Flattener questions

[skogpingvin]

Hi folks, Bill here

I've got a flattener that is giving me (slight) 'Star Trek warp speed" shots (see https://stargazerslounge.com/topic/199452-flattener-reducer/ and my test shot at www.flickr.com/photos/skogpingvin/35322362416).

Question - from the images in the stargazerslounge thread, I'm too close to the flattener. But can anyone tell me if it's possible to tell HOW MUCH too close?

[Atmos]

From something that Sauvi posted a little while back (trying to remember the thread at the moment) it looks like it may be 5mm too far back. Just a rough uneducated guess :P

[skogpingvin]

Wait - what? Too far "back"? Do you mean the DSLR sensor is too far away from the flattener? That's not what the Teleskop-Service images in the stargazerslounge thread would suggest?

I'm confused. I thought I have to add distance between the flattener and the DSLR.

[Atmos]

You're right, it is kinda difficult to see on my phone (Flickr isn't iPhone friendly) but I'd still guess at 5mm.

[Slawomir]

Hi Bill,

How far is your sensor away from the flattener at the moment? It does look like it is a few mm off the optimal spacing.

As for the images showing distortions when the sensor is too close and too far, curiously I found last night that when I moved my sensor further away from the 0.75 Riccardi reducer, stars in the corners got worse and looked more like in your image - a 'Star Trek warp speed' type, and when I moved the sensor closer to the reducer, star shapes improved. So I'm not quite sure whether the images showing different distortions when too close/too far apply to all reducers/flatteners :shrug:

It would be best to add a small spacer and see if star shapes get better or worse, so then you will know which way to go.

[skogpingvin]

Yeah, I'm not entirely convinced that the two images are diagnostic in every case - it might depend on other things like whether the design focal length is longer or shorter than your focal length or something. Really the only thing I can do is lengthen it or shorten it and see how that affects the photos. I don't think it's going to be a quick process. :(

I've got an M42-M48 stepdown ring in the train, and this adds about 5.6mm to the train length. I've ordered a 10mm one (they're not expensive) so I'll take more test shots with the longer train. With luck, it'll give the other effect, and then I can simply shorten the stepdown ring using a bit of wet-or-dry sandpaper and a vernier caliper. If, on the other hand, I find that it makes the "warp drive" effect worse, I'll need to shorten the train, which is more problematic. I've got a mate with a lathe who can probably help with that!

I can't actually tell you how far away I am at the moment - I'm at work and all the photos and scribbles are at home. From memory it's about 61mm, which is significantly (like 5mm or so) longer than the published back focus for the flattener. I'd put the difference between the images saying too close and the published numbers saying "too far" as being because the design focal length was different to my focal length.

So thanks for the help - your suggestion that it might not be totally diagnostic actually gives me some comfort.

Bill

[LewisM]

Do what I did with my custom flattener spacer - buy a non-rotating helical focuser as the "end piece" (assuming your flattener has sufficient distance). I simply added a small T spacer to mine, then the T threaded locking helical focuser (Borg and TS sell these) and that way I could very precisely try differing distances until it was flat. Only issue is the terminator is 1.25" (fine for smaller sensors and 1.25" filters).

Then again I have never bought an off-the-shelf spacer - always had them custom machined to my design (in Brisbane Suavi, by an astro-nut like us :) ). I didn't use the helical in the SW Esprit 80ED, because it was right in the first place

[lazjen]

Where did you get this done? I need to get some stuff done too and if there's someone in Brisbane that can handle this, that would be great. I was just going to have a stab with a google search, but this would give me a greater chance of success.

[LewisM]

You need to design it yourself and send him the drawings, measurements, thread sizes and preferrably the mating threads too. Send a message to Ubique here on IIS or messagte me for his email address.

They will be raw - not anodised - so be orepared to paint

[lazjen]

Ok, thanks. Painting doesn't scare me but I'm not good with drawings, etc. I was going to take my parts to someone show them what connects where and what I want that's different. :D

[LewisM]

OK, he lives on an island, so meeting up will need to be pre-arranged.

Give him a try - I've been using his skill for around 5 years now.

[skogpingvin]

OK here's a photo (from underneath the DSLR) and some (rough) measurements. I do use a vernier caliper but it's hard getting it perfectly square in some of those gaps. At the moment it looks like the flattener flange to the sensor is 61.9mm (assuming the Pentax K mount has a mount flange to sensor distance of 45.5mm). The design distance from the flattener flange to the sensor is a tiny 56.04mm for the 815mm (I think) focal length, but the flattener is said to work for my 700mm focal length refractor.

So it won't surprise me either way if I find that I have to get closer or further away! I'm just going to have to test and see.

Now we wait for bits to be delivered...

[skogpingvin]

He he. That's how I deal with my lathe mate. I gave him lunch on Sunday and he did some drawings!

Bill