Jump to content

  •  

CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.

Photo

60mm or 72mm Zenithstar for EAA?

  • Please log in to reply
17 replies to this topic

#1 Lornet

Lornet

    Vostok 1

  • ****-
  • topic starter
  • Posts: 100
  • Joined: 27 Sep 2017

Posted 15 September 2021 - 10:24 AM

On an Ioptron GEM28, would I get pleasing EAA images from a 60mm Zenithstar (doublet APO)?  How about a 72mm Zenithstar?  Would they be too small?  Also, how much would it affect my image if I didn't use a field flattener and focal reducer?  As an entry into this field, would I immediately find it unsatisfying or would it be a decent setup to get started? 

LT



#2 barbarosa

barbarosa

    Gemini

  • *****
  • Posts: 3,169
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 15 September 2021 - 10:58 AM

Go to Astronomy Tools, Field of View Calculator. Select Imaging Mode. Pick a target. Find your scope (both scopes are already on the list) . Pick a camera, add a reducer or not (a flattener has no effect on the image scale). Repeat for your second scope. Compare the relative target size/field of view. The smallest scope I've used was an 85mm, but there are a lot of very enthusiastic owners of the Red Cat 51mm. You will enjoy either of the scopes you mention.

 

Here is M8 w/ a 533 camera. Larger sensor = wider field of view. Smaller sensor = greater magnification.

 

astronomy_tools_fov (29).png


Edited by barbarosa, 15 September 2021 - 10:59 AM.

  • Lornet likes this

#3 GazingOli

GazingOli

    Viking 1

  • -----
  • Posts: 605
  • Joined: 13 Oct 2019
  • Loc: Stuttgart GERMANY

Posted 15 September 2021 - 11:06 AM

It depends a little on what you want and which camera you are using. I got a selfmade triplet apo and get great results for EAA when I mount it on an AZ-Gti. So a douplet gives you a good start. I personally would prefer the 72 mm because I prefer to ge closer to the celestrial objects. But you can figure out with Stellarium, for example, which objects fit into the FOV of the desired camera.

 

CS.Oli

PS. you might check out my gallery (link in my signature). There is a photo of the apo on the AZ-GTi. The images after that where captured with that setup.


Edited by GazingOli, 16 September 2021 - 12:47 AM.

  • Lornet likes this

#4 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 125,249
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 15 September 2021 - 12:18 PM

I use a 72mm f/6 AT72EDII, which is similar to the 72mm Zenithstar, and a small ASI290MM mini camera. The field of view of this combo is about 45' x  25'. That's big enough for quite a number of smaller targets.

 

If you want to cover large emission nebulae, you probably want a bigger camera than mine for this scope.


  • Lornet likes this

#5 barbarosa

barbarosa

    Gemini

  • *****
  • Posts: 3,169
  • Joined: 11 Apr 2010
  • Loc: 139 miles W of the Awahnee Hotel

Posted 15 September 2021 - 01:03 PM

Just a note on refractors and doublets vs triplets. I have an 85mm ED doublet (unknown ED glass type), labeled by the maker as a "semi-apo". I have a more expensive triple with FPL-53 glass. It is all around the better scope, but the difference in image quality  is not so large as to make the smaller scope unsatisfactory. By far the largest difference in the price.

 

Williams Optics, SharpStar, Sky Watcher and others sell ED doublets that have many well satisfied buyers. The image quality of the Zenithstar is good. That said larger sees a bit deeper and resolves a bit better, but not drastically so in this case. Price, portability, the mount and the camera (sensor size, pixel size, cooled or not, color vs mono)  are have to be considered as well.

 

The mount is very important. Alt az or EQ and payload (not just the first scope you buy but the next one), style of use (only 30s or less frames or will you want to add a dual band filter and go longer, 1min to 5 min). When I caught the bug it was a basic Alt az go to mount and small scope. Four mounts later it is a tandem setup on a pier carrying 60lbs of scopes and cameras--- and I still do EAA. It just gets better.smile.gif (and more expensive). The point is that if I had looked ahead and looked past the immediate need and cost, it could have been two mounts, and a significant savings in time, effort and cash.


