Homemade Hyperbolic Astrograph - 1, Optical Layout

[Chanlunlun]

I have no experience in making or designing Newtonian astrograph. I just want to learn more from you on the design.

I am no better than you. I am still learning. this is a complete new area for me. Worse than U, I am gambling.

Your focal plain has a radial distance of 110+55=165 from the principal axis. The mirror has a radius of 102/2 = 55. So is there a reason why the MPCC is not moved much closer to the principal axis? Moving the MPCC closer to the principal axis also allows the use of a small secondary mirror and reduce central obstruction.

The lowest profile focuser I found has a minimum height of 55 mm, this is a design constraint. Except than I make my own focuser ( which is not quite possible considering my poor craftmenship ) 165 mm is the minimum I can go for.

There are many cheap 4 to 5 inch Newtonians around, why don't you use their cell/body and just focus your effort on the optical part?

A Newtonian you mention is not suitabe :-

1. they have a focuser not up to astrograph requirement - not low enough profile, not zero backlash and incapable of housing a 2 inch MPCC.

2. their tube is not strong enough to hold the tight alignment precisely under long exposure. Don't forget my Epsilon has a speed of f/3.5

3. Their tube is too long, too fat to be carried around.

4. Circular shape OTA is very difficult t9o be packed inside a suitcase.

5. I like things to be flat packed. So the OTA will be rectangular in shape, ready to be dismantled and flat packed inside my suitcase- mimimum space for maximum capacity. Rectangular OTA still not seen on the market.

Thanks for your questions and suggestions. As I always maintain, your voice is always my best teacher. Looking forward to hearing again from you soon.

Best regards
Chan Yuk Lun

[梅西爾]

鏡神，

小弟無知，請問為什麼你會加插一些透鏡，而不用甩凹面鏡？

謝指教 [跪拜禮]

梅西爾

[A/45]

This is a very interesting topic. I once contemplated getting myself a small 4 to 5 inch newt and convert it to an astrograph. I searched the web and found these examples, all producing reasonable images on DSLRs with a MPCC. The challenge I think is the optimal size of the secondary to achieve the minimum back focus required yet limiting the amount of vignetting; otherwise the secondary mirror will become ridiculously large given the fixed distance between the CCD and the MPCC with the height of the focuser.

Judging from the photos you can size up the distance they moved the focuser towards the primary mirror, with reference to the standard Vixen dovetail.

[梅西爾]

Thanks for yuor shairing

[Chanlunlun]

,,

[Eddy So]

期待鏡神前輩的出品！加油！

[Chanlunlun]

Dear Angus,

There already existed some excellent commercially made correctors/reducers for parabolic systems (such as Philip Keller), why you choose to build a hyperbolic mirror instead? Sure the MPCC is cheaper, but I am not sure about its quality for serious imaging. I've tried it on my SN-10 (f/4) and very disappointed at the results. Perhaps as you mentioned, it only works well for slower systems.

Basically there are two types of correctors for parabolic mirrors on the market - the Ross type and the Wyyne type.

MPCC and the one used in Takahashi are basically Ross type. The advantage is that they are relatively cheap to make and allow bigger field with smaller lens diameter - being suitable for small Newtonians. However the Ross type correctors all suffer spherical abberation ( under-correction ) which degrades the image quality when used with parabolic mirrors. Depending on f/ratio, the severity ranges from insignificant (f/5 or slower ) to objectionable (f/4 or faster ). Normally when applyinmg in f/4 or faster system the Ross type corrector will give rather dissapointing results, except that we change the primary into hyperbolic form to compensate the S.A.

The corrector sold by Philip Keller is a Wyyne type corrector. This is a perfect corrector for parabolic mirrors -no S.A., corrects for coma and gives very little color error. However being a wide space 3 element design such corrector gives very bulky apperance and carries a very high price tag. (A 3 inch diameter Wyyne type corrector cost about HKD 7000 at the moment ). Another disadvantage of such corrector is that they easily produce ghost image on the CCD, especially in prolong exposure of areas with bright stars. In addition such system cannot be used in ordinary Newtonians because the giant corrector lens ( Wyyne corrector, in its smallest size available, is about the size of a 80 mm refractor ) requies a huge focuser and very big diagonal mirror. However Wyyne type corrector is commonly used in observatory ( such as teh CHFT ) with very big ( up to 200 inches aperture ) parabolic primary mirrors because they don't have this focuser constraints.

