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Your thoughts: 10" F/3.2 Astrograph

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#1 Lognic04

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Posted 18 September 2020 - 08:25 AM

Hi All,

Been a while since I've been on these forums! Good to be back. I have lately had the idea of making a 10" F/3.1 imaging newtonian. I was inspired by Rolf Olsen, who takes amazing images with his equipment: https://www.rolfolse...Serrurier-Truss

The idea of a serrurier truss piqued my interest, and I daresay it looks achievable to a mere mortal without a CNC. Rolf has done some great work with his scopes, and the idea of using Aluminium for the central box is intriguing.

At the moment my plan is to use:

AA Grade Marine ply for the lower and upper OTAs. (Single ring lower and double ring upper)

A moonlite focuser.

An Explore Scientific HRCC Coma corrector, which has a 1.06x barlow effect, meaning the initial mirror focal ratio would sit around f/3.

An Orion Optics UK mirror cell, inc homemade mirror. Would plate glass be adequate?

A 3.5in secondary in a commercial (Astrosystems I believe?) secondary holder and spider.

 

Does anyone have any thoughts or opinions they would be able to chip in. One thing of note is the serrurier truss assembly would be quite short at f/3, but it seems like the best choice for imaging regardless.

Would it be of use to hyperbolize the primary mirror to use with the HRCC? I've heard things about hyperbolic astrographs but not sure if that carries any meaning in this application.

Apologies for the rather messy thread. Just writing my thoughts down really!

Cheers,

Logan


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#2 Augustus

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Posted 18 September 2020 - 09:16 AM

I think you mean the MPCC; the HRCC doesn’t modify correction or at least not significantly. The Baader MPCC adds spherical aberration which people hyperbolize the mirror to counteract - that’s my understanding anyways. It also doesn’t add any magnification like other CCs and IIRC it does a better job at correcting coma overall, at the expense of the SA.

I don’t see why plate wouldn’t work.

Edited by Augustus, 18 September 2020 - 09:18 AM.


#3 Gipht

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Posted 18 September 2020 - 09:43 AM

One of the  issues with the very fast  reflectors is the size of the image field at focus at the sensor.   The second is tilt in the image.   Wish I had the technical knowledge to advise, but just wanted to be sure you were aware of these issues.


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#4 tommm

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Posted 18 September 2020 - 11:25 AM

Logic is correct, the HRCC has 1.06 multiplication factor. The MPCC MKIII has a factor of 1.  There was a thread by Ed Jones on using a hyperbolic primary with the MPCC, nothing I know of on the HRCC.  In that thread he gave for a 10" f/3.55 a conic constant of -1.252.

 

A Surrier truss hardly seems necessary at f/3 or so for a 10" since normal truss poles would be quite short and stiff if of large enough diameter. But if you just want to build one...

 

You will want to think about image scale and pixel scale, and what range of object size you plan to image if you haven't already.


Edited by tommm, 18 September 2020 - 11:31 AM.

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#5 Lognic04

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Posted 18 September 2020 - 06:49 PM

I think you mean the MPCC; the HRCC doesn’t modify correction or at least not significantly. The Baader MPCC adds spherical aberration which people hyperbolize the mirror to counteract - that’s my understanding anyways. It also doesn’t add any magnification like other CCs and IIRC it does a better job at correcting coma overall, at the expense of the SA.

I don’t see why plate wouldn’t work.

No, I mean the HRCC  and as tommm said it does indeed have a factor of 1.06.

 

 

One of the  issues with the very fast  reflectors is the size of the image field at focus at the sensor.   The second is tilt in the image.   Wish I had the technical knowledge to advise, but just wanted to be sure you were aware of these issues.

Thanks! - For sure...Tilt is a big one. I think having the plate design similar to Rolf's it would mean i could slip in a few spacers to hopefully aid tilt

 

 

Logic is correct, the HRCC has 1.06 multiplication factor. The MPCC MKIII has a factor of 1.  There was a thread by Ed Jones on using a hyperbolic primary with the MPCC, nothing I know of on the HRCC.  In that thread he gave for a 10" f/3.55 a conic constant of -1.252.

 

A Surrier truss hardly seems necessary at f/3 or so for a 10" since normal truss poles would be quite short and stiff if of large enough diameter. But if you just want to build one...

 

You will want to think about image scale and pixel scale, and what range of object size you plan to image if you haven't already.

Thanks Tom. On the website it says the HRCC goes down to right about f/3 so i'm hoping it will correct properly in this case?

