correct distance (also named "backfocus") will depend on it.

(you can PM me in Italian if that's easier for you)


The FR that i use is this:
Solomark -

Attached Thumbnails

  • 8614EDDE-6B85-4FBC-8FF0-455D5A768CBB.jpeg

Edited by nettdi, 01 July 2020 - 03:48 PM.


#12 Rickster

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Posted 01 July 2020 - 05:25 PM

Yes. Could you say which focal reducer you use exactly? The correct distance (also named "backfocus") will depend on it.

 

(you can PM me in Italian if that's easier for you)

How many languages do you speak?!smile.gif


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#13 nettdi

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Posted 02 July 2020 - 03:30 AM

How many languages do you speak?!smile.gif

I'm Italian.

I can't write in italian on this forum



#14 Clouzot

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Posted 02 July 2020 - 03:56 AM

How many languages do you speak?!smile.gif

Three: French, English, Italian (I'm pretty close to the border so I thought it useful to learn, just like a lot of people learn Spanish in California I'm told). And a bit of the local Provence/Ligurian dialects for good measure.

 

 

The FR that i use is this:
Solomark -

It looks like the usual Svbony/Orion reducers, so I assume it is quite similar. You should have a spacing of approximately 50 to 55m between the middle of the reducer (look for where the lenses are) and the sensor plane to get the nominal x0.5 reduction, and less coma on your stars.



#15 nettdi

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Posted 02 July 2020 - 04:59 AM

Three: French, English, Italian (I'm pretty close to the border so I thought it useful to learn, just like a lot of people learn Spanish in California I'm told). And a bit of the local Provence/Ligurian dialects for good measure.

 

 

It looks like the usual Svbony/Orion reducers, so I assume it is quite similar. You should have a spacing of approximately 50 to 55m between the middle of the reducer (look for where the lenses are) and the sensor plane to get the nominal x0.5 reduction, and less coma on your stars.

I will try it



#16 mclewis1

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Posted 02 July 2020 - 10:40 AM

Nettdi, What is the black ring at the front of your camera marked with? C or CS?

 

C = 17.5mm back to the sensor

CS = 12.5mm back to the sensor

 

The 1.25" nosepiece for this camera is usually 20mm but sometimes 25mm in length.

 

There are a lot of different numbers about the correct spacing for these (and similar) .5x 1.25" focal reducers. What Clouzot has mentioned is the common information for the GSO sourced reducers (50-55mm), but I've seen a few similar reducers with different spacing. 

 

I would actually check the focal length of the focal reducer. Measure the focal point with just the reducer from a bright infinite source (the moon is a good choice - do not use a light bulb in the same room) and then use half of that measurement as the spacing the reducer needs.

 

Something else to check. The focal reducer should when mounted on your camera have the lens that faces outward (away from the camera) curved outwards (a convex surface). There have been examples of inexpensive focal reducers being assembled incorrectly. While the curve is not obvious it's quite easy to check, just gently place the end of a small ruler (or other flat surface, even a small piece of paper or cardboard can be used) across the lens and you should see a small space towards the edges .... or the ruler should be able to rock back and forth a little bit. 

 

If the lens is assembled wrong the lens surface facing forward will be concave (curved inwards) and a flat end of a ruler will not rock when placed on it (or you will notice a space in the middle of the lens to the surface of the ruler).

 

If you need to flip the lens over the thin retaining ring can usually be carefully un threaded using two small (jewelers) screw drivers or other small sharp ended tool. If you do remove the lenses to flip them make sure to be careful to keep the lens together (they are often in two pieces) and don't rotate them, just ease the lens out, flip it over and return the retaining ring. Be gentle with the lenses and keep them straight when removing and re inserting them to prevent chipping the edges.


Edited by mclewis1, 02 July 2020 - 10:44 AM.

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#17 nettdi

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Posted 02 July 2020 - 06:13 PM

Nettdi, What is the black ring at the front of your camera marked with? C or CS?

 

C = 17.5mm back to the sensor

CS = 12.5mm back to the sensor

 

The 1.25" nosepiece for this camera is usually 20mm but sometimes 25mm in length.

 

There are a lot of different numbers about the correct spacing for these (and similar) .5x 1.25" focal reducers. What Clouzot has mentioned is the common information for the GSO sourced reducers (50-55mm), but I've seen a few similar reducers with different spacing. 