So the only option left to me is the Ross Corrector + Hyperbolic primary mirror design.

Best regards
Chan Yuk Lun

[PTS]

Dear Star Shooters,

Hyperbolic astrograph has been around for some decades but it’s still not very popular. There exists commercial available ones ( the most famous one may be the Takahashi Epsilon ) but the price is unusually high. No wonder the difficulty in manufacturing of the deep hyperboloid primary mirror contributes greatly on the price tag of such system.

Things become interesting recently. The emergence of the MPCC in a very reasonable price has triggered vast discussion about coma-free Newtonian in various forum. However a deeper investigation leads to the tragic conclusion that to eliminate coma, using a MPCC, the price to pay for is to allow under-corrected spherical aberration coming into the system. Most importantly this is unavoidable if we insist on using parabolic primary mirror.

Fortunately this is not entirely the case. If the speed is limited to slower than f/5 the MPCC is quite a piece of marvel. At faster speed spherical aberration becomes so obvious that star images start to get very fat and resolution decreases appreciably. At faster speed, we need to change to shape of the primary into hyperbolic, in order to correct for the S.A. of the MPCC. Thus this leads again to the hyperbolic astrograph design, which is exactly the same as the “Epsilon”.

The experience in Komeko tells that in star shooting speed is a very key factor upon success. The speed of f/2.8 is very desirable to capture both “deep” enough and to finish fast enough before clouds come. So I wonder whether such a system could be built, at home, in an amateur sense.

Attached please find the optical layout of the proposed system. The first thing that may catch attention is the unusually big secondary diagonal mirror. The system is built specially for QHY8 or APS-C DSLR. To allow for such a big field a 50% diagonal is a must. ( Actually it is already a bit too small but design constraint cannot allow a bigger diagonal ).

The primary mirror is a hyperbolic mirror which must be custom made to well match the behavior of the MPCC. How do I know how deep a hyperbola should be made ? I don’t . Being an amateur I plan to work in an amateur way – to figure the primary mirror upon a star until a null figure appears. In this way at least we eliminates spherical aberration. How about coma ? I don’t know, I just hope for the best.

Here is the specification of my homemade “Epsilon” :-

Aperture : 102 mm
Focal length : 350 mm f/3.5
Optical layout :- Hyperbolic Astrograph.
Image circle : 26 mm at about 70% vignetting.
Target camera type : QHY8 or any APS-C CCD or DSLR.

Frankly speaking there is no optical design involve in anyway. I just make a gamble. Being an ATMer why not build an astrograph for fun ?

Best regards
Chan Yuk Lun
17 June 2010.

Chan Yuk Lun,
Very glad to hear about your project.
To get to the final desired figure, you can simply make the tube assembly first, with the corrector, etc in place, and put the main mirror in place for tests. This way, you are in fact making "matching" for all the optical components.

I trust, with your skill, this should not be too much a problem.

Look forward to see your new masterpiece!

PTS

[Chanlunlun]

Very glad to hear about your project.
To get to the final desired figure, you can simply make the tube assembly first, with the corrector, etc in place, and put the main mirror in place for tests. This way, you are in fact making "matching" for all the optical components.

I trust, with your skill, this should not be too much a problem.

Look forward to see your new masterpiece!

PTS

Dear PTS,

Thanks for your suggestion. This is exactly what I've planned to do.

Best regards
Chan Yuk Lun

[anguslau]

Dear Angus,

There already existed some excellent commercially made correctors/reducers for parabolic systems (such as Philip Keller), why you choose to build a hyperbolic mirror instead? Sure the MPCC is cheaper, but I am not sure about its quality for serious imaging. I've tried it on my SN-10 (f/4) and very disappointed at the results. Perhaps as you mentioned, it only works well for slower systems.