Good point regarding the truss as well. I'm not sure otherwise how I would go about fitting a dovetail to the scope otherwise however as the COG would probably be right about the middle, is bolting the dovetail right to the truss poles a possibility?

 

Image scale...Hmm. Pretty up close and personal for "medium to small" nebulae (your horsehead, lagoon, etc) and a tad on the wider side for galaxies. I think it's an interesting range of targets if not a bit limiting. Pixel scale would be right on 1" with my asi1600 waytogo.gif

 

Cheers guys!

Logan



#6 a__l

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Posted 18 September 2020 - 07:33 PM

The astrograph does not forgive mistakes.
What caught my eye. MoonLite focuser. Pay attention to how the bearings work. Steel or aluminum.
AstroSystems spider-holder. I do not recommend it. Usually, for astrographs, blades are made thicker so that they can hold the weight.

 

https://astronomy.ru...c,166577.0.html

I think it makes sense to study this. Google to the rescue. The optics (and this thread) are made by the renowned optician from LOMO.


Edited by a__l, 18 September 2020 - 07:42 PM.

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#7 coinboy1

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Posted 19 September 2020 - 09:35 AM

Hello Logan,

 

If your just wanting to fully illuminate/correct an ASI1600 camera then a 2" coma corrector will suffice. In which the Televue Paracorr II has better performance although it gives a 1.15x factor. 

 

There are many different coma correctors you can get off the market that will work for an F/3 primary, some of them include the:

 

1) TS-Optics NEWTON Coma Corrector 0.95x Riccardi Design - M68 Connection

2) TS-Optics NEWTON Coma Corrector 1.0x Riccardi Design - 3" Connection

3) TS-Optics NEWTON Coma Corrector 1.15x for big sensors - 3" Connection

4) Televue BIG Paracorr Type 2 - 3" connection 1.15x

 

Look at the spot diagrams and what kind of multiplication factor you want. Serrurier truss astrograph is nice but so can a solid aluminum tube. The truss design is great but more complicated to construct, tubes are simple and easy! You can even make a CF tube or fiberglass tube for fairly low cost. 

 

Size your secondary mirror based on how large of a fully illuminated image circle you want. I personally would want an astrograph to fully illuminate/correct an image circle of 44mm (to cover full frame sensors). 

 

Make everything as stiff as possible, when all is said and done your astrograph will probably weight between 20-30lbs so you will need a mount that can handle that much and track accurately. 

 

If you choose the 3" coma correctors you will need to find a 3" focuser, which there are many available from Teleskop Service . They ship internationally I have never had issues buying from them. They are FAST! 


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#8 Augustus

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Posted 19 September 2020 - 09:40 AM

No, I mean the HRCC  and as tommm said it does indeed have a factor of 1.06.

I said spherical correction, not focal length or ratio.


Edited by Augustus, 19 September 2020 - 09:41 AM.


#9 coinboy1

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Posted 19 September 2020 - 09:53 AM

Logan with your mirror making skills you can hyperbolize the primary as given by tommm above with the link  with the Baader MPCC III which is a very affordable coma corrector. Ed jones reverse engineered the MPCCIII and ray traced the design. I have the MPCCIII saved in OSLO if you need it. 

 

It will be a custom system but it can be done. Scott Tucker of Starizona even did it and proved it works! 


Edited by coinboy1, 19 September 2020 - 09:55 AM.

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#10 tommm

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Posted 19 September 2020 - 12:18 PM

Whoa logic, you're already well-invested in this I see with an ASI1600! I guess you are aware of the importance of a stiff, rigid mount, and as a__I said a rigid OTA, and good tracking.  And of course the folks in the AP forums here are the ones with expertise on this.

 

 

...Size your secondary mirror based on how large of a fully illuminated image circle you want. I personally would want an astrograph to fully illuminate/correct an image circle of 44mm (to cover full frame sensors). 

 


 

The ASI1600 sensor has a 22.2mm diagonal size, so he will want to use that or maybe the about 13mm smaller dimension of the rectangular sensor to figure his plate scale = theta*f/57.3 where theta is the angular size of the object and f is the scope f.l.  Image scale is pixel size/plate scale and gives the arcsec/pixel. From what I've read (I don't do AP) that is good for the moon and DSOs. More like 1/4 is used for planets.

 

I agree with coinboy1's comments on the Surrier Truss. It is usually better to keep things as simple as possible and meet the requirements.

 

You may want to look at acer's thread logic.


Edited by tommm, 19 September 2020 - 12:20 PM.