  • Lornet likes this

#6 jkmonroe

jkmonroe

    Sputnik

  • -----
  • Posts: 47
  • Joined: 22 Mar 2012

Posted 15 September 2021 - 06:30 PM

I have the ZS73 on an AZ-GTi and have never had as much fun. I’ve got an asi385 but will probably get the 290mm. The small sensor of the 385 (or 290) seems to work well with no flattener.
  • Lornet likes this

#7 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 125,249
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 15 September 2021 - 08:01 PM

Yep, at f/6 the AT72EDII also doesn't need a flattener with a small sensor camera.


  • brentknight and Lornet like this

#8 chilldaddy

chilldaddy

    Vostok 1

  • -----
  • Posts: 186
  • Joined: 10 Jul 2018
  • Loc: Norman, OK

Posted 16 September 2021 - 02:34 PM

LT,

Will you get pleasing images with either of those scopes?  Yes.

 

Will they be too small?  No.  I use a 60mm because it's small and gives a wide FOV.

 

How much is your image affected without a FR?  Follow Barbarosa's link to Astronomy.tools FOV calculator and spend a lot of time framing the objects you are interested in with different OTA/camera combinations.  It will be your best friend.

 

Will it be unsatisfying?  Hardly!  In fact, it seems that many people start with long focal length OTAs, like an 8" SCT, then pair it with small sensor cameras like the ASI224 or ASI385, only to find that the FOV is very small.  They then add either a small aperture OTA like you are suggesting, or a larger sensor camera, or both!  (that's exactly what I did grin.gif )  The ASI385 works well with the AT60ED, but the larger FOV of the 533 is even better.  More than just being a decent setup to get started, it has become a favorite tool for quick and easy wide FOVs and some of my favorite viewing is with this portable setup.

 

You can use a small aperture OTA with or without a FR.  Surprisingly, my F/6 AT60ED works great with an F/6.3 SCT reducer giving me something around F/3.8.  I add a flattener when at F/6, but it's not necessary.  You can always do that later.  I'm not concerned with the speed of the systems and base all of these decisions on the FOV desired for the target.  The larger the FOV, the easier it is to get started and have quick success.  It is a highly recommended setup in my opinion.

 

Here are some examples of images taken with an AT60ED and ASI533.  The first two were at the native F/6 and the others needed the FR for approx. F/3.8 to better fit the object. 

 

With a good mount like the GEM28, I would not hesitate to put a small refractor on it and start having a blast!

 

Greg

 

(click for larger image and details)

M42, Orion

150x 8.0s 1200s

M42
 
M81, M82
40x 15.0s 600s
M81, M82

 

IC 1805, Heart
61x 30.0s 1830s
Heart Neb, L-Extreme, 30min
 
NGC 1499, California
80x 30.0s 2400s
California Neb, L-eXtreme, 40min
 
NGC 2237, Rosette
90x 30.0s 2700s
NGC 2237, Rosette Nebula

Edited by chilldaddy, 16 September 2021 - 02:57 PM.

  • Ptarmigan, Agnes, alphatripleplus and 3 others like this

#9 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 125,249
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 16 September 2021 - 02:54 PM

I have done the same as Greg, using a f/6.3 SCT reducer/flattener with my AT72EDII. Greg's captures demonstrate that it works very well for EAA with these refractors, even though it is designed for a different optical system.


  • brentknight, Lornet and chilldaddy like this

#10 Agnes

Agnes

    Sputnik

  • -----
  • Vendors
  • Posts: 28
  • Joined: 21 Jun 2011

Posted 16 September 2021 - 02:59 PM

Lovely images @chilldaddy! I am using a 6.3 reducer with my Zenithstar 66 - it works surprisingly well.


  • Lornet and chilldaddy like this

#11 MarMax

MarMax

    Surveyor 1

  • *****
  • Posts: 1,517
  • Joined: 27 May 2020
  • Loc: Los Angeles

Posted 18 September 2021 - 11:21 AM

Thanks for the post Greg, I've wondered about using the SCT reducers with the small refractors. I've been having a lot of fun with an f/3.3 reducer with the C11 but it does introduce a lot of CA around stars. I'll have to try the f/6.3 with the 61mm and 92mm.

