Field Flattener

[Spad]

Yay here again pestering 😀

This time it is field flatteners!

I was look at getting a big refractor, 125 or 150 .... but then my finances died in hysterical fits of laughter when I saw the prices. This is when my Yorkshireman gene kicked in. This sent me scuttling off on a tangent. A cheaper tangent.... a much much cheaper tangent!

So I looked, and read... as it turned out, not enough reading! About using a non reducing field flattener.... so I took the plunge and bought this https://www.firstlightoptics.com/reducersflatteners/founder-universal-field-flattener.html

All excited I got it, just in time for the clear night on Thursday. So I slapped it on me scope. With the intention of trying my first mosaic, the target was Andromeda..... the mosaic turned in to an utter disaster, but that is by the by....

What I did notice was the stars on the periphery of the images were not round. I had the correct back focus spacing (55mm). So I went on their website and saw the atrached image. It had 16.2mm of Focus Shift.... does this now mean I have to mess about with back focus to get it correct for the field flattener, or is it just a naff one?

Scope used was an SVBony SV503 70ED. If I used this on my 102ED would I have do the same with the back focus on this as well?

Camera was an ASI183MC Pro.

 

Edited September 15 by Spad

[Elp]

Not really sure what the focus shift is referring to, you don't get that on other drawings. The BF of 55 is usually a guide and you have to fine tune it backwards or sometimes but rarely forwards, you'll get variation due to manufacturing tolerances and I very much doubt they 100pc check such products, it can also vary due to the scope you're using.

If the stars are elongated at 55mm, you'll have to add more spacing, try increments of 1mm, it's common to add up to 3mm+ at times, if you use a filter you also have to add typically 1/3 of filter thickness too.

I usually crop, but for mosaics it can be a pain to blend panels due to edge star elongation, with blur exterminator I'm sure it can fix it in post.

[Elp]

The flattener is also very common, it comes in at least half a dozen different brands, I've got a reducer version branded Stellarmira.

[Spad]

27 minutes ago, Elp said:

The flattener is also very common, it comes in at least half a dozen different brands, I've got a reducer version branded Stellarmira.

The Stellamira 0 6x FF/Red is my next acquisiton.

Elp.... i need to put you forawrd for an award for helping useless tight fisted yorkshiremem!

I take it, it is a case of - add shim, shoot, check, then repeat? 😁

Edited September 15 by Spad

[Elp]

5 minutes ago, Spad said:

take it, it is a case of - add shim, shoot, check, then repeat?

Yes. If you want to make it easier there's the excellently made Askar adjustable BF adaptor, but it's not exactly pocket money priced.

Having sets of delrin rings at hand helps, I've got the Baader metal 3 set which I use more often as they're easier to handle. Think there's other metal ones now too.

Edited September 15 by Elp

[Spad]

Just now, Elp said:

Yes. If you want to make it easier there's the excellently made Askar adjustable BF adaptor, but it's not exactly pocket money priced.

They never are in this hobby!

[Elp]

You may struggle with a 0.6x reducer, there's a reason most stop at around 0.75-0.8x. I still haven't tuned my 0.65x, same sensor as yours, but I'm normally using a different setup.

[Spad]

Just now, Elp said:

You may struggle with a 0.6x reducer, there's a reason most stop at around 0.75-0.8x. I still haven't tuned my 0.65x, same sensor as yours, but I'm normally using a different setup.

Ah.... ok.... I'll get the adjuster instead. £129 is doable.

[RafalT73]

2 hours ago, Spad said:

 

Scope used was an SVBony SV503 70ED. If I used this on my 102ED would I have do the same with the back focus on this as well?

Camera was an ASI183MC Pro.

 

Back focus  of the field flattener has nothing to do with diameter or focal length of the telescope. Specified back focus of your flattener refers to the distance between M42 connection and sensor of the ASI183MC PRO. Please pay attention to back focus of the camera itself 6.5mm. It means that you need adapters for 48.5mm (55 - 6.5 = 48.5mm) between FF and ASI183MC. Anyway, some extra precise adjustment will be necessary based on real results.

 

Edited September 15 by RafalT73

[Spad]

Ok and will do 😊

[Budgie1]

This is a handy guide for Back Spacing: https://optcorp.com/blogs/deep-sky-imaging/how-to-set-the-correct-back-focus 

[iantaylor2uk]

I can recommend the HoTech SCA flattener which should work on any refractor from f/5 to f/8.

[Louis D]

I'm guessing that the 16.2mm of focus shift is the additional inward focus distance relative to the distance without the flattener needed to achieve proper focus.

[Spad]

11 hours ago, Louis D said:

I'm guessing that the 16.2mm of focus shift is the additional inward focus distance relative to the distance without the flattener needed to achieve proper focus.

Ok..... that made zero sense to me!! 🤣

Is there a ham fisted BLONDE oaf version of it? 😄

Edited September 16 by Spad

[Louis D]

@Spad The best analogy I can think of is the use of a Barlow.  Have you ever used a Barlow with either an eyepiece or a camera?  Once you've focused with the eyepiece or camera alone and then inserted the Barlow, did the focus position change or was the image still in focus?  Usually, the focus position changes in my experience.  Often with a Barlow, you have to rack the focuser outward from the objective.  With a focal reducer or flattener, you often have to rack the focuser inward toward the objective.  That's what I'm thinking the 16.2mm distance is about with your flattener.

With my TSFLAT2 flattener, I have to rack my focuser inward to reach focus as compared to when it is not present.  I've never measured the actual amount, but it is on the order of 10mm to 15mm.

With my GSO coma corrector, I have to rack my focuser inward 11mm to reach focus as compared to when it is not present.

Does that help to clear things up for you?

[Spad]

Yes it does, and thank you! 😊