Astromod DSLR - still ok with lens?

[bobro]

As generally advised, I am planning to remove the IR filter (tomorrow!) of my Canon 1000D to enhance the red end of the spectrum in images I capture with my telescope. Following this I understand that autofocus and colour balance will be different, but this is not important to me as the camera is used only for astrophotography. Nevertheless, I have recently purchased a 135mm lens to try out imaging without a telescope  and wonder if the focus will change after the mod, affecting the ability of infinity focus with a lens. Please advise if you can help.

[iapa]

If you just remove the filter I believe that the autofocus is affected.

I can't comment on what would happen if you do the modification yourself, but those who do this as a service often replace the filter with either shims or replacement glass which does not filter out the IR to maintain focus capability.

If the filter is replaced, then, focus is generally OK, and all you need to consider is white ballet -without the filter there is obviously more red in your images.

[Merlin66]

If you remove and do do replace the internal filter(s) the standard camera lenses will no longer focus at infinity.....'

I added an Astronomik CLS - clear clip in filter to correct this.

 

[ronin]

I think it should still achieve focus on an infinite object, the additional weavelengths may make the image less sharp as the 135 lens did not have the IR end of the spectrum as part of the design criteria. In effect trying to get good focus on an Ha rich area/object may mean the blue stars are a bit rounded not point like. But it would need a good lens to differentiate. SImply if you go adding spectrum at one end then the other end is not handled correctly.

As it reads that the idea is to use the modified camera and 135 for astro imaging then the DSLR will se set to fully manual anyway and you will be setting the focus so autofocus is irrelevant, as basically are the rest of the camera auto functions.

[Merlin66]

No.

The removal of the internal filters changes the optical position of the CMOS chip relative to the lens.

An optical window is required to correct the back focus distance. This can be the replacing the removed filter with a similar thickness filter or adding a clip in equivalent filter.

 

[sharkmelley]

Using refractive optics (e.g. a lens) you will need something to filter out the near Infra-red beyond H-alpha because those wavelengths will not come to focus in the same focal plane as the visible light.  It would lead to problems such as star bloat.

Mark

[bobro]

Thanks for all of your comments. The Canon 1000D IR filter has now been removed with nothing in its place. The result is the 135mm lens cannot quite achieve (sensor) focus at infinity. To try and correct this I am going to insert a shim on the lens mount to move the lens slightly farther away from the camera - quite easy as the lens is a 42mm screw type. If this doesn't do the trick then plain glass in front of the sensor looks the best bet.

[sharkmelley]

Well done on a successful conversion!

You need antireflective plain glass (or antireflective IR/UV filter) somewhere between the lens and the sensor or you need to move the lens nearer to the sensor, not further away.

Mark

[bobro]

Ah! Thanks for the clarifications Mark. It looks as though the lens can be moved about 1mm closer to the sensor by taking some material away from the lens adapter. If I understand correctly the change in focus due to the IR filter being removed should be about 1/3rd the thickness of the filter, so 1mm should be enough (I hope!).

Bob

[sharkmelley]

That's right, 1mm will be sufficient.

Mark

[sharkmelley]

11 hours ago, sharkmelley said:

That's right, 1mm will be sufficient.

Mark

If you want it more accurate you can calculate the precise adjustment required from the size of the defocused stars at infinity, knowing the focal ratio you used.

Mark

[bobro]

Focus seems to be good now - thanks! Just need the moon to spend a while elsewhere to allow an attempt at better imaging - though you can just see the Flame Nebula in this one if you look closely...

[Stub Mandrel]

I chose a more brutal route, which will deeply uspet camera buffs.

My 450D is strictly astro only (my Nikon Bridge is far more flexible for terrestrial pictures even though it shoots jpegs). I had a nice stock of lenses from my M42 Pentax SP1000 days, so I modified them so they have a little bit of extra travel by doing evil things like removing stops or turning them round by one start of the focuser thread.

Next step is removing the mirror on the 450D :-)

[bobro]

Shouldn't be any problem with mirror flop vibrations then!  Great for super short exposures - do we use them???

[Stub Mandrel]

I'm more suspicious that it's creating reflections or not always fully flipping up when it gets sub-zero. My SP1000 didn't like being used in freezing conditions either and finally gave up the ghost on Helvellyn at about -2, I still think of it as 'recently refurbished' even though it was fixed somewhere around 1986...

[Andy274]

Let me know how its gone as im planning on doing something similar - ive got hold of a 1000D so am not overly precious whether i scrap a few in the hunt of refining the process...

Omelettes & eggs & all that jazz

Cheers

Andy

[bobro]

Hi Andy,

The mod is straightforward if you take care and time and are happy doing this sort of fiddly work. The instructions by Gary Honis are excellent : http://dslrmodifications.com/rebelmod450d1.html

I started off following his youtube video but ended up preferring the web pages as there was more detail there.

Some things I suggest if you decide to do the mod :

• Make sure you have good screwdrivers that fit the screw heads and also have a fat handle. A couple of the screws were really tight and couldn't be removed with a jeweller's screwdriver. Push down well on the screws to prevent the screwdriver turning and destroying the heads.
• Take photos of the ribbon cable connectors before disconnecting the cables. That way you will know how far the cables need to be re-inserted when you put the camera back together. A couple of the cables in my camera were a bit reluctant to be fully inserted, so required a bit of wiggling with a toothpick.
• Ensure there are no cables trapped underneath the circuit board when putting the camera back together as it's frustrating to have to remove the circuit board if a ribbon cable is overlooked.
• A dust blower is useful to help ensure there is no dust on the sensor.
• Cotton gloves help stop fingerprints should the front filter be accidentally touched.

If you are just removing the rear filter (in front of the sensor) without replacing it with another, focus will be affected very slightly. This is unimportant for telescope use as a telescope focusser has a lot of travel but may affect infinity focus with a lens. Whether your lens will come to infinity focus will depend on the lens used and tolerances in your setup. 

The camera is certainly more sensitive to the red end of the spectrum with the filter removed. Images need the red reduced in processing to achieve colour balance, but the Ha emissions at the high end of the spectrum should still be more pronounced and make for improvements to emission nebulae images. That's the theory - if only the clouds and Moon would go away to allow full testing of the modified camera!!

Good luck with the mod if you go ahead.

Bob

[Stub Mandrel]

Wickes, of all places, do a superb set of fine-tipped screwdrivers on long double-ended hexagonal shafts with a big soft grip handle. Several zillion times better than 'jeweller's screwdrivers'.

Worth the £12 to help ensure success and unlike cheapies still usable afterwards!

http://www.wickes.co.uk/Wickes-Precision-Reversible-Screwdriver-Set-8-Piece/p/167315