Petzval from two doublets ...

[miketheobscure]

OK, just seeing if I understand this correctly (ie so this is a vague notion rather than a project yet) ... would mounting the objectives out of two achro doublets in line make a Petzval?

http://www.andrewscom.com.au/site-content-section-10-skywatcher.htm has some candidates: The 150 f6 skywatcher could perhaps be followed by the 120 f5, costing $1348 together (if those prices haven't changed), with the aim to get a fast, relatively cheap 6" that was shorter and (perhaps) had a flatter field & less false colour.

Is it a possibility?

Thanks, Mike

[anj026]

As far as I know an f5 petzval consists of an f12 achromat doublet objective combined with a two element reducer/corrector. It is difficult to collimate and will still have chromatic abberation unless expensive exotic glasses are used. These comments are based only on things I have read not actual experience.

I would suggest spending $1800ish for a Vixen NA140 neo achromat at f5.7.

[Karls48]

You can buy 150mm f5 Petzval OTA for $1100. I assume it is Petzval as it uses two-doublet lens. I’m considering purchasing 6”achromat and I’m looking what is available and for how much.

[miketheobscure]

I made the mistake of looking through a friend's Megrez 90 again after I wrote that post, and I'm feeling hard pressed to not spend my money on one of those now ...

Where's there a 150mm f5 Petzval for $1100 though?

And yeah, I think I was wrong about the second lens group being just another positive doublet, I think the whole point is that it's negative. Oh well, it was an idea. :P

- Mike

[Karls48]

Have look on http://www.myastroshop.com.au/products/details.asp?id=MAS-087

[miketheobscure]

Interesting, haven't been able to find any reports/reviews of them, have you? - Mike

[Karls48]

No, I did not found any review so far. I believe that this scope comes from same factory in China as 127mm APO that some members of this forum imported. Bit more expensive then 6” Skywatcher achromat, thought. I think that one member of this forum had this scope, but he did not post any comments about it.

[toyos]

I bought that 6" Petzval directly from China for FAR less than $1100, just for testing. In brief, I won't recommend this particular Petzval to anyone at all. Better get the 6" Skywatcher refractor if you're only after a cheap 6" achro.

[miketheobscure]

Definitive review - thanks for the heads up - Mike