#11 Garyth64

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Posted 19 September 2020 - 08:36 PM

Lognic04, for what you said in your first post, and if you had a 1" diameter image plane, a 3.5" secondary may be ok.  Maybe . . . Texereau's chart only shows image plane diameter's down to f/5. 

 

But if you want a 44mm (1.7") diameter image plane, with a secondary to image plane distance of 8", a 3.84" secondary should be used.  This is calculated from Texereau's secondary size formula.


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#12 Lognic04

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Posted 19 September 2020 - 08:37 PM

Hello Logan,

 

If your just wanting to fully illuminate/correct an ASI1600 camera then a 2" coma corrector will suffice. In which the Televue Paracorr II has better performance although it gives a 1.15x factor. 

 

There are many different coma correctors you can get off the market that will work for an F/3 primary, some of them include the:

 

1) TS-Optics NEWTON Coma Corrector 0.95x Riccardi Design - M68 Connection

2) TS-Optics NEWTON Coma Corrector 1.0x Riccardi Design - 3" Connection

3) TS-Optics NEWTON Coma Corrector 1.15x for big sensors - 3" Connection

4) Televue BIG Paracorr Type 2 - 3" connection 1.15x

 

Look at the spot diagrams and what kind of multiplication factor you want. Serrurier truss astrograph is nice but so can a solid aluminum tube. The truss design is great but more complicated to construct, tubes are simple and easy! You can even make a CF tube or fiberglass tube for fairly low cost. 

 

Size your secondary mirror based on how large of a fully illuminated image circle you want. I personally would want an astrograph to fully illuminate/correct an image circle of 44mm (to cover full frame sensors). 

 

Make everything as stiff as possible, when all is said and done your astrograph will probably weight between 20-30lbs so you will need a mount that can handle that much and track accurately. 

 

If you choose the 3" coma correctors you will need to find a 3" focuser, which there are many available from Teleskop Service . They ship internationally I have never had issues buying from them. They are FAST! 

Hi,

Thanks for that! I was hoping to give the HRCC a try as I already had it on hand and thought it would save me quite a bit, considering the website says it goes down to f/3 as well.

 

At the moment I don't quite plan on having the cash for a FF mono camera lol.gif  but yes something to consider for sure! Mel Bartel's site says that a 3.5 in secondary would juuust about adequately illuminate a 50mm field which seems to be good enough? But I would certainly appreciate if someone could check those numbers!!

 

Hm. The issue of tube vs truss. Now while on the one hand a tube may seem like the obvious choice we must consider wind as a factor.  A big tube is going to be pushed around on even my NEQ6, therefore ruining any photos. On the other hand my thinking is a truss like structure would allow wind to flow over and within itself hopefully causing it to be less of a sail? But again. Pure speculation and if someone had numbers they would be greatly appreciated!! waytogo.gif 

 

The hope is the NEQ6 will be able to handle about 25lbs (11kg) for imaging which is half the advertised payload, which is pushing it but at 800mm, I'm really hoping it's adequate  smile.gif
 

 

Logan with your mirror making skills you can hyperbolize the primary as given by tommm above with the link  with the Baader MPCC III which is a very affordable coma corrector. Ed jones reverse engineered the MPCCIII and ray traced the design. I have the MPCCIII saved in OSLO if you need it. 

 

It will be a custom system but it can be done. Scott Tucker of Starizona even did it and proved it works! 

Thank you for that link! I wonder if a similar thing could be pulled off with the HRCC? I pulled it apart to clean it and I counted 3 elements. That seems typical for more "higher end" correctors? I will definitely keep that in mind however.

 

Whoa logic, you're already well-invested in this I see with an ASI1600! I guess you are aware of the importance of a stiff, rigid mount, and as a__I said a rigid OTA, and good tracking.  And of course the folks in the AP forums here are the ones with expertise on this.

 

The ASI1600 sensor has a 22.2mm diagonal size, so he will want to use that or maybe the about 13mm smaller dimension of the rectangular sensor to figure his plate scale = theta*f/57.3 where theta is the angular size of the object and f is the scope f.l.  Image scale is pixel size/plate scale and gives the arcsec/pixel. From what I've read (I don't do AP) that is good for the moon and DSOs. More like 1/4 is used for planets.

 

I agree with coinboy1's comments on the Surrier Truss. It is usually better to keep things as simple as possible and meet the requirements.

 

You may want to look at acer's thread logic.

I would call telescope making my side hobby laugh.gif  https://www.astrobin...users/Lognic04/ <some of my images but I'm ever grateful to have found ATM! I'm hoping to finally have the two converge.

 

 Thanks for the link, I'll have a look!


