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A Cassegrain reflecting telescope contains a negative optical element between the main mirror and its focus. This increases it's focal length. enter image description here

Is there a reflecting telescope like as a Newtonian telescope but with the negative lens before the diagonal mirror? enter image description here

This question is not about a Barlow lens.

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    $\begingroup$ Optically it makes no difference whether you place such component in the place you indicate or outside the tube after the secondary mirror. It makes a difference for the usable aperture as it would require a larger secondary mirror or obstruction that the primary light does not pass the negative lens, thus it would reduce the light gathering capability of the telescope. Thus: likely not, as it only has disadvantages and not advantage (as far as I see) $\endgroup$ Aug 20, 2023 at 10:04
  • $\begingroup$ @planetmaker "likely not, as it only has disadvantages and not advantage" I don't quite follow your reasoning can you elaborate? $\endgroup$
    – uhoh
    Aug 21, 2023 at 23:39
  • $\begingroup$ I seem to remember a commercial telescope in the 1980s or 1990s that had this optical design. A cheap brand, too, like Tasco or something similar. The mirror was spherical instead of parabolic, and the lens was there to correct the spherical aberration. $\endgroup$ Aug 22, 2023 at 0:34

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Yes! Those with a Coudé focus and others

The following

show and link to a wide variety of reflecting telescope optical designs that include negative focal length mirrors and off-axis deflection. (Check them out, but two examples are shown below).

As for a proper, classical Newtonian with its truly 45° diagonal, it will certainly have been tried by some amateur, somewhere, sometime, if for no other reason than they didn't have access to a primary mirror (blank, or figured) with a hole in the center.

Another reason would be viewing convenience - that side-viewing option is pretty handy when your DIY portable telescope is big and heavy and for mechanical reasons a tall mount high enough to make bottom viewing with an external diagonal (inverting or fancy non-inverting) a pain.

Just for example, LOptics.com's Building an open-tubed 12.5" F/12.5 Classical Cassegrain By Michael E. Lockwood

LOptics.com's "Building an open-tubed 12.5" F/12.5 Classical Cassegrain" By Michael E. Lockwood http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html

LOptics.com's "Building an open-tubed 12.5" F/12.5 Classical Cassegrain" By Michael E. Lockwood http://www.loptics.com/ATM/telescopes/12in_cass/cass_article/cass.html



from https://astronomy.stackexchange.com/a/49247/7982

From Design and development of a freeform active mirror for an astronomy application (also here):

"Design and development of a freeform active mirror for an astronomy application" http://dx.doi.org/10.1117/1.OE.53.3.031311


From https://astronomy.stackexchange.com/a/43363/7982

enter image description here


from https://astronomy.stackexchange.com/a/43977/7982

optical diagram of ESOs Extremely Large Telescope from the ESO ELT optical diagram page, somewhat better diagram in Figure 4 of Hippler 2018.

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  • $\begingroup$ It looks like. But this is closer to the Cassegrain reflecting telescope with an additional diagonal mirror. The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror. $\endgroup$
    – Imyaf
    Aug 23, 2023 at 6:20
  • $\begingroup$ @Imyaf well "like a Newtonian" is not a very well-defined term. You haven't laid out your criteria in the question so answers have to judge for themselves what "like a Newtonian" does and does not mean. It would have been better if your comment "The path length of the rays after the secondary mirror is approximately equal to the focal length of the primary mirror" was in the original question if that's what you were thinking. $\endgroup$
    – uhoh
    Aug 23, 2023 at 9:37

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