 

I've wanted to give M45 a try but my small sensor camera is limiting. If I use the f/6.3 reducer with the 61mm it should work.

 

gallery_332504_17489_65099.png

EDITED to say the camera is the Neptune-CII (IMX464).


Edited by MarMax, 18 September 2021 - 11:22 AM.

  • chilldaddy likes this

#12 MarMax

MarMax

    Surveyor 1

  • *****
  • Posts: 1,517
  • Joined: 27 May 2020
  • Loc: Los Angeles

Posted 18 September 2021 - 11:28 AM

Apologies to the OP for the slight off topic follow up.

 

Greg, Errol and Agnes, are you using the f/6.3 reducer with 105mm of back spacing (shoulder to sensor)?



#13 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 125,249
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 18 September 2021 - 11:59 AM

Apologies to the OP for the slight off topic follow up.

 

Greg, Errol and Agnes, are you using the f/6.3 reducer with 105mm of back spacing (shoulder to sensor)?

No. For EAA with my 72mm f/6 refractor,  I'll use the reducer at about 0.73 to yield about f/4.4. The 105mm back spacing is to get f/6.3 with a  f/10 SCT optical train. For these small f/6 refractors, you may want to experiment with shorter back spacings.



#14 MarMax

MarMax

    Surveyor 1

  • *****
  • Posts: 1,517
  • Joined: 27 May 2020
  • Loc: Los Angeles

Posted 18 September 2021 - 02:38 PM

No. For EAA with my 72mm f/6 refractor,  I'll use the reducer at about 0.73 to yield about f/4.4. The 105mm back spacing is to get f/6.3 with a  f/10 SCT optical train. For these small f/6 refractors, you may want to experiment with shorter back spacings.

That makes sense. I use the f/3.3 at 53mm (f/4) instead of 60mm (f/3.3) with the C11. Do you recall your spacing with the 72mm f/6 refractor for the 0.73x?

 

And Greg or Agnes, please share your spacing if you recall.



#15 alphatripleplus

alphatripleplus

    World Controller

  • *****
  • Moderators
  • Posts: 125,249
  • Joined: 09 Mar 2012
  • Loc: Georgia

Posted 18 September 2021 - 02:52 PM

That makes sense. I use the f/3.3 at 53mm (f/4) instead of 60mm (f/3.3) with the C11. Do you recall your spacing with the 72mm f/6 refractor for the 0.73x?

 

 

63.5mm from the sensor to the rear face of the f/6.3 reducer yields f/4.4 with my 72mm f/6 AT72EDII.



#16 Agnes

Agnes

    Sputnik

  • -----
  • Vendors
  • Posts: 28
  • Joined: 21 Jun 2011

Posted 18 September 2021 - 05:42 PM

I'm not very scientific about the spacing, but it's in the region of 50 to 60 mm. That gets me down to F4.4 to F4.0. I think the reducer is stronger in a faster scope.

I don't have the backfocus for 105 mm spacing with the Zenithstar 66 by the way.

#17 MarMax

MarMax

    Surveyor 1

  • *****
  • Posts: 1,517
  • Joined: 27 May 2020
  • Loc: Los Angeles

Posted 18 September 2021 - 08:01 PM

Thanks for the back spacing info guys! And again apologies to the OP for the distraction. I'll give the f/6.3 a try on both the 61mm and the 92mm.



#18 chilldaddy

chilldaddy

    Vostok 1

  • -----
  • Posts: 186
  • Joined: 10 Jul 2018
  • Loc: Norman, OK

Posted 18 September 2021 - 09:38 PM

I think I had it around 80mm recently and am still experimenting.  I haven't calculated the exact F/ratio yet.

 

Greg




CNers have asked about a donation box for Cloudy Nights over the years, so here you go. Donation is not required by any means, so please enjoy your stay.


Recent Topics






Cloudy Nights LLC
Cloudy Nights Sponsor: Astronomics