Basically there are two types of correctors for parabolic mirrors on the market - the Ross type and the Wyyne type.

MPCC and the one used in Takahashi are basically Ross type. The advantage is that they are relatively cheap to make and allow bigger field with smaller lens diameter - being suitable for small Newtonians. However the Ross type correctors all suffer spherical abberation ( under-correction ) which degrades the image quality when used with parabolic mirrors. Depending on f/ratio, the severity ranges from insignificant (f/5 or slower ) to objectionable (f/4 or faster ). Normally when applyinmg in f/4 or faster system the Ross type corrector will give rather dissapointing results, except that we change the primary into hyperbolic form to compensate the S.A.

The corrector sold by Philip Keller is a Wyyne type corrector. This is a perfect corrector for parabolic mirrors -no S.A., corrects for coma and gives very little color error. However being a wide space 3 element design such corrector gives very bulky apperance and carries a very high price tag. (A 3 inch diameter Wyyne type corrector cost about HKD 7000 at the moment ). Another disadvantage of such corrector is that they easily produce ghost image on the CCD, especially in prolong exposure of areas with bright stars. In addition such system cannot be used in ordinary Newtonians because the giant corrector lens ( Wyyne corrector, in its smallest size available, is about the size of a 80 mm refractor ) requies a huge focuser and very big diagonal mirror. However Wyyne type corrector is commonly used in observatory ( such as teh CHFT ) with very big ( up to 200 inches aperture ) parabolic primary mirrors because they don't have this focuser constraints.

So the only option left to me is the Ross Corrector + Hyperbolic primary mirror design.

Best regards
Chan Yuk Lun

Thanks very much for your explanations! You are a real master on this subject and I've learned a lot from you!

[Chanlunlun]

鏡神，

小弟無知，請問為什麼你會加插一些透鏡，而不用甩凹面鏡？

謝指教 [跪拜禮]

梅西爾

Dear Messier,

Parabolic mirrors form coma. Lens is used to cancel it.

[梅西爾]

Dear Mr. Chanlunlun,

Thanks for give advice from you.

Messier

[鄧登凳]

Dear Star Shooters,

Hyperbolic astrograph has been around for some decades but it’s still not very popular. There exists commercial available ones ( the most famous one may be the Takahashi Epsilon ) but the price is unusually high. No wonder the difficulty in manufacturing of the deep hyperboloid primary mirror contributes greatly on the price tag of such system....

Attached please find the optical layout of the proposed system. The first thing that may catch attention is the unusually big secondary diagonal mirror. The system is built specially for QHY8 or APS-C DSLR. To allow for such a big field a 50% diagonal is a must. ( Actually it is already a bit too small but design constraint cannot allow a bigger diagonal ).

The primary mirror is a hyperbolic mirror which must be custom made to well match the behavior of the MPCC. How do I know how deep a hyperbola should be made ? I don’t . Being an amateur I plan to work in an amateur way – to figure the primary mirror upon a star until a null figure appears. In this way at least we eliminates spherical aberration. How about coma ? I don’t know, I just hope for the best.

Here is the specification of my homemade “Epsilon” :-

Aperture : 102 mm
Focal length : 350 mm f/3.5
Optical layout :- Hyperbolic Astrograph.
Image circle : 26 mm at about 70% vignetting.
Target camera type : QHY8 or any APS-C CCD or DSLR.

Frankly speaking there is no optical design involve in anyway. I just make a gamble. Being an ATMer why not build an astrograph for fun ?

Best regards
Chan Yuk Lun
17 June 2010.

Dear god of mirrors,

I started recently learning to do astro-imaging using Newtonians. I guess it is too late for you to modify the design but my comments may help me to check whether my understanding is correct.

