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Cassegrain Mirror Sets and a Word about Cassegrains Design Tips
and Prices for Both
Classical and Dall-Kirkham |
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Cassegrain systems can embody a multitude of configurations designed to suit individual needs. Some people want a very large focal ratio for planetary and binary star work and some want more moderate ratios for general astronomy. It is not possible in a limited space to give a complete design and use course but a few tips and some general rules will help most people get what they want and need.
You might want to consider why you want a Cassegrain. If your interest is deep sky astronomy, a Newtonian is likely your best choice. For higher power work a Cassegrain may be an advantage. Some people want them because the tube is shorter, or just because they like to sit at the bottom end of the telescope. The reasons are many and varied. What I care about is that if you buy something from me you will like the results and feel the money was well-spent. That's how I build a customer base. While most amateurs are likely to opt for a Newtonian there are those for whom the Cassegrain is the object of their desires and an ideal instrument.
Well-made Cassegrain optics are more expensive for several reasons. Not only are Cassegrain systems significantly more time-consuming to make well than single parabolic mirrors, but anything less than a first-rate Cassegrain will perform badly; far worse than a less than first-rate Newtonian - hence their less than stellar reputation. If you want a Cassegrain that works superlatively, it must be made carefully and with attention to detail. They, especially, can not be rushed. Further issues arise due to the flexibility of the design. The permutations of size, focal ratio of primary, and effective focal ratios are endless. And everyone wants their own design. As a result, I can not have 20 of anything made for me in one order so I can make a few ahead, it's all one at a time and each part unique. I can make four or five Newtonian mirrors of any given standard size at a time but I can't do that with Cassegrains. And the test setup is more complex and sensitive and has to be erected and broken down for each set. Not conducive to quantity production.
I advise potential purchasers who are not familiar with the peculiarities of Cassegrain systems, and their advantages and disadvantages, to read my article entitled A Cassegrain Primer - Answers to questions about Ritchey-Chretien, Classical and Dall-Kirkham Systems in order to more fully understand the more complex aspects and applications of the Cassegrain. Here you will see what happens with regard to the size of secondary obstructions and off-axis aberrations and why Dall-Kirkhams work well in some embodiments and not in others. However, for a quick reference and general guide, I offer the following:
Classical
Cassegrain: Parabolic Primary and hyperbolic secondary
General Observing:
Effective focal ratio of 15. Primary focal ratio of 4.
Planetary: Effective focal ratio of 18 to 25. An EFR of 20
and primary focal ratio of 4.5 if possible is recommended.
Dall-Kirkham
Cassegrain: Elliptical Primary and spherical secondary
General Observing:
Effective focal ratio of 18. Primary focal ratio of 4. Coma
excessive at faster focal ratios, excellent at slower ratios.
Planetary: Effective focal ratio of 18 to 25 recommended and primary focal ratio of
4 or greater.
Dall-Kirkham systems can give stunning results when designed
properly.
Ritchey-Chretien:
Hyperbolic Primary and hyperbolic secondary
A specialized system for moderately wide field imaging. Fast
systems (f/8 or 9 EFR) require very fast primaries of less than f/3 and often
down to f/2.5. Very difficult to make and make work well.
Prices for Ritchey-Chretien systems are quoted on an individual basis depending
upon the specific configuration. For example, systems such as very fast
Ritchey-Chretiens requiring f/2.8 primaries are significantly more costly than
classical systems with f/4 or f/4.5 primaries.
Prices for Standard Dall-Kirkham two Mirror Sets
The prices given below are for two mirrors only, Pyrex, coated with protected aluminum. EFR = effective focal ratio. These systems are at the specifications for my tube assemblies and are a standard set for which I have ready-made tooling, mostly involving test plates for the secondary mirrors. Other ratios or configurations will be quoted separately. Obstruction size is total full size and allows for an unvignetted field.
|
Diameter and |
Price with Standard or Conical Mirror* |
Deposit Required |
| 8" Primary f/4 - EFR f/22.5 (180") Physical Obstruction 25% Backfocus 10.5" |
$1,800* | $800 |
| 10" Primary f/4 - EFR f/20 (200") Physical Obstruction 25% Backfocus 11" |
$2,500* | $900 |
| 12.5" Primary f/4 - EFR f/20 (250") Physical Obstruction 24% Backfocus 11.5" |
$3,200* | $1,000 |
| 14.5" Primary f/4 - EFR f/18 (261") Physical Obstruction 25% Backfocus 14" |
$4,300* | $1,000 |
| 16" Primary f/4 - EFR f/18 (288") Physical Obstruction 25% Backfocus 15" |
$5,300* | $1,000 |
Prices for Custom Classical Cassegrain two Mirror Sets
The prices given below are for two mirrors only, Pyrex, coated with protected aluminum. EFR = effective focal ratio. These systems are custom made at the customer's specifications within the guidelines shown.
|
Diameter and |
Price with Standard and Conical Mirror* | Deposit Required |
| 10" Primary f/4 to 5 EFR f/15 to f/30 |
$2,800* | $1,000 |
| 12.5" Primary f/4 to 5 EFR f/15 to f/30 |
$3,500* | $1,000 |
| 14.5" Primary f/4 to 5 EFR f/15 to f/30 |
$4,500* | $2,000 |
| 16" Primary f/4 to 5` EFR f/15 to f/30 |
$5,500* | $2,500 |
Dall-Kirkham systems are recommended with slower effective focal ratios of f/20 or higher inasmuch as this reduces the impact of coma and will give better performance as planetary telescopes. I know some makers are making faster D-Ks but I strongly advise against this. Please read the article referenced above for an explanation of the specific uses and recommended configurations of the Dall-Kirkham. The Dall-Kirkham is not an all around telescope. It's a specialty instrument.
* Prices subject to change if energy costs increase cost of Pyrex