 

I would actually check the focal length of the focal reducer. Measure the focal point with just the reducer from a bright infinite source (the moon is a good choice - do not use a light bulb in the same room) and then use half of that measurement as the spacing the reducer needs.

 

Something else to check. The focal reducer should when mounted on your camera have the lens that faces outward (away from the camera) curved outwards (a convex surface). There have been examples of inexpensive focal reducers being assembled incorrectly. While the curve is not obvious it's quite easy to check, just gently place the end of a small ruler (or other flat surface, even a small piece of paper or cardboard can be used) across the lens and you should see a small space towards the edges .... or the ruler should be able to rock back and forth a little bit. 

 

If the lens is assembled wrong the lens surface facing forward will be concave (curved inwards) and a flat end of a ruler will not rock when placed on it (or you will notice a space in the middle of the lens to the surface of the ruler).

 

If you need to flip the lens over the thin retaining ring can usually be carefully un threaded using two small (jewelers) screw drivers or other small sharp ended tool. If you do remove the lenses to flip them make sure to be careful to keep the lens together (they are often in two pieces) and don't rotate them, just ease the lens out, flip it over and return the retaining ring. Be gentle with the lenses and keep them straight when removing and re inserting them to prevent chipping the edges.

My Black ring is Marked CS

I want to i know how can i measure the correct distance?

I focus and measure the focuser's excursion
this point is not clear for me



#18 mclewis1

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Posted Yesterday, 02:51 PM

Ok, with a CS ring the sensor is 12.5mm from the front surface of the ring. I would make a small mark on the side of the camera at the sensor location. I would also then measure the distance from the back of the camera to the sensor location in case you'd like to measure the focal point with the camera deep in the focuser. I assume that with the big Dob that the camera sits down into the 2" focuser tube in a 2" to 1.25" adapter.

 

Without the focal reducer the sensor will sit at the telescope's focal point. With the focal reducer on the camera the camera will need to be moved inwards (towards the telescope). The amount of inward movement required is dependant on the focal length of the reducer, the spacing it is set at, and the f ratio of the telescope. The amount of inward position could be 15mm or as much as 35mm. I use this website http://www.wilmslowa...rmulae.htm#FR_a for quick calculations about the spacing and reduction factors of focal reducers and the inward focal point changes.

 

We would really like to know the focal length of the SvBony focal reducer, but I can't seem to find a number anywhere I've looked. It would be helpful to actually measure it using the idea I mentioned in an earlier post (point the bare reducer at the moon, note where it makes a sharp image on a piece of paper, measure the focal length, then divide by 2 for the spacing needed).

 

I also notice that SvBony makes a very inexpensive .5x reducer that is a singlet lens ... yikes! (along with a slightly more expensive doublet version) I wonder just what kind of optical quality this single element lens would produce (not very good I would guess)? All the .5x reducers I've seen or worked with have been doublets and while most of these are not incredibly good optical quality they at least produce reasonable images.

 

So with all of this investigation it might be that the easiest solution to the flared star images would be (if your reducer is indeed make from only a single lens element) to use a different better quality focal reducer. 


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#19 mclewis1

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Posted Yesterday, 02:57 PM

When measuring for the spacing back to the sensor (or the focal length) you usually start with a point that is inside the lens element(s) (often hard to accurately approximate) and not the bottom of the metal that holds the lens. Of course it is easier to measure from the bottom edge where the threads start, but then just remember to add about 5mm.


Edited by mclewis1, Yesterday, 02:58 PM.


#20 nettdi

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Posted Yesterday, 03:56 PM

The photos that you ask me

Attached Thumbnails

  • 62638559-D526-42E5-8B76-11B16FDF6C3A.jpeg
  • 6AE50980-C836-469D-87C0-F63D6BE56626.jpeg
  • 0B91556E-5475-4F58-A6A4-04019A3278C1.jpeg

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#21 nettdi

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Posted Yesterday, 05:58 PM

I check my focoal reduce. It is mounted wrong!!!! It has the curved face into not out.

 

I tryed to install the fr correct with scotch tape but i could not get focus...

I tryed to install  the correct face of focal reduce after the cs ring and i get the focus

I attached you a photo.

Now, I ask you, Can you suggest my a good focal reduce for my telescope and my cam?

 

Capture_00001 00_08_23.jpg




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