1. MPCC from Baadar is optimized for f/4.5, it should "work" for f/4 to f/6. Making it work for f/3.5 is a bit risky.

2. The designed focal ratio is 3.5. However, with a 50% diagonal, 25% of the incoming light is blocked or 75% is remaining. Ordinary aluminium plating by Zhi Tung is around 88%~92% reflecting only, so with primary and secondary mirrors, the light remining is no more than 85%. The resulting light reaching the surface of the 4-elements MPCC (light reduction ignored as refractors have same/more elements) is 75% X 85% = 64%. Based on that the photographic effective f-stop is SQRT(3.5^2/0.64) = 4.4. So in reality, it is a f/4.4 instrument (FL=350mm).

3. Manual focusing f/4 ~ f/4.5 300mm camera lenses using ED elements are not difficult to find in the market. They are not heavy and can be transported easily. The price is comparable with a MPCC and a good focuser. But, of course, buying one will never give anyone the joy and satisfaction of making the scope himself/herself.

Regards,
Tang

[PTS]

Dear Star Shooters,

Hyperbolic astrograph has been around for some decades but it’s still not very popular. There exists commercial available ones ( the most famous one may be the Takahashi Epsilon ) but the price is unusually high. No wonder the difficulty in manufacturing of the deep hyperboloid primary mirror contributes greatly on the price tag of such system....

Attached please find the optical layout of the proposed system. The first thing that may catch attention is the unusually big secondary diagonal mirror. The system is built specially for QHY8 or APS-C DSLR. To allow for such a big field a 50% diagonal is a must. ( Actually it is already a bit too small but design constraint cannot allow a bigger diagonal ).

The primary mirror is a hyperbolic mirror which must be custom made to well match the behavior of the MPCC. How do I know how deep a hyperbola should be made ? I don’t . Being an amateur I plan to work in an amateur way – to figure the primary mirror upon a star until a null figure appears. In this way at least we eliminates spherical aberration. How about coma ? I don’t know, I just hope for the best.

Here is the specification of my homemade “Epsilon” :-

Aperture : 102 mm
Focal length : 350 mm f/3.5
Optical layout :- Hyperbolic Astrograph.
Image circle : 26 mm at about 70% vignetting.
Target camera type : QHY8 or any APS-C CCD or DSLR.

Frankly speaking there is no optical design involve in anyway. I just make a gamble. Being an ATMer why not build an astrograph for fun ?

Best regards
Chan Yuk Lun
17 June 2010.

Dear god of mirrors,

I started recently learning to do astro-imaging using Newtonians. I guess it is too late for you to modify the design but my comments may help me to check whether my understanding is correct.

1. MPCC from Baadar is optimized for f/4.5, it should "work" for f/4 to f/6. Making it work for f/3.5 is a bit risky.

2. The designed focal ratio is 3.5. However, with a 50% diagonal, 25% of the incoming light is blocked or 75% is remaining. Ordinary aluminium plating by Zhi Tung is around 88%~92% reflecting only, so with primary and secondary mirrors, the light remining is no more than 85%. The resulting light reaching the surface of the 4-elements MPCC (light reduction ignored as refractors have same/more elements) is 75% X 85% = 64%. Based on that the photographic effective f-stop is SQRT(3.5^2/0.64) = 4.4. So in reality, it is a f/4.4 instrument (FL=350mm).

3. Manual focusing f/4 ~ f/4.5 300mm camera lenses using ED elements are not difficult to find in the market. They are not heavy and can be transported easily. The price is comparable with a MPCC and a good focuser. But, of course, buying one will never give anyone the joy and satisfaction of making the scope himself/herself.

Regards,
Tang

Just completing the optics is a great challenge. It is great courage for Chan Lunlun that everyone of us should admire! Being one of the early day equipment makers, I never have the courage nor consistency to attempt these "high tech" optical project!

From what I can understand, these astrographs are no match by the ED camera lenses. If you count the number of optical surfaces in these lenses, and use your formula to calculate the "effective" f/ratio, you will be mostly disappointed!

Look forward to seeing Chan's astrograph first hand!


pts

[Subaru]

I believe Chan sir's 100mm Astrograph is just a starting experiment to figure out the conical index of the hyperboloid in minimizing the SA caused by the Ross corrector.

The real stuff is a 10" or even 12" high speed Astrograph after the field test.