Incredible thanks to all who took the time to reply. Your help will be invaluable as this project hopefully begins at some time soon!! bow.gifbow.gif

 



#13 msheald

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Posted 20 September 2020 - 02:20 PM

Hello! I have thought of this design.

 

    If you put the camera and focuser in the tube itself, the mirror has to be VERY large in order to handle the filter wheel that would handle a full frame sensor. A 20" to 22" primary mirror can just get the central obstruction to 40% with most filter wheels that can handle 2" filters needed for full frame coverage.

 

    Even coming off the secondary mirror to a traditional Newtonian focuser, the secondary mirror has to be about 40% obstruction to make it worthwhile - see Takahashi Epsilon HD for an example. Mel Bartel's site suggests such large obstruction for reasonable illumination of the focal plane. The larger the primary, of course, the small the secondary - but even then, getting to 33% central obstruction and good illumination requires a primary of at least 18"

 

    The problem becomes engineering an appropriate secondary holder that can handle the weight of camera and focuser should you choose to go the route of having the camera and focuser at the traditional site of the Newtonian secondary mirror. It has be be really beefy, but designed so that vibrations do not set up a resonance that would kill any astrophotos. The big Ritchey-Chretians can give an idea of what those manufacturers had to do with their secondary holders in order to make sure they are stable. This had me very concerned since it would be a one off design for me, and I usually mess things up the first time I do them!

 

    If you plan to use a full frame DSLR or a one-shot color cooled camera at the position of the secondary, why not consider the Celestron Rowe-Ackerman or Celestron EdgeHD? Both cover a full frame. The former would have to be used with a DSLR or one shot color camera (like QHY600) at f2, the latter at f10 (of f7 with a full frame focal reducer). The engineering mechanical details have been worked out and they are plug and play. The 11" Rower Ackerman is fairly reasonable (the 14" is much higher priced!). The 14" EdgeHD is fairly reasonably priced as well. Of course, as has been stated in the past, the scope is just the first step - the mount becomes the limiting factor.

 

    If you wanted to stay wit a smaller frame camera, say with a diagonal of 20-22mm, perhaps a Night Owl 0.4 focal reducer on a traditional SCT giving f4? or Optec Lepus focal reducer on Meade f8 ACF giving f5? Those combinations would be match fairly well according to their literature.

 

    Additionally, you could try a slumped mirror design - significant weight savings. For a 10" primary mirror, you could use an aluminum tube or phenolic tube whether you do a slumped mirror or traditional thickness mirror. And, with a traditional secondary mirror, it would get you up and running fairly quickly. If you then decided to try the camera and focuser inside the tube, the focuser and coma corrector would not need to be changed, just repositioned once you assure yourself of a good secondary holder and tube for them.

 

    In summary, it comes down to the type of astrophotos you want to make. OSC or filter wheel? Full frame or smaller? Astrophotos for beauty or science (? photometry) or both. Finally, how much can you experiment with a design - does it have to work the first time (cost constraints?) or can you experiment, sometimes over several iterations and over possibly an extended length of time to get the design right? Best regards.

 

Mike


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#14 don clement

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Posted 20 September 2020 - 04:09 PM

This may be of interest: http://hbastro.com/F...iderations.html

 

Don


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#15 rbacci

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Posted 20 September 2020 - 05:46 PM

I have a 400 mm f 3.2 but i use it for astrometry  with a ccd qhy22 ,  for astroimaging is really hard  . Now i have purchased a qhy294C but  the first test are not good i have a superfast telescope but  i have a lot of aberration on images coma  and whit baader mccIII i have more aberration than without .

if i can post here images i show you

Best regards

Roberto


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#16 Oberon

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Posted 21 September 2020 - 03:49 AM

Good point regarding the truss as well. I'm not sure otherwise how I would go about fitting a dovetail to the scope otherwise however as the COG would probably be right about the middle, is bolting the dovetail right to the truss poles a possibility?

No. Very bad idea. ohlord.gif ohmy.gif ranting.gif shameonyou.gif


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#17 MitchAlsup

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Posted 21 September 2020 - 08:16 PM

Good point regarding the truss as well. I'm not sure otherwise how I would go about fitting a dovetail to the scope otherwise however as the COG would probably be right about the middle, is bolting the dovetail right to the truss poles a possibility?

Very bad idea.

 

If you want to attach an axis in the middle of the truss poles you need to build an upper truss and a lower truss and bolt the dovetail to the intermediate structure.

 

Truss poles are carrying fairly heavy loads from top to bottom and if you add another set of forces to the poles in the middle you will end up with collimation and vibration problems (to say the least.)


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